EUROPEAN SOUTHERN OBSERVATORY . VLT REPORT No. 44

VERY LARGE TELESCOPE

INTERIM REPORT Presented by the ESO Study Group

January 1986 VERY LARGE TELESCOPE

Interim Report

Presented by the ESO Study Group

January 1986 - 3 -

Foreword

The ESO conference "Optical Telescopes of the Future", held in December 1977, was the starting point for intense discussion and studies at ESO during which several concepts were compared. Even at this time an array was considered as a favourable candidate.

The Cargese workshop in May 1983 showed a very positive reaction from the European astronomical community in favour of a very large telescope project. At the end of 1983, a dedicated Very Large Telescope study Group was created within ESO and by October 1984 the initial group was almost complete. A parallel set-up of scientific working groups and a scientific advisory committee was also created and began to function in early 1985.

The initial task of the study Group was the definition of a provisional detailed concept for subsequent technical and scientific analysis. The baseline concept that emerged was an array of fixed independent telescopes. The reasoning behind this selection is given in this document. Briefly, the array concept resulted from the following considerations:

1. trade-off between various scientific requirements; 2. sufficient flexibility to satisfy the needs of the broad European astronomical community; 3. exploitation of ESO's in-house experience in telescope, instrumentation and control technology; 4. best use of European industrial technology; 5. deliberately limiting high risk developments to only those areas that promised sUbstantially improved performance or cost savings; 6. early availability of at least part of the collecting power; 7. the need for alternative concepts and technologies for critical components (mirror, building).

The base line concept has been given to the scientific working groups for critical assessment. The Study Group has analysed the technical feasibility, cost and leadtime. This preliminary analysis is largely completed but some key experiments related to mirror technology are still in progress. Even without the results of these remaining experiments it can be stated that the project is technically feasible and its cost is compatible with the proposed budget.

This document is a report to the European astronomical community and gives the present status of the project. Its form is that of a summary of the critical studies and experiments that have been carried out by ESO or by various industrial firms or institutions under contract with ESO. - 4 -

In Chapter I the main technological issues such as mirror technology and the various means to optimize image quality are discussed. In Chapter II there is an attempt to compare the characteristics, performance and cost of the 3 basic possibilities, i.e., a segmented mirror telescope, a MTT and an array. Chapter III presents the ESO basic concept, its various design alternatives and the result of a preliminary cost analysis.

The authors of this document are:

D. Enard F. Merkle M. Sarazin M. Schneermann R. Wilson L. Zago M. Ziebell

The VLT Study Group was guided by the advice and council of the scientific advisory committee and the scientific working groups. other individuals who have made important contributions to this report include: W. Bauersachs, T. Bohl, M. Cullum, B. Delabre, F. Franza, C. Jauch, P. Giordano, G. Hess, D. Hofstadt, K. Madsen, S. Milligan, K. Mischung, M. Moresmau, L. Noethe, J. Roucher, R. Scharrer, M. Tarenghi, I. Weber

Discussions with many colleagues from ESO's telescope, instrumentation, electronics and scientific groups as well as scientists from other institutions were important for helping the VLT Study Group in their choice of experiments and directions.

Finally, grateful thanks are due to J. Wampler who has critically reviewed this report and himself made important contributions to it.

Daniel ENARD Head VLT Study Group - 5 -

CON TEN TS

PAGE

Foreword 3

Chapter I General Aspects 7

1 Basic requirements for a VLT for the 1990s 7

2 segmented and monolithic mirrors 8 2.1 Segment fabrication 9 2.2 Segment position control 10 2.3 Cost aspects 10 2.4 Optimum size of a monolithic mirror 12

3 Options for a 8-10 m monolithic mirror blank 14 3.1 Zerodur 15 3.2 Borosilicate glass 18 3.3 Aluminium 18 3.4 Non-corrosive steels 20

4 Optical figuring of large mirrors 21 4.1 General 21 4.2 Building and test tower 22 4.3 Milling, grinding and polishing machine 23 4.4 Mirror support during figuring 25 4.5 Nature of the blanks and thermal aspects 25 4.6 Optical testing 27 4.7 Lead time 28

5 Optimisation of image quality 29 5.1 Wind effects on large telescopes 30 5.2 Active optics control system 34 5.3 Seeing limitations 40 5.4 Adaptive correction of the atmospheric perturbations42 5.5 ESO's strategy for optimisation of image quality 50

Chapter 11 concept Selection 53

1 Mirror technology and mirror figure control 54 2 Optical efficiency 54 3 Wide field imaging 55 4 IR observing 55 5 Interferometry 56 6 Flexible scheduling 57 7 Mechanical structure and bUilding 57 8 Redundancy, flexibility 57 9 Cost comparison 58 10 conclusion 60 - 6 -

Chapter III VLT Concept PAGE

1 Concept drivers considered for the fixed array 61

2 Optics of the 8m unit telescopes 65 2.1 Optical characteristics 65 2.2 primary mirrors 69 2.2.1 Options for the mirror blank 69 2.2.2 Steel option 70 2.2.3 Mirror supports 87 2.3 Active correction 91 2.4 Coatings 94

3 Mechanical design of the 8m unit telescopes 94 3.1 General approach 94 3.2 Tube structure 98 3.3 Fork structure 104 3.4 Support structure 109 3.5 Bearings 109

4 Beam combination 114 4.1 Combined Coude focus 119 4.2 Fiber optics for beam combination 123 4.3 Interferometry 125

5 BUilding concept 127 5.1 Optimisation of cost, seeing, wind load 127 5.2 pillars, laboratories, auxiliary buildings 128 5.3 Telescope enclosure 133 5.3.1 Thermal aspects 135 5.3.2 Telescope shelters 137 5.3.3 Service platform 143 5.4 Wind screen 145 5.5 Handling equipment 147

6 Control system 149 6.1 Distributed intelligence 149 6.2 Main servos 151 6.3 Image analysis 151 6.4 Instrument control and data acquisition 152 6.5 User's end and remote control 152

7 Site requirements and site testing 153 7.1 Particularities of an ideal VLT site 153 7.2 Existing sites in Chile 154 7.3 New possible sites 154 7.4 Present investigations 156

8 Summary of VLT characteristics and performance goals 159

9 Provisional cost analysis 165 - 7 -

CHAPTER I GENERAL ASPECTS

1.1. Basic requirements for a VLT for the 90's

A collecting area equivalent to a dish of 16 meter diameter, thus providing a gain of 20 with respect to a 3.6 m telescope, has for a long time been considered an appropriate target for a telescope to be operated in the 90's (1)(2). However, the efficiency of a telescope depends as much on its dimension as on its imaging performance. Therefore, the increase in telescope size should not be allowed to relax requirement for atmospheric limited image quality. Up to recent times it was thought that subarcsec seeing in the visible was rather exceptional. Experience with the existing large telescopes located at high elevation sites suggests that the ultimate limit of the atmosphere may lie somewhere below 0.5" consequently the telescope quality should be improved at least by a factor 2. Often, existing telescopes are as much limited by the thermal inhomogeneities of their immediate environment and by poor support and collimation of the optical elements as by the atmosphere. If ultimate performance has to be obtained, an important effort to better control the thermal environment and optical alignment of the telescope has to be undertaken.

Finally, the cost of the project must be kept under control. This alone would justify the development of new technologies because the extrapolation of the classical solutions would certainly lead to prohibitive costs.

As shown in this report the target of a 16 meter aperture cannot be obtained with one single mirror and in one way or another it must be achieved by combining several smaller mirrors together. TWO directions can be considered: one consists of assembling small contiguous elementary mirrors so as to restore a continuous surface (segmented mirror), the other is to build several telescopes with the largest single mirrors that can be produced and recombines the beams of these telescopes. They can be either mounted together in one structure or have independently steerable mounts.

Unit telescopes with 8 m apertures are seen as providing the best compromise between the various conflicting requirements. The scientific needs, cost optimisation and reduction of system complexity suggest the largest dishes, while the wish for early availability, lowered risks and ease of handling and transportation argue for smaller sizes. - 8 -

Under best seeing conditions and at a wavelength of 10 urn, 8 m dishes would provide a diffraction limited image slightly larger than that corresponding to the residual errors of the telescope, so that an optimu~ combination is then achieved; only 4 beams need to be combined together to provide the 16 m equivalent aperture; handling and transportation can still be managed and a reasonable extrapolation of a factor of 2 in size with respect to the previous generation of telescopes give confidence that all technical problems will find an adequate solution.

The question of adequateness of the different concepts to the observing goals is only briefly discussed in Chapter II.

I.2 Segmented and monolithic mirrors

In view of the difficulty of getting large mirror blanks of glass, the idea of dividing up the collector in small parts to be assembled together so as to restore the ideal surface, is extremely attractive, and has been suggested and discussed many times. The first experimental 4 meter segmented mirror telescope has been built around 1970 by P. connes and al (3). By lack of funding, this telescope never reached an operational state. A 10 m segmented mirror telescope is under construction at CALTECH, and University of california.

A segmented mirror would have 3 main advantages:

a) The blank requirements are limited to the size of the segments not of the final telescope aperture.

b) The equivalent mirror thickness is that of the segment. Because the internal support of each segment must be passive, the aspect ratio of the segments should be close to conventional. The mirror thickness is then only determined by this ratio and the segment size. A 16 meter segmented mirror made of 1 to 2 meters segments could have a thickness of 100 to 200 mm and a final aspect ratio of 80 to 160. The relatively light weight of the primary mirror has also favorable consequences on the telescope structure and performance. The total cost of the primary is proportional to the mirror area and goes as D2 , since the quantity of glass, the number of elements to polish and the number of supports scale in proportion to the mirror area.

c) There is in principle no limit in size. This may not be important for ground based telescopes because the size may be set by other factors such as the bUilding size or the aerodynamic load on the structure. For space based telescopes, these limits do not exist and the possibility to fly a large collector made of small and transportable elements i~ indeed attractive.

Nearly all telescopes have aspheric primary mirrors, the only notable exception being the Schmidt telescope. parabolic or hyperbolic shapes give optimum field correction. It is in principle possible to use a spherical primary mirror which in the case of a segmented mirror would allow identical axisymmetric segments. The compensation of spherical aberration alone can be achieved by means of a single element corrector either refractive or reflective. Correction of field aberrations is a much more difficult problem and although a number of solutions have been - 9 -

proposed, none really provides more than a few arcminutes field of view. TO maintain an adequate optical quality over a larger field of view, the primary mirror can hardly be faster than F/3 to F/4 so that the telescope tube would be much longer than with a classical combination for which a fast primary is not a fundamental limitation. Designs have been proposed for correctors of very fast primary mirrors (4) (5) (6) (7) (8) (9) but the corrected field of view does not go beyond 2 arminutes (without providing access to large field intermediate focii unlike a MMT or an array) and the centering tolerances of the corrector elements are very tough. Though a spherical segmented primary could possibly be considered for some specialised applications (e.g. transient telescope) it does not provide a real alternative to an aspheric mirror for a general purpose telescope.

There are 2 major technical problems associated with the development of a segmented mirror: the fabrication of aspheric segments and the figure control. Both present a terrific challenge.

I. 2.1 Segment fabrication

Stressed polishing has been widely used, first by Schmidt and further developed by Lemaftre (10) to produce deformed axisymmetric plates with low optical tolerance. An associated idea, hardly applicable to a segmented mirror, that of modifying the shape of a mirror by applying after polishing a controlled stress has also been used and found an application recently to correct the curvature of the secondary mirror of the CFHT (11). It is also the basic principle of active optics.

In 1979, J. Nelson demonstrated the application of the stress­ polishing method to a 14 inches aspheric mirror (12). The problem is however that a turn-down effect is present at the edge of the mirror like with any other polished optical surface. This turn-down cannot be tolerated in a segmented mirror because, present at many places in the pupil, it would have disastrous effect on the image quality. A solution is to cut the mirror after polishing at its definitive dimension and then remove the outer region of bad quality. Unfortunately this operation generally deforms the optical surface by relieving internal stresses which, though extremely low for high quality glasses, are still present. An attempt by KPNO to produce a 1.8 m segment with 0.1 wave accuracy and substantial correction was not successful (13).

A second possibility is to use a small polishing tool whose movement is computer controlled with the extreme accuracy. It is in particular through such a method that one of the 2 alternative primary mirrors of the space telescope has been produced. A few U.S. industrial firms claim to have mastered the technology and it is the solution finally adopted for the 10 m Keck Telescope. Though possibly appropriate, the method is rather slow and expensive.

A third solution could consist to pre-cutting the segments at their final shape and assemble them together in a stiff and stable structure as if they were in the telescope and polish the complete mirror in a conventional way. Indeed, the polishing machine has then to be adequately dimensioned. This could well be the cheapest way, if the problem of the support system during figuring can find a satisfactory solution. Apart for blank procurement, this method would be similar to the processing of a monolithic mirror. The problem of turned-down edges at the segment interspaces may remain. - 10 -

I. 2.2 Segment position control

Since the segments are likely to be much larger than the Fried parameter r o (see chapter I. 5.3), a co-alignment of the segments is in principle sufficient in the visible. That means the relative phasing of each segment with respect of its neighbours is only determined by the tolerance on focus. In the IR however, ro can be larger than the segment size and the whole array may possibly be phased, and produce a diffraction limited image. In that case, the relative phasing becomes the limiting parameter. This IR requirement translates to an individual center of curvature requirement of one part in 105 • NO off-axis segment with this tolerance has ever been produced.

In practice, the positioning accuracy of the control system must be of the order of 10 to 20 nm. TO obtain such an accuracy on elements whose weight may approach one ton is a challenging task. Much progress in meeting this goal has been obtained by the team of the Californian 10 m Keck telescope (14) using capacitor position sensors and high accuracy mechanical actuators. Remaining problems include improving the response time, isolating the servo system from structural resonances and insuring high reliability. A 16 m telescope would have several hundreds of sensors and actuators.

A major difference with a monolithic mirror active support based on a distribution of forces is that an error anywhere in the segmented system will show up as a local deformation of the wavefront, whereas for a monolith, low order modes, above all astigmatism, will be generated. computer modelling and experiments have shown that an accuracy of 1/1000 for the force based supports is sufficient (approx. 1 N) and easily achieved by simple means. By way of comparison, the 10 or 20 nanometers accuracy necessary for a segmented mirror (see chapter 11.4) would be more difficult to achieve.

I. 2.3 Cost aspects

cost of blanks: For traditional passive support systems, the aspect ratio is generally kept constant leading to a variation of the mass of the mirror with 0 3• We will assume that the cost of blanks varies linearly with the mass. For active supports, the limiting factor is the sag between supports so that for a constant density of supports, the thickness only should be conserved and the cost would then vary as 02 (see chapter 1.3). Therefore, the cost of an active mirror blank may vary'as 02 , that of a conventional passively supported mirror as D3 and for a segmented mirror as 02 and d3 (d being the segment size). We will consider here only solid blanks, not lightweight structures for which cost extrapolations are hazardous.

cost of figuring: Analysis of the cost of various mirrors as well as recent studies performed in the frame of the VLT project (15) (16) show that the cost of figuring an astronomical large mirror may vary as 01• Investment and tooling are not considered here; since we assume that several mirrors will be produced, this part of the cost will be shared on several units. -11-

It is clear, however, that if they were considered, the curve of figure 1 will probably exhibit a step increase at 4 meters diameter. Also we have not attempted to considered the cost of the supports because this depends very much on the type of requirement one considers, for instance with respect to wind load.

On the basis described before, one can attempt a cost comparison between the 2 solutions by assuming that the cost for producing an off-axis segment is, at best, comparable to the cost for producing an on axis surface of identical dimension. A non-axisymmetric optical surface is in practice far more difficult to produce so that this assumption is definitely not correct. Nevertheless, it can provide an extreme lower limit for the cost of a segmented mirror relative to a monolith. For a low expansion glass blank and a conventional aspect ratio of about 8, experience on previous projects shows that the cost for figuring a mirror becomes roughly equivalent to the cost of the blank for a diameter of about 2 meters.

Let C2 be the total cost of a 2 m mirror.

The cost for figuring a mirror or a segment is then:

C 2 (0 or d) C f =--2 2

The cost of the blank(s) would be:

C 3 2 =-- (Q) for a monolithic blank and a passive support. Cb 2 2

C 2 2 (Q) for a monolithic blank and an active support. Cb 2 2

C 3 2 2 =-- (£ ) (Q) for a segmented mirror. Cb 2 2 d

The total cost of a monolithic active mirror is:

2 C 0 2 1 1 C = (- + -) MA 4 o 2

For a segmented mirror we have:

(-1+ -) d 4 d 4 - 12 -

Fig.l shows the variation of the cost per unit of collecting area for the 2 solutions. The cost of the NTT mirror has served as calibration. Surprisingly, and despite the crudeness of this analysis, the current estimates for the Keck 10 m telescope and the ESO VLT fit reasonably well with the predictions.

The conclusion is that a segmented mirror is bound to be more expensive for large diameters than an active monolith and should optimally be made of segments of about 2 meters in size. very large segments would inevitably drive the cost of the mirror up as well as indirectly the cost of the structure because the weight of the mirror will increase linearly with the segment size. A reduction of the complexity of the control system cannot compensate for the very steep increase of the cost function. This analysis does not take account of the technical difficulty to figure aspheric off-axis segments which will considerably raise the cost of the segmented mirror. Also, one has assumed the blank technology to be identical in both cases, i.e. based on low expansion glass. New possibilities indeed exist which, coupled to an active correction, would once more increase the difference. Indeed very large blanks have not yet been produced. The use of a new technology or the extrapolation of an existing one implies necessarily a certain risk but in none of the 2 solutions can the risk be avoided; they are simply of a different nature.

compared to a classical mirror with a constant aspect ratio and whose blank cost would increase as D3, the segmented option is extremely competitive, but it is evident that for reasons of cost and weight, a classical blank of diameter larger than say 5 to 6 meters is simply out of consideration. Therefore for large diameters the options that can realistically be considered are only the segmented mirror, or a lightweight monolith.

A number of technologies discussed in I.3 could be used to produce an 8 m mirror with a final weight of 15 to 20 tons. A segmented mirror of comparable size and composed of 2 meter segments of aspect ratio 10 would weigh 25 tons. The segmented mirror is lightweight only with respect to a conventional mirror not to an active or lightweight monolith. It is indeed possible to select a greater aspect ratio so as to decrease the mass. The Keck telescope has for instance an extreme aspect ratio of 20 but then, the segments internal supports which can only be passive becomes complex. EVen in that limit case, the mass per m2 is only 40% less than for a lightweight monolith.

I. 2.4 Optimum size of a monolithic mirror

We have defined a size of 8 m as a nearly optimum value for the following reasons:

i) The analysis of mirror production problems (r.3) has shown that 8 m mirrors could be produced within a reasonable time and with good prospects for excellent optical quality. -13-

MDM

.8

.6

VL T ESTIMATE .4 ___G)~1::.- ~I-=ZE~R~ODUR BLANK)

.2 o d

2 3 4 5 6 7 8 m SEGMENT SIZE Id) OR MIRROR DIAMETER (0)

Figure 1: cost per m2 of mirror (blank and figuring). curve 1 corresponds to an active thin mirror and is plotted against mirror diameter. curve 2 corresponds to a segmented mirror and is plotted against segment size.

Thermal Specific t Material Thermal Specific Thermal Thermal Young's Poisson s Mech. expansion conductivi- Heat Mass. Diffusivity Sensitivity Modulus Ratio bending ty resistance a A C P 6 • A/C 'P 6/a E P p V K'P~I'_V"I 8 [Ko'] "0- ['tt'm.K] [J/ Kg. KI [ Kg/ m3] [m2/ ••c],C18 [m 2 K;'.c] [N/mm2) 103 [-] [m].03

Aluminium 23 :27 879 2700 95 4.16 72 0.34 3074 Steel 13/4 11 25.1 502 7750 6.45 0.59 210 0.28 2997 Invar 1 10 500 8130 2.46 2.46 145 0.30 1998 Borosilicate 3.2 1. 13 1047 2230 0.48 0.15 68 0.20 3238 Ule 0.05 1. 31 770 2200 .0.77 15.4 66 0.17 3149 Zerodur 0.05 1. 64 821 2530 0.79 15.8 91 0.24 3891 CFRP 0.2 10 712 1800 7.8 39.0 105 0.32 6625 (UHM-quasi- isotropic)

I!'igure 2: comparison of thermal and mechanical properties of various materials. - 14 -

ii) Transportation within Europe of an 8 m blank appears possible with minimum inconvenience. An extension of the diamete~ to 10 m would probably lead to much greater difficulties.

iii) Because the atmospheric seeing varies as A-lIS and the diffraction limit as A/D, image quality in the IR is usually limited by diffraction for existing telescopes. A larger aperture can therefore provide a better resolution, up to a limit set by the optical quality of the telescope and the seeing. Under best atmospheric conditions, an equal contribution from seeing and diffraction are obtained at 10 microns with an aperture of about 8 m. Efficient use of a larger aperture necessitates an adaptive correction of the atmosphere phase distortions. Yet the remaining errors of the primary mirror, particularly high spatial frequency errors which cannot be corrected either with active or adaptive optics, will then determine an ultimate limit. Whereas some gain could still be expected at 10 microns and beyond with a larger aperture, even a very ambitious goal such as an instrinsic image quality of 0.1 arcsec will not permit taking full advantage of an 8 meter aperture below 5 microns for which the diffraction limit would be 0.12 arcsec. Increasing the size of the aperture is likely to be linked to a degradation of the optical quality and it is therefore doubtful whether an aperture much larger than 8 meters would really produce at the end any better imaging performance.

1.3. Options for 8-10 m monolithic mirror blanks

AS indicated in the last paragraph, the cost of conventional blank procurement grows more rapidly with size than figuring costs. This explains immediately the intense activity in blank development which has emerged with projects markedly exceeding for the first time the diameter range of 3,5-5 m of the last generation of "conventional", equatorially-mounted telescopes.

Modern methods of active control permit the use of lighter and more flexible blanks, but "conventional" materials such as glass ceramic or ULE-quartz are still bound to be expensive for sizes of the order of 8 m. The physical nature of glass ceramic (a remarkable technical development to achieve effectively zero expansion) is such that the production of lightweighted structures presents a problem of formidable technical complexity. Both cost and technological aspects are therefore drivers in the current activity concerning new methods of blank production and a re-appraisal of the merits of different materials.

very low expansion Zerodur glass-ceramics were the result of a 100 year long, systematic development in the use of glass blanks for telescopes. These followed a 200 year long development of speculum metal. In view of the modern possibilities of opto-electronic control discussed below, it is essential to consider objectively what physical properties of the blank are really necessary for a modern telescope. Although a zero or near-zero expansion coefficient is clearly an advantage, modern control systems in the telescope and figuring techniques in the optician's factory can handle materials with an expansion coefficient significantly - 15 -

higher than that of sophisticated quasi-zero expansion glasses. Angel's work on lightweighted blanks (18) in cheap BSC glass is precisely in this direction.

ESO has investigated in depth the possibilities of metal for blanks. Already for the NTT it was clearly established that it was perfectly feasible to manufacture an aluminium blank coated with polishable nickel of 3,5 m diameter. For the VLT we consider that aluminium remains a serious candidate but further studies have shown that non-corrosive steels, such as ferritic chrome steels or nickel steels seem particularly promising. The favourable thermal diffusivity of such materials compared with glass makes such steels excellent candidates for the VLT blanks from a technological viewpoint and the evidence we already have shows that the essential properties of polishability, corrosion resistance and form stability can be adequately met. The cost and leadtime advantages over glass are formidable, particularly with the use of a new manufacturing technique initially developed for the manufacture of pressure vessels (see chapter I.3.4). An additional advantage of steel blanks is that their expansion coefficient matches that of the mirror cell, simplifying the support system.

The above comments are illustrated by Fig.2, which gives the significant physical properties of the materials most interesting for blank production. Attention is drawn particularly to the column 6 ( = diffusivity/expansion coefficient) which may be considered as a rough figure of merit for thermal distortions of the blank, as has been confirmed by a recent finite element study (15). The thermal link with the ambient air (affecting "dome seeing" - see chapter I.5.3) will depend in a complex way on the effective cooling surface, thermal capacity, and diffusivity. A detailed discussion of the desirable physical properties in blanks, with an analysis of their impact on blank development in the light of the current rapid evolution of telescope technology in general, is given in (17). The four most interesting blank materials will now be considered in more detail:

I. 3.1 Zerodur (Glass ceramic)

The possibilities of producing such blanks in Zerodur have been the sUbject of a study performed by Schott for ESO.

It emerged clearly in this study that the production of blanks of this size by techniques similar to those used for existing 3,5 m blanks, i.e. by casting massive blocks and machining down to required meniscus dimensions would require investments that are commercially unacceptable. (An amount of 100 MDM has been mentioned).

Since ESO had considered a structured (lightweighted) blank to be more favourable (without necessarily rUling out a meniscus) and since Schott considered the market to. be more interesting for such blanks, emphasis was placed in the study on structured blanks. For such blanks, Schott has analysed the following methods of manufacture: - 16 -

I. 3.1.1 casting

The technology is very difficult, the difficulty increasing with decreased wall thickness. 40 mm has been achieved, 20 mm may be possible.

TWO basic techniques are possible:

- Direct casting whereby the liquid glass flows round the mould structures.

- Plunging a form into the liquid glass.

Because of -fold- effects where glass flowing from two sides meets, the first technique seems less promising than the -plunge- method. For 8 m blanks, casting seems feasible, assuming the required temperature control during and after pouring can be achieved. The difficulty of achieving this increases rapidly with diameter.

I. 3.1.2 Bending and high temperature fusion (HTF)

plates of 4 mm thickness of vitreous Zerodur can be bent and welded to form a structure. Such plates are, however, considered too thin for an 8 m blank. It is assumed that thicker plates can be HT welded: the upper limit of thickness is, however, not yet known. The problem with HTF is that the allowable heat-up time to the melting temperature is only a few seconds, otherwise unacceptable pre-nucleation occurs. For this reason HTF is not promising for an 8 m mirror with relatively thick ribs.

I. 3.1.3 Low temperature fusing (LTF)

The bonding by this process takes place at lower temperature than HTF. The fundamental problems of LTF (e.g. sufficient bond strength while maintaining acceptable stress values) can be considered to be solved. It follows that the connection of a cell structure to a faceplate, both consisting of vitreous Zerodur, by LTF to manufacture a lightweight blank of 8 m diameter and weighing 15 tons will be feasible with high probability. Tests have been performed with several different wall and faceplate thicknesses. Nevertheless, a number of detailed experiments would have to be carried out. This would lead to tests up to 4 m diameter. Fig.3 shows a test blank of about 400 mm diameter realised with the LTF process.

For ESQ's VLT, LTF may therefore be considered as easily the most promising process. It also promises to be the cheapest and most rapid process.

LTF was developed for bonding structures to faceplates made by spincasting or slumping. However, it may also be possible to produce faceplates of smaller sections. Under this aspect, new technologies for the manufacturing of solid meniscus - 17 -

Figure 3: Lightweight Zerodur mirror blank from SCHOTT.

Figure 4: Build-up of a steel mirror blank. The front-plate and the ribs are produced by addition of welding seams. This experimental 500 mm blank has been produced in less than 3 days (courtesy of SULZER). - 18 -

blanks might come up. This possibility requires, however, further investigation.

I. 3.2 Borosilicate glass (BSC, pyrex, Duran 50)

The work of Angel et al.(18) has re-awakened interest in a material which had been abandoned for telescope blanks by the professional glass making companies in favour of ULE fused silica or glass ceramic. (Schott, for example, will no longer supply Duran 50 in diameters more than 40 cm). The interest of BSC glass is, of course, its low bulk price compared with zero expansion materials, about an order of magnitude cheaper. The experience of the USSR 6 m BSC blank has shown clearly that a massive block of such material has a thermal inertia so great that unacceptable distortions are inevitable in a passive telescope. The experience at Lick Observatory with a 3 meters blank of BSC is somewhat confused but there are indications that the blank is only marginally capable of producing sub-arcsecond images when there are temperature gradients across the blank of one degree (32). The palomar 5 m, on the other hand, was a brilliant development in the pre-computer age whereby a lightweighted structure of BSC was able to adapt thermally, to a sufficient degree for the specification at that time, provided great care was taken to match the structure and mirror temperatures(4). It is thus plausibly argued by Angel that a thin-walled, lightweighted mirror in BSC can give the performance demanded of modern telescopes if air cooling is provided. His open-mould spin-casting technique has produced blanks with a diameter up to 1.8 m. Whether the technique will work up to 8 m diameter can only be proven by actually doing it, but it seems reasonable to suppose that the chances of successful manufacture are quite good.

Much more serious are the thermal problems associated with BSC. Its thermal sensitivity is easily the worst of the materials considered here and its value for diffusivity is bad, as for all glasses. There must therefore be doubts about both thermal distortion of the blanks and about thermal inertia affecting "dome seeing". In a solid blank, thermal distortion would certainly be only in low spatial frequency modes. Such modes may also exist in a lightweighted blank due to radial gradients and might be corrected by an active optics system (see chapter 1.5.2). More serious is the doubt concerning" print-through": the appearance of the structure in the polished surface. The danger is probably higher during the polishing phase than during operation in the observatory. Only experiments can satisfactorily answer these questions and give clear results.

Similarly, from the point of view of "dome seeing" effects, the degree of success of the active cooling will determine whether the weakness of low diffusivity can be overcome.

I. 3.3 Aluminium

Since the disappearance of speculum from the reflecting telescope about 1865, aluminium is the only material apart from glasses which - 19 -

has been used for telescope blanks of 1 m diameter or more. Aluminium, including somewhat harder alloys, is too soft to be polished directly. Its use therefore only became practicable with the industrial development of nickel coating, usually deposited by chemical means. Such coatings, to which the trade name ·canegen" is usually applied, consists of about 90% nickel and 10% phosphorus. canegen coats are softer than glass but have been successfully figured to the tolerances required by astronomy. Electrolytic coating is also possible but is unproven for practical mirrors.

In the 60's an ambitious project using aluminium blanks for a series of 1 m telescopes was launched by H. Johnson. As reported by Forbes(19), the results at first seemed promising, but sUbsequent warping of the mirrors resulted in their successive withdrawal from service because of poor image quality. The failure of this bold experiment was particularly unfortunate since it gave metal in general a bad name in the USA as a material for blanks without any proper analysis of the reasons for the failure. We believe the failure was due to an unfortunate choice of blank form (a "vase" shape instead of sticking to a conventional meniscus or flat-backed blank) combined with inadequate heat treatment.

A much more successful project was carried out in Italy in 1968 with the 1,37 m telescope at Merate(20), which has an aluminium primary. ESO tested this telescope in 1983, after 14 years of normal use, and found its quality to be completely comparable with that found for similar telescopes with glass primaries. We found an astigmatism coefficient of about 1 wavelength which mayor may not have been partially or entirely due to warping. With active control (see chapter 1.5.2), the removal of such a term would be trivial.

We may therefore conclude that the Merate telescope, in operation now for 16 years, has proven that an aluminium primary of 1,37 m diameter is quite capable of fUlfilling a "normal" specification for a passive telescope and that, in an active mode, considerably larger warping defects than have appeared could be corrected without difficulty.

In view of the positive test result for Merate, experiments in warping of aluminium blanks due to thermal cycling were carried out in the ESO laboratory(2l)(22). The results indicate that a variety of alloys of aluminium, or the "pure" metal, made by a variety of processes, can give very low warping provided correct heat treatment has been applied. The warping range would then be well within the range of active correction. It was therefore decided to tender formally to industry for an aluminium blank for the ESO 3,5 m NTT. Several responses were received that gave us great confidence that such a blank could be manufactured to a very severe technical specification in at least 4 member states. Delivery times and price including the canegen coating were of the order of 35% of that of a conventional Zerodur blank: It was finally decided not to order the aluminium blank for the NTT because of timing problems. Although the delivery time was much shorter than that for a Zerodur blank, the latter had, in fact, been ordered far earlier as the basic blank giving minimum technical risk. The aluminium blank had - 20 -

been seen as a second, experimental blank hopefully to be delivered sooner. Since the order could not be placed soon enough, it would, in fact, have been delivered after the glass blank in spite of a much shorter leadtime.

The main (but certainly not fatal) technical problem that emerged was not the procurement of the aluminium blank but -breakthrough­ problems due to the thinness of the canegen coating during the figuring phase. For the VLT, therefore, attention turned to non-corrosive steels as a more promising alternative. Since these can be polished directly they avoid the problems associated with coatings.

The NTT aluminium blank was therefore abandoned with considerable regret to concentrate on steels for the VLT. However, reference to Fig.2 shows that thermally aluminium is easily the best of the cheap metals. Furthermore, its low density (comparable with glass) is very favourable. In view of the new possibilities of manufacture by the so-called build-up process (see chapter I.3.4), aluminium must still be considered as a very interesting candidate for the VLT. production of an 8 m aluminium blank would be less easy than in steel, but is certainly feasible. The problem with the thin canegen coat remains.

I. 3.4 Non-corrosive steels

In the previous chapter, it was pointed out that non-corrosive steels seemed a more interesting candidate for 8 m VLT blanks than aluminium because there was evidence that some such steels can be polished directly and do not require a coating such as canegen. In addition, casting and other manufacturing technologies are available in larger dimensions for steels than with other metals.

The first step was to establish which non-corrosive steels were the most promising from the point of view of polishability, corrosion resistance and thermal properties. AS mentioned above, a general review of metal blank materials according to their physical properties and methods of manufacture is given in Ref.l7. For VLT 8 m blanks the current state of our development work for steel blanks is as follows:

The most promising steels seem to be certain ferritic or martensitic steels. Small samples have shown that with correct heat treatment, these can effectively be polished as well as glass. Austenitic steels can also be polished but have inferior thermal properties. Invar is a very interesting extreme case and 'can also be polished. It has a thermal sensitivity ratio similar to aluminium.

Of classical production techniques, casting seems the most feasible for 8 m blanks unless welded joints are accepted. Welded structures would require further work to test their stability but, apart from that, weld seams could present a polishing problem. For this reason, methods such as casting that give a continuous faceplate are preferred at this stage. - 21 -

However classical casting in these dimensions raises considerable doubts concerning the homogeneity of the material from the point of view of polishing. A new technique of manufacture, originally developed for the production of high performance pressure vessels, has been proposed at ESO for making either whole blanks or faceplates. This method, in its most promising form called "build-up (BU) welding", seems to solve completely the homogeneity problem, for any dimension. The method consists basically to piling-up welding seams in such a way that a structure is progressively built-up. The machine consists essentially of a computer controlled movable welding head. Because a very small amount of material is deposited at each pass, any variation of homogeneity of the material is automatically distributed into the blank in a random way. Whether a whole blank would be made by this process or only the top-plate is still open: it is merely a matter of technical optimisation. Fig.4 shows an experimental 500 mm BU blank being manufactured. The building-up of the ribs did not give particular problems. With simple equipment a blank of nearly any size can be built, the process by its nature being easily scalable. The experimental blank was produced within a few days.

With the encouraging results of the first samples, a number of 500 mm blanks have been ordered from 3 firms, some produced by classical casting, some by BU welding, some by conventional welding. These will be polished spherical by optical firms. If these results are successful, thermal cycling will be performed. The principal question to be answered is the degree to which these mirrors can be polished and figured. If this is successful, the prospects for 8 m steel blanks will seem to be very good. Of course, a larger test blank will be necessary. If a blank of 1 - 1,5 m can be successfully produced with BU, the way to 8 m seems relatively clear.

If the methods investigated are successful, the costs and leadtimes for steel blank production cannot fail to be very favourable when compared with glass.

1.4. Optical figuring of large mirrors

I. 4.1 General

AS has been stated above, the cost of figuring large glass mirrors grows less rapidly with size than that of conventional blank procurement. Since cost is also a measure of difficulty, it follows that the technical problems engendered by size extrapolation are ­ while still considerable - less serious for figuring than for conventional blank production. The pressure for radical technological innovation is therefore also less for figuring.

This is an important general conclusion: for it implies that current technology is basically able to cope with the figuring of 8 m glass mirrors. Of course, this does not mean that considerable efforts will not have to be made in certain specific areas. - 22 -

Furthermore, it does not mean that financial investments of a major character may not be required in order to extend the technological facilities to 8 m diameter.

There are only two suppliers of large precision optics in western Europe. These are REOSC in France and CARL ZEISS in West Germany. Both companies independently performed feasibility studies for the figuring of the VLT mirrors under contracts with ESO (15)(16). While there are significant differences in their technologies depending on experience and preference, both firms feel entirely capable of producing the VLT optics. The optical performance that can finally be produced depends much on our active optics concept (see chapter I.5) and the limits will above all be set by the test methods.

It seems inevitable that major investments have to be made in buildings and technical installations. The question of funding for such an investment and the assessment of a possible market becomes central.

From the technical and financial analysis performed independently by the 2 firms, it comes out clearly that the cost for polishing a large mirror depends on the following functions of 0 (diameter) and of the focal ratio (A): 01 A2 whereas the capital investment for buildings, machinery and tools would vary approximately as 02.A-O,5.

Considering that the figuring of such l~ge mirrors will require new techniques to be developed, the first mirror may be more expensive and also if some fraction of the specific tooling is not considered as capital investment and is affected to the project, the average cost of each VLT mirror may then vary as 01 •5 A2. This relationship is in itself a strong justification for a project based on large monoliths, and also indicates that going to a fast primary would become rapidly very expensive. Adding to this, the increased lead time and the risk of not attaining our stiff optical requirements, it may not be desirable to go much faster than F/2. This is the aperture considered at the present time for the preliminary study although technological improvements may lead to consider F/l.7 as a possible trade-off between the increased cost of the mirror and savings in the control tower as well as in the telescope building, and mechanics.

The following chapters (I.4.2 to I.4.7) will deal with the main aspects affecting the total operation of optical figuring.

I. 4.2 Building" and test tower

Neither firm has existing building facilities suitable for 8 m blanks. Such infrastructure costs are always relatively high but most of the infrastructure, machinery and test tools can, of course, be re-used. An important matter is the question of a - 23 -

quasi-vacuum test tower. It seems probable that the additional investment for this is justified. REOSC however intends to use Helium as a substitute. Helium has an index of refraction about 10 times lower than air and is therefore 10 times less sensitive to thermal gradients. A vertical conical tube made of modern plastic fabrics would have a leakage of less than 2 liter/m2/day. Between tests, Helium which is much lighter than air will sta~ in the upper part. The cost of Helium which is less than 30 DM/m makes this solution attractive at least for the final polishing phase. It is assumed by both suppliers that neither vacuum nor Helium will be necessary till the mirror is close to its final shape.

Handling of the mirrors and question of overlap in the production schedule (to improve total leadtime, for example) will have a major impact on building layout.

I. 4.3 Milling, grinding and polishing machine

It is probable that a single machine, adapted to the different processes including the polishing, will be the best solution. It is a matter of opinion and debate whether standard machines on the market are adequate or not.

The most interesting modification of existing technology is the question whether it is possible, and if so the most economic solution, to mill the surface to a fair approximation to the final aspheric. The main requirement here is to maintain axial symmetry, above all to avoid a residue of astigmatism which could not be easily removed in the polishing phase. A standard high accuracy machine could provide a final accuracy of 30 to 50 ~m. A better accuracy could be obtained if the azimuthal errors are kept under control. CARL ZEISS has proposed a scheme in which the run-out and tilt errors of the turntable are controlled with a laser interferometer (Fig.5). If the mirror is adequately supported an accuracy of a few microns could be obtained.

The smoothing, polishing and figuring processes are essentially conventional. An important aspect is the size of tools and laps which can be used. More because of the higher value of the relative aperture D/f with modern concepts (around f/2 instead of f/3) than because of the large diameter, it will not be possible to use fUll-size 8 m tools: 4 m will be about the maximum that can be envisaged. Of course, the larger the tool, the greater the smoothing effect. The high quality requirements of the VLT specification (see chapter I.5) will require exceptionally smooth surfaces. Highly flexible tools, or possibly active tools, will have to be considered. computer control methods for determining lap form, pressures and speeds will.clearly be used as required by the phase of the work.

The relaxation of low spatial frequency tolerances, as with the NTT, will have much more significance for the VLT, since the dynamic range of active correction will be much greater and the relaxation correspondingly more. This should produce a considerable saving both of time and money. - 24 -

4

High precision machine 7 - mirror 2 - grindmg spindle with support 3 - reference flat 4 - laser interferometer Figure 5: principle of an interferometrically controlled grinding machine according to C. ZEISS.

Figure 6: ZEISS optical facility project for the production of 8 m mirrors. A vacuum control tower is located above the grinding and polishing machine. - 25 -

Figs. 6 and 7 show CARL ZEISS and REOSC plans for their optical shop with machine, test tower and handling facilities.

I. 4.4 Mirror support during figuring

The design of an adequate support system for the 8 m blanks that have, in any case, a flexibility an order of magnitude higher than the NTT 3,5 m blank and two orders of magnitude higher than those for conventional telescopes like the ESO 3,6 m, is mandatory for the successful production of the mirrors.

The studies of the two firms show that the support problem during figuring for an 8 m VLT blank may be considered to have been solved. In both cases, the relaxation of certain optical tolerances because of active optics control will simplify support requirements. REOSC proposes a pressure variation control which is, in the manufacturing phase, to some extent similar to the concept of our own active control system for the final telescope in function. For acceptance, the supports, will have to be able to simulate the support conditions in the final telescope cell.

I. 4.5 Nature of the blanks and thermal aspects

Whatever the blank material - even in Zerodur - thermal aspects will almost certainly set the limitations of what can be achieved. CARL ZEISS has done a thermal analysis in some depth on the problems associated with the figuring process for blanks of different materials and with or without structure. AS would be expected from Fig.2, the quality of materials from the point of view of thermal warping during polishing goes from Zerodur (easily the best) through beryllium, invar, aluminium, ferritic non-corrosive steels, austenitic non-corrosive steels (·stainless steels·) to BSC glass (pyrex) as easily the worst material. This will apply to both long-wave (low spatial frequency) distortions and to ·print through·. This does not mean that BSC glass will notwork: it means simply that if the thermal problems can be solved for BSC (which would have to be proved by experiment) then these problems will be easier to solve for any of the other materials envisaged. print-through is above all dependent upon the internal thermal conductivity of the blank. A possibility to improve the thermal conductivity of a steel structure by dp.positing, through a spraying technique, an aluminium layer is presently considered at ESO (see II.2).

If unacceptable ·print-through· occurred for a given structure and material, this would be fatal for that choice: either the structure or the material, or both, would have to be changed. LOng-wave warping may also be serious for a given combination because of the ·cooling-down delay· for figure stabilisation. This was responsible for the long leadtime for the palomar 5 m mirror. Excessive increase of the leadtime for thermal reasons would effectively rule out such a combination for the VLT. It has still to be proven experimentally that fine, highly-lightweighted structures in any material/even zerodur) will not lead to unacceptable print-through. FA~ADESUll-EST FA~AOESUO- OUEST

N 0'1

AXOrlOI1ETAIE

Figure 7: REOSC optical facility project for the production of 8 m mirrors. A plastic fabric -tent- is located above the machine. For the final tests it is first evacuated and then Helium filled. - 27 -

I. 4.6 Optical testing

If aspherics are produced by milling or grinding, test methods will be required at this stage. Various methods have been proposed by the two manufacturers, ranging from beam spherometry to interferometry in the visible or IR-interferometry (possibly also millimetric interferometry). ESO has the impression from the studies that both manufacturers are fully capable of solving the technical problems involved in view of the fact that the precision required in this stage is not very high.

In the polishing phase, the test procedure is of fundamental importance: one can only reliably correct what has been reliably ~easured. A two-track approach to testing is the best. For the acceptance, the following two basic methods could be used:

SHACK-HARTMANN (S-H) testing, as introduced by ESO for off-line telescope tests(23). Interferometry (although wave-shearing or other types of interferometry might also be acceptable, both suppliers prefer LUPI interferometry, the form also preferred by ESO).

In both cases, following the approach laid down for the NTT, ESO is of the opinion (and both firms have agreed) that a set-up should be used in each case with so-called compensation or null systems. The set-up is therefore, in principle, identical and only the image analysis end is different. It should be noted that the price paid for the advantages of compensation testing of aspherics is an inevitable pupil distortion which leads to a distortion of the measured residual defects. But this can be corrected by a modification of the image analysis software.

Of course, cross-checks by other methods, including Hartmann test with compensators, are possible and will be welcome, but the above two methods are seen as fundamental. The technology of compensators is therefore very important and cross-checks with different types of compensators will be required. For the matching of primary and secondary, a supplementary method (pentaprism test) may be required. Ripple and high frequency effects will require supplementary analysis based on interferometry.

The specification of the ·intrinsic qualitY·(24) (the quality determined by high spatial frequency effects alone) which will be of the order of 80% geom. energy in 0,10 arcsec will stretch any test method to the limit and will require extreme care in its performance. This is one reason why a quasi-vacuum or Helium test tower is probably a necessary investment, since air disturbance is one of the two major sources of error. The other major source is vibrations. Shack-Hartmann has a big advantage over conventional interferometry here, because vibration (and high time frequency air turbulence) can be integrated out. Heterodyne interferometry or equivalent dynamic procedures may bring a big gain in the precision and convenience of interferometric testing and overcome the disadvantages of static (conventional) interferometry. - 28 -

It appears from the studies that the fundamental methods above also form the basis of workshop tests during fabrication, but a range of other well-known methods may be also used at the choice of the manufacturer.

At this stage, it would seem that both manufacturers have the means, with the proposed test procedures, to manufacture the optics to the specification envisaged.

I. 4.7 Leadtime

The leadtime for figuring the mirrors is a steep function of the degree of optimisation of the testing and figuring processes. Generally, a shorter time (hence a lower cost) can be obtained through a higher investment either in development or in machinery. For the first time in astronomy it is proposed at the beginning of a project to manufacture several mirrors. This increases the need to reduce the leadtime, but also permits the consideration of a higher initial investment which could not be considered for only one mirror but which could payoff in terms of cost and lead-time if several mirrors have to be produced. Since the VLT will set another standard in astronomy, it is likely that additional telescopes of 8 m size, national telescopes for example, will be constructed. Thus the optimisation of the production schedule is all the more important.

Estimates of the delivery time range between 2 and 3 years, but cannot be expected to be less than 3 years for the first mirror. The relaxation of tolerances on low spatial frequencies permitted by the active compensation of the mirror should normally lead to a shorter lead-time but this is difficult to evaluate accurately as there is no experience available yet on this type of mirror. The results of the figuring of the NTT mirror will certainly be decisive.

The nature of the mirror blank material will have an impact on the time schedule. Zerodur is viewed as the best material and in any event, the only one for which the suppliers have a direct experience. It is difficult to forecast how the lead time may be affected by the material. The suppliers are of course very cautious and foresee only additional difficulties with, for instance metal or borosilicate glass. Test can rapidly reduce the uncertainties. We have an optimistic opinion, based on the fact that with a material of higher thermal conductivity than glass, it should be possible to actively eliminate the thermal gradients very rapidly. We foresee a similar way during operation of the telescope. Testing time may then be even reduced compared to low expansion glass for which the time constant is expected to be longer, although less critical.

By reducing the test time or by performing parallel figuring and testing of two mirrors with frequent exchanges between the polishing machine and the test tower, it should be possible to bring the manufacturing time of sUbsequent mirrors down to 2 years. This is viewed as an absolute minimum, and will require - 29 -

great efforts. The optical facility has to be designed according to a certain fabrication scheme, and no easy modification can be provided later on. Frequent swaps of 8 m mirrors may also be costly and dangerous, and one can wonder whether the best practical possibility is not to consider a double facility. In such a case, an additional delay during the figuring phase could to some extent be compensated by a fUlly parallel processing so that the project schedule would remain within reasonable limits. This may be the best solution if one considers the limited accuracy of the present estimates and the corresponding risk cast on the project schedule and cost.

1.5. Optimisation of image quality

The image quality of telescopes can - and normally is - reduced from its theoretical diffraction limit for visible light by the following sources of error or disturbance:

a) Optical design (residual aberrations). b) Optical manufacturing errors. c) Theoretical support errors (those predicted by the analysis), both for individual mirrors (or optical elements) and for the structure positioning them to each other: theoretical tracking or slewing induced errors of decentering or focus. d) Maintenance errors of c). e) Errors due to thermal distortions of optical elements or structure. f) LOng term mechanical deformation due to structural or material changes of optical elements (e.g. long-term warping of mirror blanks). g) Thermal effects of the ambient air: -dome-, -telescope- or -site- seeing (turbulence). h) Wind buffetting deformations of optical elements. i) Atmospheric seeing (turbulence). j) High frequency tracking errors (image motion) due to motor vibration, structural or mirror resonance or wind shake.

It is important to consider the time bandpass of these error sources:

a) and b) are effectively dc: no change takes place with time. c) is dc for theoretical support errors, but time varying with telescope movement for the structure errors (focus and decentering). with tracking the bandpass is very low (of the order of lO-3HZ ) with re-pointing (change of observing object) of the order to lO-2HZ or less. d) represents slow changes which cause deterioration over weeks or months. very low bandpass. e) represents errors of very low bandpass because the thermal capacity and inertia of the system is too high to allow rapid changes. f) is by definition extremely low bandpass. g) includes effects which may cover a wide bandpass from slowly varying effects which may be stable for hours to high frequencies of many Hz similar to i). h) has the frequency spectrum of wind buffets filtered through the prime mirror (the deformation of other optical elements will - 30 -

probably be negligible in comparison with the primary). For the VLT B m primary it is expected that the effective bandpass will lie roughly between 0,1 HZ and 2 HZ. i) has the large bandpass from about 0,02 Hz to 1000 HZ. j) has a bandpass roughly in the range 5 HZ to 100 HZ. Telescope designers always try to push up the lowest eigenfrequencies to reduce the amplitudes.

I. 5.1 Wind effects on large telescopes

Wind has been for a long time considered an enemy of telescope observations. The design of conventional domes has been largely driven by the need to reduce the wind disturbance of the telescope inside. In the latest telescope projects, however, the familiar semi-spherical domes are giving way to lightweight box-like buildings largely open to the wind. The choice was initially dictated by cost considerations but the positive experience made with the MMT is leading to a radical change of philosophy in the concept for telescope enclosures, which recognises that in many cases the open air environment is more favourable to optimum seeing conditions than the nominally stable and controlled environment inside a classical dome.

In fact, if the telescope is exposed to the wind, whatever heat is transferred from the surfaces to the air is quickly swept away and does not affect the optical properties in the field of view. Minor temperature differences between surfaces and air, of the order of a few tenths of degree, which in a closed dome are sufficient to create unfavourable seeing effects, become irrelevant within a continuous air flow.

However, the wind, which is favourable for the seeing properties, will also cause important loads on the telescope structures and drives. The dynamic effect of wind turbulence may significantly affect tracking accuracy. Also, wind loading on the primary mirror can affect its optical properties.

I. 5.1.1 Wind loading on telescope structures

The characteristics of wind loading on the telescope are very much dependent on the enclosure configuration. A few of the many types of telescope/enclosure arrangements that could be in principle considered for the VLT are illustrated in Fig.B.

We shall here mainly consider the three arrangements that are general~y being considered for the new generation of large telescopes: exposed, embedded, flushed.

In the exposed configuration the telescope is essentially sUbjected to the site conditions. This implies that whatever structure is used for sheltering the telescope, can be removed during observations and also that whatever service structure is required around it is designed so as not to affect the air flow through the tUbe. In this case the design conditions for the telescope are set by the wind data. Of particular importance for - 31 -

determining the dynamic behaviour of the telescope are the data giving the turbulent velocity and the gust spectrum. (The turbulent velocity is the standard deviation of the wind velocity). At a particular site it is very much dependent on orographical characteristics and on wind direction.

In order to evaluate the dynamic response under gust loads the gust spectrum, i.e. the spectral distribution of wind kinetic energy, must be determined. The typical wind gust spectrum in the low atmosphere is shown in Fig.9. Only the last peak (with a period of approx. I minute) and the slope of the right-hand curve are relevant to the dynamic response of buildings and structures.

A preliminary evaluation of wind loading and its effects on the VLT telescope structure has been done (25). This analysis shows that if the drives and the control system can be designed to compensate both the quasi-static effects and any dynamic effect of the wind with frequency up to, say, 2 HZ, an acceptable tracking accuracy should result. A wind screen in front of the telescope would only slightly improve the tracking accuracy but significantly reduce the static torque to be taken by the drives. A point in favour of telescopes in open air is that the environmental conditions are essentially less complex and easier to evaluate than within a dome and are thus more predictable.

The MMT is the first modern telescope building designed with very large openings. (Fig.8.3). Here the telescope is exposed to the wind through a large front opening, but the back wall of the building is closed. It appears that the highest wind loading occurs when the telescope is directed into the wind. Reference (26) concludes that for the worst case a MMT-type design requires that the telescope be capable of operating in the open. In fact it is probable that the wind loading at the top of the tube is the same as if the telescope were in the open. However, as the back wall is closed, wind loading is considerably reduced in the region of the primary mirror.

In spite of the low winds of Mt. Hopkins the vibrations of the MMT is said to be just marginally permissible. The installation of wind screens on the front opening should improve this situation but then air exchanges are reduced and -dome seeing-, which is very low for the MMT, may unavoidably reappear.

The MMT experience suggested that possibly a -flushed- building could be designed to provide adequate wind protection, while the air flow through it would eliminate dome seeing. This was the concept chosen for the ESO NTT (Fig.8.4). The building is open both on the front and back side, so that the wind may go through, -flushing- the telescope.

A new phenomenon, however, appeared during the wind tunnel tests of the building. When the building is rotated with respect to the wind direction, the mean velocity of the flow is reduced but high turbulence intensities are created at the location of the telescope. The measured gust spectra show a peak frequency which - 32 -

/ I

1. OVERSIZED DOME 2. MINIMUM SIZE DOME

--j-----l I---~----- , I -(, I /' I I I /' I I -(' I I ~ I ( I -r I

3 OPENED ROT ATING BUILDING 4. FLUSHED ROT ATlNG BUILDING S. OPEN AIR

--~---­ ~ I

6. CO- TRACKING PROTECTION 7. BOULE 8. WIND DEFLECTORS FIXED ON TELESCOPE STRUCTURE

Figure 8: various types of telescope buildings. - 33 -

depends on the width of the opening. It can be very close to the first eigenfrequency of the telescope and affect its dynamic response. Therefore, in case of moderate to strong wind, wind screens become necessary in order to keep the induced turbulence under control by reducing the mean flow velocity.

The same analysis mentioned above (25) can be used to extrapolate to the size of the VLT the results of a wind tunnel tests performed for the NTT. The results show that in fact, from the point of view of dynamic response to wind loads, an open configuration would be definitely much less critical by about one order of magnitude, than a -flushed- bUilding arrangement. The reason for this is that high frequency turbulence is usually generated by surrounding structures whereas the natural wind has itself a very small influence. For instance any openings more or less facing the wind will generate turbulences with frequencies that are inversely proportional to the width of the opening. This, and not the intrinsic wind turbulence is the likely cause of the wind induced vibrations that are known to affect telescopes unless they are deeply embedded within oversized domes. The same problem is likely to affect the co-tracking protection concept and the different concepts presented in Figs. 8.2, 8.6, 8.7, 8.8.

This emphasises a major point that must be considered when dealing with wind loading of structures. Wind shields that decrease average wind velocity and absolute turbulence amplitude can usually be designed without major problems. However, it may be more difficult to avoid turbulence in the incoming flow induced by the protecting structure itself. Although their amplitude is very small, these generated turbulences will usually have a much higher frequency than the original wind turbulence. Thus the efficiency of a wind shield may often be much lower for the dynamic behavior than it is with respect to the static wind loading.

I. 5.1.2 Wind loading on primary mirror

Here we consider only the case when the telescope is directly exposed to the wind. In fact for an embedded MMT-like configuration the wind loading on the main mirror is known to be low and for a flushed NTT-like arrangement wind screens are anyway required to limit turbulence and wind velocity near the telescope.

TWO different effects are likely to be important for the wind loading of the mirror: 1) the site wind directly incident on the mirror surface, 2) the turbulence created by the telescope structure in the air flow.

In the case of natural site wind the time-varying pressure on the mirror can be considered to consist of two terms, a static term corresponding to a 10-20 minutes average and a fluctuating term. An analysis has shown that the static term can add up to 5% of nominal mirror gravity load with a fluctuating term of 1-2%. However, if one assumes that the support system of the mirror can - 34 -

be made such as to react faster to the wind (a bandwidth up to 1-2 HZ appears feasible) the actual fluctuating term is decreased to values of the order of 0.2% of the mirror weight.

The second effect is related to the drag of the telescope structure. The tube structure of the VLT will be made of a truss-type framework largely permeable to the wind.

Nevertheless, the structure and particularly the most massive parts such as the elevation bearings may create turbulent vortices. The magnitude of the fluctuation can reliably be evaluated only in wind tunnel tests and if it is found to be a significant problem it may constitute an important justification for a wind screen.

I. 5.2 Active optics control system

This system had its origins about nine years ago in telescope testing work done for the setting up of the ESO 3,6 m telescope (27). out of this has evolved the active optics control system of the ESO New Technology Telescope (NTT), which will also be the base technology for the VLT active control.

The purpose of the active optics system is to correct all the low bandpass errors, from dc up to a frequency set by the dynamic response of the system. The most important impact of this system is the active removal of error sources b), c), d), e), and f) (in chapter 1.5). Optical design errors (a) are, in practice, negligible except for field limiting aberrations which are fundamental and can only be influenced by field correctors, not by any active optics system.

In practice, the active optics correction is limited to low spatial frequency aberrations. Of the error sources b), c), d), e), f) only b) can produce high spatial frequency errors, ripple being the commonest example. If a severe tolerance is applied to such high spatial frequencies for the optics manufacturer, that for the low spatial frequencies can be relaxed. The consequences for the NTT may be summed up as follows:

A modest specification of optical manufacturing quality except for high spatial frequencies, giving major economies in manufacturing cost and leadtime. AS an example, a low astigmatism value is normally very difficult to achieve but a rather large amount of astigmatism can be tolerated in an active mirror because it is possible to correct it in situ.

A final functional specification far better than the manufacturing specifications giving a continuously maintained diffraction limited performance in the visible. (80% of the geometrical energy within 0,15 arcsec).

With our favourable experience with the 1 m experimental test bench - see below), we expect to be able to use a much wider dynamic correction range with the VLT than for the NTT and to aim for an even tighter functional specification of 80% geometrical energy within 0,1 arcsec. - 35 -

The active optics system is shown schematically in Fig.lO. The principles of this system have been described in refs. (24), (28), (29). The essential features are:

A wavefront sensor analyses the image of a guide star in terms of a quasi-Zernike polynomial using a small computer. There is no disturbance to the observation and the astronomer will be unaware that an analysis (or sUbsequent image correction) is taking place.

The image analyser is of the SHACK-HARTMANN type, as developed at ESO for off-line telescope testing, but using here a CCD as detector. The system will work with stars down to ca. l4,Sm, more than ample for availability of stars within the field of the VLT unit telescopes (see chapter II 2.2.2).

The system is a force-based (soft), not position-based, system (30). We believe it is far easier to measure and control forces with sufficient accuracy than to try to control the position to a small fraction of a wavelength. The correct form is assured by the natural elastic properties of the monolith under the influence of force changes.

Decentering coma is corrected at its source by appropriate movement of the secondary mirror. All the other low spatial frequency aberrations are corrected by the active primary support.

AS with passive support systems, the fundamental law on which the active system is based is the Linearity (Superposition) Law which is simply Hooke's LaW in elasticity theory.

Also fundamental is the application of the principle of st. venant in elasticity theory which leads to an analysis of the elastic modes of a telescope mirror in terms of a converging series originating from a Fourier equation mathematically identical with that leading to Zernike polynomial terms. The practical consequence is that disturbances in the system lead essentially to low spatial frequency terms which can therefore be corrected. High spatial frequency terms with significant amplitude can neither be provoked nor corrected by the support. This is why a very hard tolerance on ripple etc. is necessary for the optical figuring.

The Orthogonality LaW of Zernike polynomial terms comes from the nature of the Fourier equation and applies equally to terms induced by elastic flexure. Combined with the linearity law, this allows linear superposition of corrections without Wcross talk w •

Our system uses pre-stored information in the computer from elasticity theory calculations. This avoids the necessity of a complex general matrix inversion for every correction and reduces the on-line calculation to a simple proportioning and addition of terms taking account of their azimuth phase. - 36 -

6

'.. N~ 5

4

3

....:...- 2

10 100 cycles/hr

1 week 1 day 1 hour 1 minute Figure 9: Typical wind gust spectrum in the atmosphere.

r/l/l/'----1-1117:1I4----"1 ---~ /. "

Focus ~ .~~~-~-~-~------­ ~ v Wavefront sensor

Computer

Figure 10: principle of active optics. Information from a wav&front sensor locked on a reference star is processed and used to control the primary mirror actuators and M2. coma and focus errors are corrected by translating M2 (x,y,z). - 37 -

a

b

Figure 11: 1 meter experimental active mirror. The mirror a) and the support b) have been scaled down from the characteristics of the NTT mirror. The mirror is 18 mm thick and is supported by 78 active axial supports. - 38 -

In physical terms, then, the procedure consists of an image analysis giving the coefficients of the lower order polynomial terms: the computer has pre-stored information of what force corrections are required to generate a coefficient of, say, 500 nm of each aberration term to be corrected by the primary support: this change is scaled linearly with the measured coefficient: the force changes for all the aberrations to be corrected by the primary support are then summed, these force changes are then applied to achieve the correction.

For reasons of simplicity, third order coma is corrected by a rotation of the secondary about its center of curvature, which does not disturb the pointing.

For the different ·levels· and frequencies of correction, the reader is referred to reference (66).

The active optics system has now been tested in the ESO optics laboratory on a 1 m model mirror only 18,5 mm thick, and has performed with even greater precision than expected. The 1 m mirror was scaled from the full-sized NTT primary according to the gravity flexure law and therefore has the same support geometry. Fig.ll shows a photograph of the support system, the final design and manufacture having been done by citterio et al (31). Load cells measure the actual forces with high precision. The hardware of this model has functioned extremely well, limits being set finally (as should be the case) by the signal/noise limit of the Shack-Hartmann image analyser. Experiments have given excellent agreement with the pre-calculations (calibrations) for the various aberrations with negligible cross-talk. We thus feel quite confident that we can go ahead with a similar system for the VLT, using an even larger dynamic range of correction.

The active optics system can be applied equally to glass or metal mirrors. Glasses are considered to have a very high level of long-term dimensional stability. More doubt has been expressed concerning metals although experiments at ESO on modest size aluminium mirrors indicate that careful heat treatment can reduce warping to levels acceptable even in a passive support mode (21)(22). With active control, warping effects can be completely removed. In this sense, the active optics and metal blank developments are completely complementary.

In modern telescopes with an advanced optical concept for image control, thermal disturbances will be - together with atmospheric seeing.-~he major source of image errors. Glass blanks have the weakness of a poor internal time constant (low thermal diffusivity). This can lead to radial gradients resulting in form distortions unless the expansion coefficient is virtually zero like Zerodur. If these remain within the dynamic range, such effects can be removed by the active optics control, provided they remain low spatial frequency effects. This will certainly be the case for solid meniscus blanks: but light-weighted structures can lead to print-through, probably even more in the optical figuring phase than in use. Metals have a higher expansion coefficient than - 39 - glasses but a far better diffusivity. The thermal distortions or print-through effects with metals lie therefore somewhere between Zerodur and BSC glass.

Apart from distortion of the form of the primary, a high thermal capacity combined with low diffusivity will make the primary a major heat source, or sink, producing telescope turbulence (72). This can be reduced by light-weighting to reduce the thermal capacity and increase the air-surface thermal link. If this proves insufficient, active cooling must be performed. For metals, either air- or water-cooling can be envisaged, but for glasses (because of the poor diffusivity) only air cooling is practicable. A "blowing" system is very bad as heated air might rise into the optical beam. A suction system, sucking ambient air round the back of the mirror and extracting it down a pipe is far better.

Wind buffetting deflection of the top end of the tube relative to the primary will produce tracking errors and coma. High frequency vibrations must be damped, as very successfully achieved in the 3,6 m telescope (33), otherwise the autoguider would not be able to cope with the high time frequencies of tracking error involved. As with any cassegrain telescope, the sensitivity to decentering coma for the VLT is far less than that to tracking error (for the probable optical geometry). If the effect is not negligible, it would have to be corrected actively by a movement of the secondary.

A problem with the correction of wind bUffetting effects producing such coma from the structure flexure or other aberrations from distortions of the primary is that the bandpass of the effect (ca. 0,1 HZ - 2 HZ) no longer allows integration of the atmosphere. In principle, therefore, there is no means of separating off the wind-buffetting effects from atmospheric seeing effects. There is evidence that for telescope sizes not greatly exceeding the Fried parameter, significant low spatial frequency terms such as astigmatism occur in the atmospheric seeing. However, the VLT unit telescopes will so much exceed the probable maximum Fried parameter values that it is expected that the atmospheric seeing amplitudes of such low spatial frequency terms will be negligible: in other words, separation of the effects would then be achieved by spatial frequency filtering instead of time frequency filtering. Of course, it can also legitimately be argued that, if such atmospheric spatial frequencies did occur at the time frequencies of the wind buffetting bandpass and could not be separated, then their correction with the wind buffetting effects is not a weakness, but a strength of the system, provided any other atmospheric seeing correction system takes account of this.

The above approach seems the most promising but requires a support system capable of functioning up to 1 or 2 HZ. This seems quite feasible. Other methods which may well give useful supporting information on wind buffetting effects are under investigation, especially measurement of the reaction of the fixed points to wind bUffetting. - 40 -

I. 5.3 Seeing limitations

I. 5.3.1 Atmospheric seeing

The astronomical community uses various ways of describing the degradations caused by the atmosphere to an otherwise diffraction limited image of an unresolved stellar object. Images show various types of behaviour, they spread out, wander, twinkle and split into parts. However, qualitative judgement is presented in terms of good or bad "seeing", which can be related to the size ro (Fried's parameter) of an equivalent diffraction limited telescope. The better the seeing, the larger roe

Because of the accumulation of refractive index inhomogeneities along the optical path down to the entrance pupil of the telescope, amplitude and phase of the impending wavefront vary randomly in time. The above mentioned ro is a statistical average diameter of the area on which phase changes can be ignored. It does not distinguish between free atmosphere, boundary layer and dome perturbations. Wavelength dependent, it may vary by a factor of two or more within a few hours and usually ranges between 5 cm and 30 cm in the visible.

At optical wavelengths a VLT aperture is far larger and thus will uncompass numerous undisturbed elementary wavefronts, producing a speckle pattern in the image plane. Each elementary speckle moves randomly and contains information from the whole aperture. Speckle interferometry allows to retrieve nearly diffraction limited images if the exposure time is short enough to freeze the motion of the fringes. This sets the limiting magnitude for resolvable objects. The longest exposure is determined by the speckle life time which is proportional to roe It has also been shown to be inversely proportional to an averaged wind velocity in the atmospheric turbulent layers, and is dependent on the wavelength; typical values of 3 ms to 30 ms have been measured in the visible. The number of speckles as well as the size of the total illuminated area are a function of the strength and thickness of the turbulence layers encountered along the optical path. The spread angle corresponding to the size of the whole speckle pattern is usually related to as "blurring".

The motion of the center of gravity of a speckle pattern, also named "image motion", represents the average phase fluctuations of the incoming wavefront. Since phase fluctuations across the aperture are random variables with zero mean, the images produced by large. telescopes will move with a smaller amplitude. The parameter describing image motion is its standard deviation, it is independent of wavelength and usually amounts to a few tenths of an arcsec, again depending on the integrated turbulence along the path. Its average time spectrum follows a -2/3 power law from 0.02 HZ to the cut-off frequency of speckle motion. It is worthwhile noting that the effect on image motion of local turbulence close to the aperture will be decreased for a VLT whose diameter is large compared to the outer scale of the turbulence (the scale out of which physical processes are not correlated). The superposition - 41 -

of blurring and image motion defines the characteristics of "long exposure images", usually called the seeing image.

other parameters are more sensitive to high altitude turbulence than to ground layer or dome effects. These are scintillation and isoplanatism. The total intensity of the image of an unresolved object in the focal plane of a telescope is a random function of time. This scintillation effect when considered in the pupil plane was also named "flying shadows". The velocities of the shadow patterns are directly related to the wind velocity in the troposphere. The latter shows a multi-layer turbulence structure whose contribution to scintillation increases with the altitude. Because of aperture filtering, the scintillation index (variance of relative irradiance fluctuation) of VLT images would be independent of wavelength and atmospheric layers under 5 km would be filtered out.

The isoplanatic angle which defines the angular extent of the sky where atmospheric perturbations may be considered correlated is independent of dome or ground layer effects. It is wavelength dependent and as for scintillation, high altitude turbulence is a major contributor.

The physics of site selection must include a detailed analysis of all the aforementioned effects. However the quality of a site may be characterised by the size of the long exposure images it produces. This parameter (FWHM) is highly variable both on a daily scale because of local influences and on a seasonal scale because of seasonal motion of the jet-streams. That is why sites can only be reliably compared statistically. In that respect one generally assumes a log-normal distribution of image size.

According to worldwide site survey campaigns, the atmosphere above a good site would be expected to deliver sUb-arcsec zenith images 80% of the time at 5000 A. worst conditions are met with strong jets, wind shear with stable temperature stratification or boundary layer turbulent activity. On a well selected site and using an instrument optimised with respect to dome seeing, one could expect the images to be smaller than 0.8 arcsec at least 50% of the time (smaller than 0,3 arcsec 5% of the time).

Then, one of the most difficult tasks is to be able to forecast atmospheric seeing for flexible scheduling of telescope time. It requires a better knowledge of the mecanisms of turbulence and a way to relate quantitatively statistical parameters such as the temperature structure function to standard meteorological phenomena such as wind shear, temperature inversion and gravity or lee wave structures.

I. 5.3.2 "Dome Seeing", "Telescope Seeing", "Mirror Seeing"

These terms are intended to cover here all those local effects of turbulence caused by the telescopic equipment or its ancillary facilities: man made effects. - 42 -

In the last generation of conventional, equatorially-mounted telescopes, such ·dome seeing· effects (using this term to cover all three terms above) has frequently been a major factor in limiting the image quality. TWO modern trends have tended to work against good image quality.

AS electronics controls for telescopes and instrumentation became ever more sophisticated, more heat sources were added to the telescope environment.

The evolution towards zero-expansion glasses for blanks made these less and less sensitive to thermal distortion, but their thermal inertia caused the ·mirror seeing· to become worse.

Learning from the experience of the 3,6 m telescope, a major effort was made with the building of the 2,2 m telescope in La Silla to maintain good dome seeing conditions. The excellent results with this telescope indicate that these efforts have been largely successful. In this case, the building and dome were still largely ·classical·. With the NTT a building was adopted which essentially came from the philosophy of the MMT: maximum natural ventilation with variable windscreens to provide the necessary wind-buffetting protection. This windscreen concept applies a principle enunciated by Woolf (26), namely that ·isothermal· turbulence does not cause deterioration of the image.

It is possible to go even further in this direction by removing the shelter completely and operating the telescope in the open. ·Dome· seeing thus no longer exists, but the other forms of local seeing degradation may still be present. EVery effort will be made to reduce heat sources from electronics and by absorption of heat into the telescope structure or site base. possible cooling of the primary mirror was discussed in chapter 1.5.1.

I. 5.4 Adaptive correction of the atmospheric perturbations

Dome seeing, mirror seeing, and atmospheric seeing degrade the imaging quality of ground-based astronomical telescopes. The reason for this degradation is a random spatial and temporal wavefront perturbation induced by the turbulences in the different layers of the atmosphere. This wavefront perturbations result in a complex phase aberration of the light beam

~(r,t) - iA(r,t).

The real part ~(r,t) represents the phase shift in the wavefront, usually called ·seeing", while the imaginary part A(r,t) is a measure of the intensity across the aperture plane, called "scintillation·.

It is possible to correct the phase shift with a technique called adaptive optics (35). The basic principle of adaptive optics is to use a phase shifting optical element, which can be controlled in space and time in order to compensate the phase shift. A correction of the scintillation would require a spatially and - 43 - temporally controllable apodisation element. A full correction system applies an aberration compensation equal to

- Ijl (r , t) + i \lA (r , t ) •

For most of the imaging problems, especially with very large telescopes the phase correction part is sufficient and the technical realisation easier. The principle of adaptive optics is equivalent to the so-called active optics used for the figure correction of the telescope optics. The field of active optics is just a special case of adaptive optics with low spatial and temporal frequencies. Therefore this separation is in some sense artificial.

During the last 10 years the techniques of adaptive optics have been developed, mainly for military applications, i.e. laser beam transmission through the turbulent atmosphere. Although most of the developments have been made in the United states (ITEK, Lockheed, United Technologies, AVCO EVerett, The Aerospace crop., and others), there are some promising activities in Europe (CGE (France), MBB, Diehl (FRG». The transfer of these techniques to the astronomical field is not straight forward, both because of the very low light levels compared with laser applications and the large apertures of the astronomical telescopes. However, progress in the design of phase-shifting hardware and in wavefront-sensor technology and new ideas, such as shooting laser beams to the upper atmosphere to creat an artificial Wstar-like w reference source with high intensity and availability of fast computers to cope with the large number of correction when large apertures are used has opened the technical feasibility of adaptive optics to astronomy.

An adaptive optical system (see figure 12) contains four basic elements: an optical train and image detector, a wavefront sensor, a servo-control system, and phase-shifting optical element. The distortion of the received wavefronts is usually compensated by reflecting the light beam on a deformable mirror. The surface of this mirror is adjusted in real-time to compensate the path length aberration. The information required to deform the mirror is obtained by analysing the light beam with a wavefront sensor. A map of wavefront errors is then derived. Using the error map, the control system determines the actuator signal required to drive the deformable mirror and to null the phase aberrations by closing the adaptive loop. The phase correction values can be determined by expanding the phase-correction function

N Ijl(r,t) = L an(t) fn(r) n = 1

in a spatially (fn(r» and temporally (an(t» dependent function. The spatial functions might represent zones or mode of the aperture, resulting in a zonal or modal correction strategy. - 44 -

The complexity and design of an adaptive system depends of the aperture size of the telescope, and the turbulence; The latter is characterised by the Fried parameter r o ' the wavelength X and the zenith distance y of the observation. The following parameters usually govern the design of an adaptive system:

* The number of degrees of freedom which is given by the number of independent modes or zones (N) to be controlled:

'V (cosy)-6/5

* The wavefront correction range:

(cosy) -1/2

* The temporal frequency is governed largely by the severity of environmental effects and site conditions to be compensated:

'V r -1 o '\. x-6/5

'V (cosy) -3/5

Typical values for the visible wavelength range with 1 arcsec seeing at 0.5 microns are:

Subaperture size: 10 cm Time constant: 5 msec

For an 8-10 m telescope this would lead to more then 6000 controlled sUbapertures working at frequencies higher then 200 HZ. At present such a system is not feasible but at IR wavelengths and the same atmospheric conditions it would be feasible to construct a system of 40 to 200 sUb-apertures and working at frequencies of 100 HZ.

I.5.4.1 strategy for seeing optimisation

The shape of an optical wavefront may be represented in two different ways: (i) using an array of independent localised zonal functions, or (ii) using a set of orthogonal whole-aperture modal functions. Analytically, the two systems are equivalent in terms of the number of degrees of freedom required to specify a given wavefront to a certain precision. However, there are major practical differences especially in the implementation of - 45 -

LIGHT FROM THE TELESCOPE

ABERRATED ADAPTIVE WAVEFRONT MIRROR CLOSED LOOP

CONTROL SYS TEM I I + I I I I I HIGH RESOLUTION I IMAGE l_ WAVEFRONT SENSOR

Figure 12: principle of adaptivt optics. A reflecting tltmtnt is uStd for the wavefront correction.

Figure 13: Adaptive mirror (manufactured by CGE, Franct): 7 piezoceramic actuators, molybdenium face-plate with approx. 10 cm diameter, cooled on the backside. The actuators are locattd at the nodts of a hexagonal lattice of 15 mm period. (With permission of CGE; pUblished at CLEO '84; sponsored by DRET). - 46 -

wavefront sensors and compensation devices. All practical wavefront sensors and most of the deformable mirrors use the zonal approach. With zonal mirrors, the main variable is the shape of the influence function of each zone, which determines the wavefront fitting error.

For modal compensation, the well-known zernike polynomials, which correspond to systematic optical aberrations such as defocus and astigmatism encountered in conventional optical components, may be employed; for turbulence compensation the Zernike polynomials are optimal only for a small number of modes. A More general set of functions, the Karhunen-LQeve (36), can be optimised for turbulence compensation of any size.

If the modal approach is used rather than a zonal decomposition, image improvement is possible even with a limited number of modes. This is one of the essential advantages of the modal correction strategy.

1.5.4.2 Elements of an adaptive system

Wavefront correction device: The wavefront can be controlled by either changing the velocity of propagation or by changing the optical path length. The former is achieved by varying the refraction index of a medium, while the latter is implemented by moving a reflective surface such as a mirror or by moving a grating as in a Bragg cell.

At the present time, reflective devices are the most successful and widely used as wavefront correctors. The problem with the other devices are mainly the limited range of refraction index change, the spectral absorption, and nonuniform transmission. On the other side mirror coatings are available with high efficiencies over wide spectral ranges and because the optical path is confined on one side of the mirror surface, a great variety of substrates and methods of deforming the mirror are available. Finally the wavefront deformation is a true optical path length change, independent of wavelength. The following scheme gives the basic types of active mirrors which have been developed:

segmented mirrors

+ piston only + piston and tilt

continuous thin-plate mirrors + Discrete position actuators + Discrete force actuators + Bending moment actuators

Monolithic mirrors Membrane or pellicle mirrors - 47 -

For the correction of atmospheric perturbation in an astronomical telescope with adaptive optics the continuous thin-plate mirrors with discrete position actuator or the bending moment actuators seem to be the most favourable ones (see figure 13).

Wavefront sensor: It is not possible to measure directly the phase of an optical wavefront, as no existing detector will respond to the temporal frequencies involved. Three techniques are commenly used to overcome this problem:

Measurements can be made on the intensity distribution of the image produced by the entire wavefront;

A reference wavefront of the same or slightly different wavelength is combined with the wavefront to be measured to produce interference fringes;

The wavefront slope of small zones of the wavefront may be measured. This can be achieved by using a shearing interferometer or the Hartmann test.

Each of these three approaches has its own advantages and disadvantages. For the application in astronomy with the severe intensity problems only the third approach has a chance of success.

The Shack-Hartmann sensor (37)(21) is based on the well-known Hartmann test for checking the figure of large optical elements. The wavefront is divided into a number of zones, usually contiguous and of equal size. The light from each zone is brought to a separate focus and the position of the centroid of each focus is measured in two dimensions by photoelectric device, e.g. a CCD camera. The position measurement reveal the mean wavefront slope over each zone. The residual wavefront curvature over each zone is not measured, and in fact degrades the signal-to-noise ratio.

In a shearing interferometer (38), the wavefront to be measured is amplitude divided into two components which are mutually displaced and recombined with each other to generate an interference patters. If the path length of the two beams is equal, then fringes are generated even with incoherent light sources, because light from each element of the source interfers with a displaced duplicate of itself. Several methods of producing a sheared wavefront are existing; one of the most useful is a moving Ronchi grating located at the focus of the light beam. control system: All slope-measuring wavefront sensors require a reconstruction of the wavefront itself. Normally, two orthogonal wavefront slope measurements are made for each actuator location. In other words there are twice as many measurements as unknown so that a least-squares fit can be performed with benefitial effect on error propagation. Several reconstruction operations have been used or proposed in literature. All of these algorithms require very high computation powers in order to meet the temporal and - 48 -

spatial requirements of the astronomical application. With special dedicated hardware or hybrid systems this problem has been successfully solved.

I.5.4.3 Performance of an adaptive system

The main sources for errors in the model of adaptive optics are wavefront fitting errors (oF)' which depend on how closely the wavefront corrector can match the actual wavefront error; the detection error (oD)' which essentially reciprocal to the signal-to-noise ratio of the wavefront sensor output; and the prediction error (op), which is due to the time delay between measurement of the wavefront error and its correction. The overall residual error is usually assumed to be:

I.5.4.4 Adaptive correction in astronomy

It is first required to measure the effect to be corrected. This is the major problem for the application of adaptive optics in astronomical observation. The observed sources are in most cases so faint that their light is not sufficient for the correction. A brighter nearby reference source is seldom available and has to be within the isoplanatic patch, which is wavelength dependent and covers only a few arcsecs in the visible wavelength range. Recently a new technique to overcome this problem has been proposed (34). With a LIDAR like technique an artificial reference source is generated by resonance scattering of light in the mesospheric sodium layer (figure 14). From the technical point of view it is in a realistic scale. First measurements on La Silla (39) (additional measurements will follow) have shown that the low frequencies in the visible MTF of the atmosphere are correlated with the infrared MTF (see figure lS). This correlation opens the possibility to measure atmospheric distortions in the visible and correct for IR-wavelengths longer than the yellow sodium lines. Additionally there will be a partial correction for visible wavelengths.

Different levels of adaptive correction can be approached:

Optical figure correction or so-called active optics. In this case the turbulence of the atmosphere is not included (I.S.2). Typical frequencies are 0 to 2 HZ including wind effects on the mirror and structure.

Image motion stabilisation (With a simple tip-tilt mirror, single channel adaptive system).

partial wavefront correction (with less sUbapertures and frequency range than turbulence sampling requires).

Full wavefront correction (with full sampling of the aperture and temporal variations for a given wavelength). - 49 -

STAR BEAM

--- -ARTIFICIAL REFERENCE SODIUM --- SOURCE RESONANT SCATTERING (BEACON)

PERTURBED LAYERS =100KM

==10KM I

t------:;f----+----_}(,'I. CORRECTING MIRROR

WAVEFRONT ASTRONOMICAL REAL TIME PARALLEL SENSOR INSTRUMENT PROCESSOR __1

Figure 14: Sch~matics of the laser probing principle. A pulsed laser beam is scattered at the mesospheric sodium layer. The wavefront of the backscattered light is analyzed and th~ information serves for the correction with adaptive optics. - 50 -

It should be mentioned at this point that the first suggestions for the construction of an adaptive optical correction device came from astronomy. In 1953 Babcock pUblished his paper with the title: -The possibility of compensating astronomical seeing- (40). First observatory results of stellar image sharpening were reported in 1977 by BUffington et al. (41) and McCall et al. (42).

At the present time there are six centers where the application of adaptive optics in astronomy is being investigated: Kitt-peak, CFHT (Hawaii), Observatoire de Meudon, and ESO for stellar observation, and LEST and Sacramento peak (New Mexico) for solar observation. NOAO already started to build an adaptive system with 37 sUbapertures. A cooperation program associating ESO and several European scientific and industrial groups is being organised. The goal of this collaboration is a 19 sUbaperture adaptive system. The technical developments of this project could then contribute to the much larger system which will be applied in the VLT. The experience with this system, which will be available in 1988, under realistic astronomical observation conditions on La Silla will be an important input for the development and final design of the adaptive system for the VLT.

possibilities for an adaptive compensation of the atmosphere should be integrated in the design of new telescopes and of their instrumentation at a very early stage for optimum operational reliability. Rapid technical progress is being made and the astronomical potential bf adaptive optics at least in the IR is high.

1.5.4.5 Adaptive optics for the VLT

It is considered to equip the individual VLT telescopes with independent adaptive optical systems. A two mirror system seems to be favourable: a first mirror compensates tip-tilt aberrations and a second deformable mirror all higher order aberrations. A system with approx. 200 subapertures, 200 HZ operational frequency, and a Shack-Hartmann wavefront sensor is the target for the future investigations. The adaptive system would offer diffraction limited observation at wavelength greater than 4 ~m and a partial correction at shorter wavelength. For interferometry the gain of the 8 m apertures is only given in combination with adaptive optics. Otherwise the signal-to-noise ratio will not be improved compared with interferometers with smaller apertures.

I. 5.5 ESO's strategy for optimisation of image quality

From the above discussion, it is clear that it is not possible to define a telescope concept in which all the image degradation effects would be eliminated in a simple way. One must compromise 2 conflicting requirements:

i) the protection of the telescope from wind and ii) the elimination of man made local seeing. - 51 -

[dB]

o --CROSS SPECTRUM /SIt••2.21t••O.S -- -INFARED SPECTRUM (2.2,um1

-50

o 1000 2000 HZ TELESCOPE FREUUENCY CUT-OFF FREUUENCY AT 2.2,um Figure 15: simultaneous observation of the atmospheric MTF in the visible at 0.5 ~m and the infrared at 2.2 ~m (alpha carinae, August 1, 1985, day-time observation, 3.6 m telescope, La Silla): The diagram shows a comparison of the cross spectrum (amplitude) of the visible and IR channel (normalized to the spectrum of the visible channel) with the MTF at 2.2 ~m.

F/3.42 R. 3900 mm

Figure 16: pseudo-cassegrain focus for polarimetry. A 7 arcminutes wide annular field of view is left available at the Nasmyth focus for active correction and tracking with a reference star. - 52 -

Two opposing strategies are conceivable: a) protection of the telescope from the wind by a closely fitted building, and minimisation of thermal disturbances by a proper design of the bUilding and an active control of the thermal environment; b) elimination of the surrounding building, hence of most of local seeing effects, and proper design of the telescope in such a way that it stands the mechanical forces induced by the wind.

Strategy a) is the classical approach and has been applied with various success to most existing telescopes. strategy b) has been partly applied with success to the MMT. It has the additional advantage of also reducing the cost of the building. A small and simple semi-fixed shelter would be sufficient to protect the telescope during day time and bad weather periods. This is the basic strategy considered for the VLT. The specific elements of this strategy are the following:

Open air operation of telescope. Design of the telescope structure so as to minimise the action of wind forces:

lightweight truss structure designed for laminar air flow up to the windspeed limit; high rigidity and eigenfrequency; stiff and oversized drives.

correction of tracking errors with the telescope drives up to 2 Hz. Correction of higher frequency tracking errors (up to 10 Hz) with the lightweight secondary mirror (if needed). Correction of wavefront deformations due to gravity, thermal effects, wind pressure with the active correction of the primary mirror (up to 1 or 2 HZ). Correction of atmospheric turbulence with an adaptive system whenever possible. Low thermal inertia of the primary mirror and temperature control through Eorced ventilation. Tight control of local environment: removal at a fair distance of major heat sources, and passive thermal control of the immediate surroundings (e.g. radiative cooling of the platform). Day time protection of the telescope ensuring a good thermal insulation, a low thermal capacity and an adequate protection against dust, storm, snow etc. and minimum air flow perturbation. If necessary, decrease of the actual wind speed with windscreens. - 53 -

CHAPTER II: CONCEPT SELECTION

The main approaches to a VLT concept proceed largely from mirror technology considerations, i.e. segmented versus monolithic mirrors. There is indeed a nearly infinite number of ways in which the 2 technologies can be combined, trading off size and number of elements. The number of realistic options is, however, limited if one considers that i) a 16 m aperture cannot realistically (i.e. within the time frame envisaged for the VLT) be obtained with one single monolithic dish. ii) if an array (here defined, in its broad sense as an unfilled aperture) is considered, it should be made of the largest possible unit mirrors.

The rationales for this latest statement are:

The necessity to have large single photon collectors for observations requiring a large field which cannot be obtained at a combined focus of independent telescopes, and to limit the number of identical instruments to be produced.

IR imaging under diffraction limited conditions.

AS discussed in chapter I.2, a size of 8 m appears as a nearly optimal trade-off for a monolith. Therefore one can restrict the discussion to concepts either based on 8 m monolithic mirrors or on a single segmented mirror, 16 m in diameter, and made of segments about 2 m large as discussed in Chapter I.2.4.

Intermediate concepts such as a combination of several segmented mirrors should not, in first instance, be considered because they would mostly cumulate the drawbacks of both options. Nevertheless such a solution could well be considered a reasonable, if not optimal, fall-back solution for an array of large telescopes.

Although the discussion between the pros and cons of various concepts has been going on for nearly 10 years (2)(43)(44)(45)(46)(47), we feel it is useful to give an overview of the 3 basic possible concepts, in their various technical and observational aspects.

We shall therefore compare:

a segmented mirror telescope (SMT) made of one mirror 16 m diameter;

a multi-mirror telescope (MMT) made of four 8 m mirrors;

an array of 4 independent 8 m telescopes. - 54 -

11.1 Mirror technology and mirror figure control

This has largely been discussed in Chapter I and our conclusion was that monolithic mirrors appear feasible and will likely be reliable once in operation, whereas a segmented mirror is a more challenging solution and in any case will offer much less redundancy and reliability.

Though an MMT and an array are based on identical mirror technology a major difference appears when active correction is considered. The combined field of view of an MMT, which cannot exceed a few arcminutes unless the combining mirrors are made exceedingly large, would be too small to find a bright enough star that could serve as a reference for active tracking and correction of the mirror. Therefore an MMT is bound to either not have any real-time active correction or to have an intermediate large field focus where the wavefront sensing can take place. In the first case, the telescope will have to rely on its mechanical and thermal stability and on modelization, in the second case the combining optical system becomes quasi-identical to that of an array and the basic advantage of an MMT over an array is lost.

The ESO approach consists to rely extensively on active control to correct for various disturbances and achieve ambitious image quality performance. Though in principle there exists alternative ways such as a modelization coupled to a periodic switch to stellar standards, we believe that on-line correction for which an array is best suited is a definitive advantage.

11.2 Optical efficiency

The SMT is conceptually similar to a classical telescope and the full collecting power is in principle available over the total field of view and with no wavelength limitation. By contrast, the MMT or an array need a complex optical system to recombine the beams and the field of view at the combined focus is necessarily small. The effective field of view is dependent upon the relative distance between the telescopes; an array will therefore tend to have a combined field of view smaller than an MMT.

The beam combining system of an MMT is located inside the telescope structure; it is therefore sUbjected to flexures and the exchange of mirrors that would be necessary for selective high efficiency coatings to be used is much more problematic than for an array for which there is more space available for the combining optics outside the tube structure.

The practical,effective field of view at a cassegrain or Nasmyth focus would be the order of 30 arcminutes for an SMT, 2 to 3 arcminutes for an MMT and 0.5 to 1 arcminute for an array (depending on its topology). In addition both MMT and arrays can have wide field capability at focii of unit telescopes. Based on an efficiency of 90% for ~uminium and 98% for high efficiency coatings. An MMT with 5 Aluminium mirrors would have 59% efficiency and an array with 3 ~uminium and 5 high efficiency coatings 66%.

If silver coatings with 98% efficiency in the visible would be used for all mirrors, the MMT would have 90% efficiency and the array 85%. By - 55 -

comparison, an SMT at the Nasmyth focus would have 61% efficiency with Aluminium coatings and 94 % with silver coatings. High efficiency coatings have therefore a crucial importance all the more with an array for which they are mandatory for the combining optics.

II.3 Wide field imaging

Because of field limitation at the combined focus, wide field imaging with an MMT or an array is only possible at the focus of unit telescopes. Post detection co-addition of data becomes then necessary whereas direct images are produced by an SMT.

Both modes lead to equal performance as long as the detector read-out noise can be neglected (which will be the case with a VLT owing to the predominance of sky background and likely improvement of detector performance). The total detector area and number of pixels necessary to cover a given field are also identical in both cases (61). The more complex data processing required for post-detection combination may be seen as the price to pay for smaller detectors and redundancy. One should also consider that it might anyhow be necessary to split long integrations with one single detector into several exposures in order to discriminate real objects from spurious events such as cosmic rays, so that both type of imaging may finally require a similar number of images to be combined.

Altogether the 3 concepts, will probably yield equally satisfying results in wide field imaging observations.

II.4 IR observing

The SMT is optically similar to existing telescopes. With 2 warm mirrors for cassegrain operation and 3 for the Nasmyth it offers the less emissive surfaces but additional emissivity is introduced by the segment interspaces.

The MMT and the array need additional mirrors to combine the beams. Either these mirrors are warm and correspondingly an additional background can be anticipated or they are cooled at the cost of a considerable complexity (very large vacuum pipes, large scale cryogeny, windows ••• ). With cooled combining mirrors, the 3 concepts would appear roughly equivalent, the emissivity of segment interspaces being balanced by the presence of extra components with the MMT on the array. The SMT remains however definitely simpler.

IR diffraction limited observation is an aspect for which the 3 concepts may be seen as very different. It would probably be easier to phase an MMT than an array but if an array were phased it would give a better resolution at least in one direction. We have conservatively assumed that the MMT would be phased and the array not. The resolution of the array is therefore that of a unit telescope. The table below shows a comparison of the 3 concepts for different cases and at 10 and 5 microns. For simplification one has considered the various error sources to be random. For cases C and D, the residual high frequency error of the mirror (not correctable by the active or adaptive optics Gystem) has been assumed to be 0.15 arcsec. - 56 -

SMT MMT- ARRAY l6m 4x8m 4x8m

A Aperture diffraction 0.125 0.1 0.25

B A + Seeing 0.5" at 0.5~m 0.31 0.3 0.38

A = 10~m C A + Seeing 1" at 0.5 ~m 0.57 0.57 0.61

0 C + 0.15 " degrad. 0.59 0.59 0.63

E A + 0.15" degrad. (adapt. optics) 0.20 0.18 0.29

A Aperture diffraction 0.062 0.05 0.125

B A + Seeing I" at 0.5~m 0.33 0.32 0.34

A=5~m C A + Seeing I" at 0.5~m 0.64 0.64 0.65

0 C + 0.15 " degrad. 0.66 0.66 0.67

0 A + 0.15" degrad (adapt. optics) 0.16 0.16 0.20

Table IR imaging performance of 16 m SMT, MMT and ARRAY (80% energy, arcsec)

It is clear from the results that the large gain shown by the MMT and the SMT if one considers only the diffraction limit (case A), largely vanishes as soon as the various deterioration sources are considered. There is practically no difference at 5 ~m and little at 10 ~m, although the problematic phasing of the SMT and the MMT have not been considered.

11.5 Interferometry

Beckers (52) distinguishes 3 categories of arrays which differ by their frequency domain coverage (u,v plane) and by their ability to maintain the internal path length difference invariant.

Both SMT and ~MT belong to type 0 where equal path length difference is maintained independently of telescope pointing and where (u,v) plane is continuously covered. Resolution is ultimately limited by the overall pupil dimension and is better with an MMT than with a SMT because of the mirrors separation.

In Beckers' classification an array of independent telescopes can be of type I if they are mobile or of type 11. In both cases, the (u,v) plane can only be discontinuously covered but the resolution defined by the largest distance between telescopes is indeed much greater than with an MMT or SMT. - 57 -

Technically, the stability requirements for interferometry are extremely severe. Roddier (53) gives a limit of 25 nm OPD within 0,2 second at visible wavelength and 0,1 micron in 1 second at a wavelength of 2,2 microns.

This suggests that interferometry at visible wavelengths will be more easily achieved with a compact mount than with an array of independent telescopes. At IR wavelengths, the compact mount offers a limited interest because of the limited base-line. Therefore, an array will be superior to other concepts for IR wavelengths whereas a common mount would offer a better chance of success at visible wavelengths. Yet very large dishes do not provide much gain over smaller apertures at visible wavelengths (54) so that an array limited to the IR may at the end prove to be the best option. These questions are still very much debated by the ad-hoc working group and preliminary analysis can be found in (55).

11.6 Flexible scheduling

This is a universally recognized requisite for a next generation telescope but its consequences on the telescope concept have not yet been fully appreciated. Flexible scheduling implies that the complete set of instruments is readily available for observing in time scales of minutes rather than hours. In that respect, the 3 concepts present fundamental differences:

The SMT would not offer more than 2 Nasmyth foci and possibly one prime focus or cassegrain: the MMT may offer 4 cassegrain at individual telescopes, 2 combined Nasmyth and several combined foci for small instruments: the array offers the maximum of possibilities with, as a typical example, 2 Nasmyths on each individual telescope and as many combined or simple Coude foci positions as requested with no limitation of size for the instruments. The possibility to use the telescopes of an array independently is also a specific advantage of that concept.

11.7 Mechanical structure and bUilding

For an equal F-ratio of the primary mirror, a SMT would have a tube length about twice that of an MMT which would have the most compact structure. structure compactness is important because it strongly influences the effective wind torque on the telescope drives and therefore determines the necessary level of telescope protection. Because of its greater sensitivity to wind, the longer tube of a SMT is likely to require a large building with minimum openings (embedded telescope) whereas an MMT may be accommodated in a semi-opened enclosure similar to the present Mt. Hopkins MMT. Individual telescopes of an array accept enclosures similar to that of the MMT but a completely opened telescope can also be considered.

11.8 Redundancy and flexibility

No matter which concept is retained, a VLT is likely to be a highly complex instrument involving rigorous maintenance procedures. Reliability and maintenance aspects should be seriously considered during the design phase and a maximum of redundancy for the telescope and its instrumentation is indeed desirable. In that respect, the SMT is - 58 -

the less attractive since any failure of the telescope or its instrumentation would preclude observation. The MMT is somewhat better since each telescope can be used independently; for instance a defect in one of the wide field instrument would not preclude observing with the 3 others. The array is obviously the most favorable solution because it offers a complete redundancy; even with one telescope out of operation, observation with 3/4 of the collecting power is still possible and repair operations could even take place while observing with other telescopes. It should also be pointed out that by its nature, a segmented mirror is entirely dependent on its servos. A failure would be catastrophic whereas a monolithic mirror could still work even with degraded performance in a passive mode.

Flexibility is also a specific and fundamental advantage of the array which can be used in many different ways: complementary observing of the same object in different modes, partial specialization of the telescopes for certain types of observation for which the full collecting power is not required, and perhaps the more important independent observing with 1 to 4 telescopes.

11.9 Cost comparison

It is indeed extremely hasardous to pretend to an accurate comparison without having real designs in hand. It is nevertheless important to establish trends and determine whether cost mayor not be a concept discriminator. Data on the development and construction of the 3.6 m ESO telescope has been recently analysed and compared with results obtained with the program "PRICE H" (62)(63). The validity of the PRICE model for evaluating the mechanics of optical telescopes has been ascertained and a power law of the cost upon the mass and the number of identical units has been established. If M represents the mass in tons of the unit telescope, the cost of the mechanics of the 1st unit has been found to be

(KDM 85)

and the cost for 4 identical telescopes

(KDM 85)

those formulae are only valid for a given technology but represent probably correctly the relative cost variation with mass and quantity quite independently of the technology.

We have attempted a comparison of the costs of the three concepts on the following basis:

The J concepts are based on identical technology for the mechanics and the building. The primary mirrors are assumed to have identical F/ratios. variation of cost of mechanics upon mass as MO.65, for the building upon volume as v O•65 • (Note that the PRICE model has only be tested on mechanics and that the application of the same exponential law to buildings may not be fully correct). - 59 -

variation of cost of mechanics and building for 4 units according to the radio C4/Cl. (This assumes a quick reproduction of fUlly identical units). Increase of mass of mechanics linear with the weight of the primary mirror. a) Optics

In chapter 11.2.3 the cost of a segmented mirror has been compared to the equivalent monolith. This analysis shows that the cost of a monolith would be about half that of the equivalent segmented mirror.

This does not take account of the mirror control system which is inevitably more complex and expensive for a segmented mirror. On the other hand, the monolithic approach would require an expensive coating plant as well as more handling equipment. We have therefore kept the ratio of 2 for the purpose of this crude analysis. b) Mechanics

Because the thickness of a segmented mirror is basically determined by the size of the segment, it may have a mass/m2 much lower than of a monolith. For instance the 10 meter Keck telescope would have a mass/m2 of 190 kg/m2 against 340 for a "lightweight" Pyrex or metal mirror. The relative mass of mechanics of an MMT would be 1.8 that of an SMT. Its cost will be 1.80.65 = 1.47 times the cost of the SMT.

The same comparison would not be fUlly correct for an array because a certain fraction of the cost will be fully recurrent with unit telescopes. This applies for instance to the drives, encoders, cabling etc. If we assume the proportion of duly recurrent cost to be 50%, we find a cost for the array of 1.69 that of the SMT. c) Building

Because of its compactness (its tube length is half that of the SMT) an MMT would have an enclosure of volume about half that of an SMT. A single 8 m telescope enclosure would be simply scaled down by a factor of 2 and would have a volume 1/8 that of the SMT. The MMT enclosure would then cost 0.637 relative to the SMT and an array, 0.668.

If we now assume for the array, a cost distribution such that optics, mechanics and building are equally weighted, (this distribution appears reasonable in view of previous projects and of current estimates) we can determine the relative cost of the two other concepts. This is summarized by the following table: I - 60 -

Array MMT SMT

Optics 0.33 0.33 0.66

Mechanics 0.33 0.29 0.195

Building 0.33 0.31 0.49

1 0.93 1.34

Within the limits of the very crude approximations used in this analysis, it is not possible to discriminate between an array and an MMT simply on a cost basis. The choice of the technology for the primary mirror and the strategy for the enclosure will have a far greater impact than the choice of the telescope concept itself.

The SMT appears however definitely more expensive, mainly because of the cost of its optics. Instrumental and scientific options will also have a determinant impact. For instance, a multi-foci approach may drive the cost of the entire project up by 20% compared with a single focus approach. The latter may be acceptable for an array for which there is ample flexibility but probably not for a single mount telescope. Interferometry may also indirectly increase the cost if large scale site preparation appears necessary, an aspect which has not been considered here.

Humphries et al reaches identical conclusions (49) (64) and establishes an optimum size for the unit telescopes of an array between 5 and 8 meter.

II.lQ Conclusion

From the various above considerations it seems that an array is in many respects the concept which offers the maximum of scientific flexibility and opportunities for the lowest risk and at a cost lower or similar to that of other concepts.

pending a final decision, the study group has concentrated its efforts in the analysis of a particular concept of array, -the linear array- (56) which appeared the best trade-off between the various scientific, technical and site requirements as they could be defined at the moment. - 61 -

CHAPTER Ill: VLT CONCEPT

111.1 concept drivers considered for the fixed array

A close-pack configuration was initially considered and was thought advantageous for minimising site requirements and preparation. A partial integration of the telescopes in the same building was also considered as an interesting possibility for reducing its cost. The main uncertainty at the time the first concept was laid out was the importance of interferometry with respect to traditional observing and the constraints it could possibly exerce on the configuration and design of the telescope.

The analysis of the limiting magnitude performed by uena and Roddier (54) showed that the increase of the collecting area would not necessarily provide a corresponding gain in sensitivity. The practical feasibility of a coherent combination of large telescopes was also questioned: the very limited experience so far available could not contribute much to raise very optimistic feelings within the community. Since it was an absolute necessity to begin the detailed scientific and technical investigations on a well defined base line, a provisional concept the "linear array concept" (56) was proposed. This concept was based on the assumption that interferometry would not be considered as the main driver for the VLT, but as an important capability in so far as it would neither significantly increase its cost nor have negative effects on the normal operation of the telescope.

A first consequence of this was that mobile telescopes were not to be considered and that interferometry should be in some way "decoupled" from normal observing so that a) rapid switching to and from another observing mode be possible thus preserving the principle of flexible scheduling. b) full time use of the interferometer be nevertheless possible using smaller auxilliary telescopes which could then better be mobile, and provide complementary full spatial frequency coverage. The question whether at least one 8 m telescope should be mobile is under review. This option is technically perfectly feasible but will indeed increase the project cost. Since no major technical problem is at issue, we have considered this option as an open question. The fact that there is little mention of it in the discussion does not mean it is discarded.

It was thought that a linear configuration would provide an acceptable sampling and redundancy at least in one direction. For the purpose of the initial technical analysis the distance between telescopes was set at a minimum on the assumption that a final decision could be taken at a later stage and would not fundamentally modify the concept. The linear configuration was also found favorable considering the characteristics of several possible sites in Chile which present usually a ridge with 150 to 300 m possible base line and an approximative East-West orientation facing the prevailing Northern wind. This is in particular the case at La Si1la and paranal (see Ill. 8). A much more limited North-South base line could also be obtained at the cost of additional ground preparation. Although possibly more favorab1e sites could be found, it is clear that site requirements for a very long base line - 62 -

interferometer could be met only with great difficulty and probably with a certain loss of quality, because good seeing is more likely to be found at the summit of an isolated mountain rather than on a plateau or on a smooth and very large mountain where ground effects will inevitably be more pronounced.

These starting assumptions are at the moment very much debated (55) and a change in the weighing of the various arguments mentioned here above would inevitably have an impact on the concept. However, the various studies so far performed are only marginally concept dependent, and as far as the fundamental choice for the optical strategy is not modified, the VLT could accommodate a different configuration.

The strategy for the unit telescopes is to rely essentially on active correction to compensate various image deteriorating effects such as temperature variations, hysteresis, wind buffets etc. This approach is on line with current ESO'S efforts to develop and apply an active correction scheme to the New Technology Telescope (NTT). Although the NTT will be able to operate in a semi passive mode, the VLT will make extensive use of active corrections. We believe this is the only way to obtain both optimum image quality and low cost.

The ESO active correction scheme requires a reference star, the brightness of which is a function of the correcting bandpass. A field of view of 30 arcminutes is therefore an important requirement. This requirement implies that in whichever way the unit telescopes are finally combined together an intermediate large field image plane must be available. This restricts considerably the freedom for telescope design innovation. Unless either optical quality is relaxed or extremely stable and rigid mirror blanks (hence heavy and expensive) are available we see little possibility to compromise with this requirement.

ALT-AZIMUTH mounting is now standard for modern telescope designs. It combines compactness, low weight and vertical symmetry of flexures. Its main drawback, i.e. the need for 2 axes driving is not anymore seen as a problem and the forbidden zone around the zenith where tracking becomes impossible has never in practice been a real difficulty. ALT-ALT mountings - Where a third servo mirror located at the cross-point of the two telescope axes reflects the beam along a fixed direction - are attractive whenever a Coude focus is required. Such a mounting has been considered but for the present time abandoned because of the much larger mechanical structure it requires. Also, the variable incidence on mirror 3 and its rapidly increasing size with sky coverage makes the Alt-Alt mount rather unattractive for normal operation (57). If the telescopes were to be mobile, Alt-Alt mounts should possibly be reconsidered in view of their potentiality for combining beams with no extra mirrors and no delay lines (particularly the boule design, see chapter 111.7). - 63 -

The unit telescopes are conceived for single focus operation. This choice results from experience with large telescopes where the prime focus and Coude foci represent a significant fraction of the cost, whilst relatively little used, partly because of the difficulties of operation. prime focus large field operation with a VLT would be even more difficult and expensive. Existing designs for prime focus correctors (58)(59) indicate that very large lenses close to 1 m diameter will be needed. Their manufacturing and supporting are likely to create difficult problems. The need for image rotation compensation and off-axis guiding would make the development of a prime focus system very expensive. Due to its complexity, its final image quality and efficiency may at the end not be superior to that of a focal reducer located at the cassegrain or Nasmyth focus. This is confirmed by the recent experience at the ESO 3.6 m telescope where a high throughput cassegrain focal reducer (EFOSC) provides more flexibility and in practice better image quality than the prime focus. With fly-eye technique, a large field of view can be covered with small and independent focal reducers: among other advantages this technique eliminates the need for very large single detectors (60).

Additionally, recent developments in broad-band high reflection coatings (65) indicate, that by the time the VLT goes in operation the coatings for M2 and M3 could be made so efficient, that the corresponding efficiency loss will be small. Also, single focus operation is desirable to comply with a flexible scheduling which we consider as a fundamental issue.

For an array, the combined Coude mode is mandatory. In order to continue with the philosophy of a single focus telescope we chose to give priority to the Nasmyth focus instead of the cassegrain. From the Nasmyth focus the beam can be easily relayed to the final coude focus with a set of fixed mirrors. Therefore, Nasmyth and coude operation can be obtained with an identical configuration of the telescope. Keeping constant characteristics of the telescope tube relative to gravity effects and wind load is a particularly important factor of reliability. An additional cassegrain focus may look attractive because it would have only two reflexions and no instrumental polarization. It would however require the change of the secondary mirror and the removal of the tertiary mirror. Both mirrors are large and this could only be a day-time operation which would increase the cost of the telescope and of its operation. This option has therefore not been retained for the time being. seeing optimization is a long debated issue for which there is no obvious solution. AS shown by N.J. Woolf (26), seeing and wind load must necessarily be traded off. Our approach is to deal in the best possible way with seeing in operating the telescope in open air. The critical problem becomes then the wind load. contrarily to dome seeing, wind loads can be measured and their effects can be objectively analysed and predicted. We have tacitely assumed that the cost increase of the telescope structure and drives required to cope with the wind load would be largely compensated by a corresponding decrease of the building cost. Thus the dome seeing optimisation would be achieved at no extra cost and with a good chance of success. - 64 -

tYT I -- I -1420 I I I I I I

N or- c: ...:t I 'E ,." u or- I l... rtI o I Nasmyth ,." Focus <11

I~:" °O!~ ~, T °o ~t~~.~.. , . °er 1

• 8000 .. I ,. 8000 ..

Figure 17: Optical design of an 8 ID unit-telescope. - 65 -

111.2 The optics of the 8 m unit telescopes

'rhe VLT concept is based on 4 x 8 meter telescopes. The optics of the individual telescopes are of the RitcheY-Chretien type in combination with two Nasmyth foci (see figure 17). The design goal of the optical system is 80% of the light within 0.15 arcsec after correction of all low frequency errors with active optics. An active correction of the primary mirror is viewed as the only way to achieve this value (see chapter 1.5.2).

111.2.1 Optical characteristics

The optical design characteristics of the unit telescopes are given in the following table:

Diameter of primary mirror: 8000 mm Radius of curvature (primary): 32000 mm Conic constant (primary): -1.00686 Diameter of secondary mirror: 1420 mm Radius of curvature (secondary): -5972.8 mm conic constant (secondary): 1.77521 Dimensions of tertiary mirror (flat): 1250 x 1700 (elliptical) F-ratio of primary mirror (F/Nb): 2 F-ratio of Nasmyth foci (F/Nb): 15 Linear scale at Nasmyth foci: 1.72 arcsec/m Nasmyth field of view: 30 arcmin (1046.5 mm) Nasmyth field curvature (R): 2750 mm

The primary mirror F-number is conservatively set to F/2. It could be desirable from a mechanical standpoint to decrease the F-number and 1.7 is considered as a goal for further studies. There is no strong argument in going much below this value because there will be no further gain in the size of the building. In addition, the cost of the optical figuring increases with the square of the inverse of the F-number and the optical quality would become uncertain since no experience exists yet on very fast and large mirrors. (For example the NTT F/2.2 mirror will be the fastest for its size in the near future). The risk would also become enormous, since the F-number is, together with the mirror size, the dimensioning parameter for the telescope structure and building. There is no chance for later modifications.

Figure 18 gives the theoretical spot diagrams for the flat and curved fields. The acceptable field of view without field curvature correction is about 8 arcmin.

The sensitivity of the optical system of 3 mirrors with respect to pointing errors and focusing is given by the following formulae: dy = - 69.82al - 11.29a2 - 4.65a3 + dy3 + 7.50dzl - 6.50dz2 + dz3 dx = - 69.82bl + 11.29b2 - 3.29b3 + 7.50dxl - 6.50dx2 dz = - 56.00dyl + 56.00dy2 + dy3 + dz3 - 66 -

CURVED FIELD (R = 2750mm) -10 mm -Smm Omm +Smm +10 mm a= 10 arcmin •• 'I" • ...... 'Y= 349 mm 'lUD':: ;1;: : E8 CB CV ct)

a = 8arcmin ... ·1···· E)j ...... D 'Y = 279mm CD.. EB ffi er'

a = 6 arcmin :;-H;-H: Illiilll @ ...... 'Y = 209 mm 'Ef0' EB Ew

a = 4 arcmin ,;; ;-.; 'Y = 140 mm (.·1'V...... @9':.::' E9 @·. Cl'·"V

axis ... ·1···· .::~.:: ' @· ...... ffi"i;J;~ ® E9 EIY

o r.cll~oa ,--~

FLAT FIELD 22.2 mm 142mm 8mm lSmm Omm de focus .;:1;'. a 10 arcmin I '~ ~;q)~: = -(t)- .. ,)- "(I:)'~...... to ,. to -~- :~j~.: ... ~ "' ... 'Y= 349 mm I

a = 8 arcmin j -"" - _{f:}_ -@nTh- -(~- · .. .p','''1'' . 'Y = 279mm l -ep- dp

•• :1:" a = 6 arcmin j(~:l:~i i- -~II~- -(~- ':\:1:1:' .-Cp- 'Y = 209 mm "I" I -$-

,:::1;::, (,~L. a= 4 arcmin - llltll'- -(~)- ..::':1:~ 'Il':: -(1)- 'Y = 140 mm '::1::' "'f" I -Ep-

::::1:::: ~I;r:.. 1,:) .. (.1.) .. ";'1::"-{'''h-" -(j)- axis . ::':1:::' ...... - --(1)-

" ()!11':),)1)

8m RITCHEY - CHRETIEN TELESCOPE (F 115) circle = 1arc sec ~ 582,u m

Figure 18: spot diagrams at the Nasmyth focus with and without curv~d focal surface.

;1 - 67 - where a and b are the mirror tilts in arcmins (around the x- and z­ aXis, respectively), dx, dy, and dz the mirror displacement in mm (the coordinate system is indicated in figure 17) and the indices denote the mirror numbers.

AS for the NTT, the correction of the decentering coma will be achieved by turning the secondary around its center of curvature. This method has the advantage of introducing no pointing error. TO compensate 1 arcsec of coma a tilt of 1.70 arcmin is needed. This value corresponds to a displacement of 2.96 mm of M2. The scheme of the secondary unit is shown in figure 19. In addition to coma correction, the secondary mirror may be used to correct for small but fast varying tracking errors. With 2 piezoelectric linear actuators the lightweighted secondary (mirror weight: approx. 200 kg; moving weight: approx. 300 kg) is envisaged to work at frequencies up to 10 or 20 HZ and an amplitude corresponding to 5 arcsec. The resulting actuator force is in this case small enough that it should be possible to avoid exciting resonance frequencies in the top unit structure. A sophisticated control system would allow to use the secondary for fine tracking, image motion compensation and to some extent as a chopper for IR observations. The recent developments in IR imaging arrays indicate that a chopping secondary mirror may however be obsolete when the VLT goes into operation.

Secondary effects of a fast tracking correction with the secondary mirror such as possible field distortions and background variation have however to be assessed.

At the present time, the availability of a prime focus has been abandoned, although the optical design leaves this possibility open. (See 111.1). TWo Nasmyth foci are available at opposite ends of the elevation axis, each of which is served by a large platform for mounting even large and massive instruments. All Nasmyth foci have a back focal distance of 3 m behind the primary vertex. Atmospheric dispersion compensation with a zero-deviation prism combination mounted on the elevation axis is foreseen for one Nasmyth fOCUS, while it is the intention to dedicate the other one to IR observation. It is questionable whether the whole field of view needs this compensation which would require prisms with 1 m diameter or whether it is possible to develop a system which compensates only a part of the field with moderate size elements. The problem of field rotation is met by rotating the instrumentation to avoid additional reflecting surfaces or refracting elements.

Sky baffles create mainly three problems: they increase the central obstruction (approximately from 3% considering only the secondary mirror to about 10% with a baffle designed for 30 arcmin field of view), they must be removable for IR observation, and they increase considerably the wind cross-section of the telescope. We are considering eliminating permanently the sky baffles. The secondary mirror will be dimensioned for the 30 arcminues F.O.V. and the mechanical unit will be smaller than the mirror so that from the focal plane, only the mirror and the spiders will be seen. Sky baffling must then be achieved inside the instrument. - 68 -

RING FOR CONNECTION WITH THE SPIDER FOCUSING DEVICE

BASE SUPPORT BARS FOR CONICAL FOR FOCUSING PENTOGRAPH

FOCUSING BARS X-Y-MOTION

CARDAN SYSTEM PIEZO ELECTRIC ACTUATORS

MIRROR M2 MIRROR CELL

Figure 19: Schematics of M2 unit. The mirror is supported from its back. Mechanical parts are entirely hidden in the shadow of the mirror. The mirror has 5 degrees of freedom: x,y,z for focus and coma correction, a,a for fast tracking errors correction. - 69 -

Matching the image scale of the Nasmyth focus to the detector, the pixel size would anyway require some kind of optical relay system (i.e. a focal reducer or a spectrometer) which usually provides ample opportunity for internal baffling. particular attention must however be paid to the quality of the pupil image and to diffraction effects of sky apertures, particularly in the IR, for which a sharp pupil image and a perfectly aligned cooled stop are essential to maintain a low telescope emissivity.

This is all the more critical with an ALT-AZ mounting because the instrument rotation may introduce pupil decentering if not perfectly aligned with the optical axis. It is probably desirable to introduce in the telescope design a possibility to check or control the instrument alignment. A possible scheme would consist of using light emitting diodes either in the center of the secondary mirror or at the rim of the primary, together with position sensors at the instrument internal stop.

Maximum obstruction of the spiders has been fixed to 1.5%. The total telescope obstruction is then less than 5%.

Accurate polarimetry is better achieved when the polarimeter is set on the telescope axis. With an additional mirror, which is placed in front of the Nasmyth mirror M3 it is possible to create a ·pseudo cassegrain focus· with a field of approximately 1 arcmin (Fig. 16). The dimension of this mirror will be such that an outer ring of about 6 arcminutes width will be available at the normal Nasmyth focus for acquisition of a reference star. This possibility could be introduced in one of the 4 telescopes.

11.2.2 PRIMARY MIRRORS

111.2.2.1 Options for the mirror blank

Various possibilities for the mirror blank have been discussed in chapter 1.3.

The 2 main options presently being investigated are:

Zerodur steel

Several other alternatives are also considered though not being investigated in detail at the present time. These are:

U.L.E Silica Aluminium Borosilicate glass Composite materials

Zerodur is an almost perfect material for making mirrors because of its quasi zero-expansion. The final geometry depends much on the technology of production. TWO possibilities are likely to emerge. - 70 -

Either a thin meniscus of thickness in the range of 150-250 mm or a lightweight structure made of hexagonal cells 350 mm wide and 400 mm high. (See figure 3, chapter I.).

Since no Zerodur blank design yet exists, the mechanical behaviour and supports for this option could not be analysed. Our efforts have therefore mainly been concentrated on the steel option. The other reasons for this are, firstly, that the ability of steel to meet optical stability tolerances is less well understood than zerodur, and therefore deserves more attention, and secondly that the manufacturing of a steel blank is the easiest of all solutions. If steel is found to be an acceptable material, it allows the possiblity to begin rapidly the manufacturing of the mirror and the construction of the first telescope. This procedure does not eliminate the possibility that the following mirrors may be made of a different material if experience reveals serious problems with the steel blank.

111.2.2.2 The Steel Option

A predesign of a structured steel mirror is described in (67). This design was used as a starting solution for a detailed structural optimization of the geometry. With respect to gravity loading in horizontal position (axial loading) the mirror blank geometry has been optimized within the design goal of 15 tons weight and a surface accuracy (peak-to-valley) of A/20. Finally a static analysis of the complete mirror has been performed with a finite element model of a quarter section of the mirror. Using the optimized geometry the gravity induced deflections in horizontal and vertical position have been analysed. In a first attempt to predict the necessary forces for an active correction of aberrations in the mirror, an analysis of the support reaction variations has been performed.

The results of the pre-ana1ysis of the first concept for a structured 8 meter steel mirror, which are described in detail in the following sections, generally confirm the feasibility of such a concept. However, no optimal solution for the design has been obtained so far, and further studies are necessary. The full mirror static analysis has shown that the density of axial supports can be reduced significantly, finally giving a total number of less than 300. The second optimization step for the geometry of a single hexagon cell, which will consider the results of the feasibility study for the manufacturing of the blank, will probably lead to an increased cell width and slightly modified thicknesses of faceplate and ribs. In the next analysis step the active support forces as well as the thermal performance of the mirror will be studied in more detail.

a) Mirror Structure preanalysis

The design goals for the mirror were:

Blank weight 15 tons Surface accuracy for the passive axial support and gravity loading A/20 (peak-to-va11ey) - 71 -

LOcal surface slope 0.05 arcsec Number of dxial supports 400

The mirror geometry was defined to be:

Outer diameter 8000 mm Radius of curvature 32000 mm (f/2)

With the assumptions that the mirror was flat and made out of stainless steel, the geometry of a honeycomb structured mirror has been calculated using the classical plate bending theory together with the sandwich analysis approximations. The resulting design parameters are summarized below (see also figure 20):

TOp and bottom plate thickness 10 mm Hexagon rib thickness 5 mm Hexagon cell height 230 mm Hexagon cell width 100 mm Number of cells 5440 Weight of the hexagon core 9074 kg Weight of the faceplates 7445 kg TOtal mirror weight 16519 kg Number of axial supports 360 Support distance (quadrilateral distance) 400 x 346 mm Material properties Young's modulus 21000 dN/mm**2 poisson's ratio 0.3 Weight density 0.00000785 dN/mm**3 b) Structure Optimization

The purpose of this detailed analysis of the hexagonal sandwich construction for the mirror blank was to optimize the structure with respect to elastic deflections under gravity loading. The analysis was contracted to Dornier-System, Germany (68). Several finite element models have been prepared to analyze the influence of:

faceplate thickness cell height and cell width

on the static deformation of the mirror surface between the axial supports. The final version of the model is shown in figure (21a). The model represents a cut-out of the mirror, which is axially supported at the four corners. The dimensions of the model are 400 x 346 x 280 mm. The model has been prepared using the finite element program SAP-V (Structural Analysis program). It consists of 324 shell elements and has approx. 900 degrees of freedom. With this model the following optimal geometry for the cell structure has been calculated:

top plate thickness 12 mm bottom plate thickness 2 mm rib thickness 5 mm cell height 275 mm cell width 100 mm - 72 -

Geometry:

D 8000 mm o D. 2000 mm 1 R 32000 mM c

T o It) ...L""

Weight:

The total weight of the structure ~ 15 tons (for a steel construction).

Surface accuracy requirements:

Maximum sag between two supports ~25 nm

-"1e'o.o· T ~===:;:I ===::::':::;:l~ o~ t.S"", -

Maximum slope at the support ~ 0.05"

Figure 20: Steel mirror predesign parameters. - 73 -

a.) undeformed shape

_or.. ~ ,,

D.) deformed shape - gravity -z

Figure 21: Finite element model for detailed analysis. - 74 -

B.C.V. progettll analysls or E.S.O. mlrror underormed shape 14118113 24/ 8/18B~ laxlli= 3 alpha= 4~.00 beta= 30.00

y

x

Figure 22: Finite element model for full mirror static analysis.

;~ - 75 -

With this geometry the weight of the mirror is 16246 kg. The elastic deformations of the mirror structure model due to gravity loading in axial direction are shown in the plot in figure (21b). The maximum deflection between the supports depured from the overall axial compression, is 25.7 nm and the maximum sag within the hexagon cell in the center of the model is 5.8 nm.

Although the thickness of the ribs in the honeycomb construction has not been varied in the optimization of the structure it is evident that a reduction of the rib thickness would improve the static performance of the mirror blank considerably. However, from a manufacturing point of view, it is doubtful that a thickness less than 5 mm can be realized. Therefore it has been decided to keep this value until manufacturing studies and test have proven the feasibility of thinner ribs. c) Full Mirror Static Analysis

Finite element model

B.e.v. progetti, Milano (69), has analyzed the static deflections due to gravity in horizontal as well as in vertical position of a quarter of the optimised hexagonal structured steel mirror. The finite element model, a complete isometric plot is shown in figure (22), was created with the SAP-V program using the membrane element, which transfers only in-plane loads. This choice is the best compromise between a sufficient numerical accuracy of the model and an economical handling on modern computers. The model consists of 12150 nodes and requires the solution of 26170 equations.

Mirror in horizontal position

In this loading condition the mirror is supported by 127 supports per quarter as shown in figure 23a. This distribution of axial supports has a 6-fold cyclic symmetry within the whole mirror. In the model the supports are simulated by boundary elements oriented parallel to the z-axis and fixed to the 6 nodes in the bottom plane of the structure as also shown in figure 23a. The boundary conditions at the free edges, i.e. the planes x = 0 and y = 0, are symmetric in the horizontal support case. In the static analysis the axial supports are considered as fixed points and the structure was sUbjected to gravity loading in the z direction. The purpose of this analysis was to assess the surface quality of the mirror under gravity loading in the axial direction. The results of this calculation is given in form of isocontour lines of the z-displacements of the mirror surface as shown in figure 24. The support geometry appears clearly on the surface of the mirror. The surface deformations can be summarized as follows:

Maximum peak-to-valley displacement 95.0 nm RMS value 6.7 nm - 76 -

a.) axial support distribution

, I , -1-, , '

J.

b.) radial support distribution

Boundary element

Figure 23: Support geometry of quarter mirror model. - 77 -

The maximum peak-to-valley displacement occurs at the outer and inner edge of the mirror, where the spacing of the supports is less favourable than in the central area of the mirror. But even in the edge regions the allowable value for the surface slope of 0.05 arcsec is not exceeded. The inner and outer edge deformed area covers only a few percent of the mirror surface. In the central region of the mirror the maximum peak-to-valley deformations are about 6 nm. The maximum support reaction is 41.2 dN and the average support force is 34.5 dN. The location of the maximum support reaction is shown in figure 24.

In a second calculation of the horizontal position a first attempt has been made to determine the behaviour of the mirror in case of active supports. With six different loading conditions the influence of an axial support force variation on the deformation of the mirror surface has been analyzed. For that purpose it was assumed that the axial supports are subdivided in 6 groups as indicated in figure 23a. Each group forms the quarter of a hexagonal ring and can vary the applied forces independently from the other groups. In order to avoid a rigid body displacement in z-direction, which would occur because the quarter model contains only one axial fix-point, it has been decided to fix all axial supports on the fourth ring, where the fix-points are located. Obviously this approach is not strictly correct, but nevertheless will provide the desired preliminary information for the case of spherical aberration corrections. The isocontour plots of figures 25 to 29 give the results for a load variation of 10% on rings 1,2,3,5,6. In an additional load case one single group of 3 supports, as shown in figure 29, has been changed by 10%. The results of these analyses can be summarized as follows: the variation of the support forces by 10% in one ring leads to a maximum peak-to-valley deflection of 3130 nm (ring no. 6, figure 28). From that preliminary result it can be concluded that a suitable active control of the mirror surface is possible without introducing too high a stress level into the mirror structure.

Mirror in vertical position

The mirror is supported in the vertical direction at 19 points per quarter which are distributed in the mirror as shown in figure 23b. This distribution has also, as in the axial support case, a 6-fold cyclic symmetry within the whole mirror and each 60 degree segment is symmetrical in itself with respect to the bissecting line. In the finite element model the supports are fixed with boundary elements oriented parallel to the x-axis. All 18 nodes of a hexagon cell, where a radial support is located, are fixed. In a vertical position the boundary conditions at the free edges of the quarter mirror model are asymmetrically, i.e. the elevation axis (y-axis) is an asymmetry axis and the x-axis is a symmetry axis of the model. The mirror model has been analyzed under gravity loading in the x-direction and the 19 radial supports' have been fixed. In this analysis no axial support is acting on the mirror, except the 3 fix-points. The isocontour lines of the z-displacements of the mirror surface are shown in figure 30. The plot of the deformed surface shows quite clearly the locations of the radial supports. The significant values are: - 78 -

Deformations Support Reactions dz max .. 95 nm Fz max .. 41.2 dN

dz rms 6.7 nm Fz av. .. 34.5 dN

Scale of Isocontours delta dz ., 5 nm

Figure 24: Mirror surface z-displacements isocontour plot for axial gravity loading.

j - 79 -

r --'

,

~------'-- \ r~_r~

/_ I --/- ~ ----..- "'-­ ,--"'-"'-""'"" _1""--- .~

./ -". ,/. /_.- _._./... - _f'-"::::'" ,_,I :.:::::: _

/ L.- 10% Force variation Supports fixed

dz max - dz min 2879 nm Scale of Isocontours : delta dz = 50 nm

Figure 25: Mirror surface z-displacements lsocontour plot. 10% force variation in support ring 2. - 80 -

L- 100/0 Force Supports variation fixed

dz max - dz m1n 1150 nm Scale of Isocontours : delta dz - 50 nm

Figure 26: Mirror surface z-displacements isocontour plot. 10% force variation in support ring 3. - 81 -

/"_-- . \.--­"'

Supports 1Q%Force fixed variation

dz max - dz min 1490 nm Scale of Isocontours : delta dz = 50 nm

Figure 27: Mirror surface z-displacements isocontour plot. 10% force variation in support ring 5. - 82 -

- ~, ...... - ~ ...... - .. -­ .... 100/o Force 6 -"- . - .. ... ~ _..... - ---?'- ----.. • ~ - -- .- ... variati on --..... - - - - ... F

--"------/"'\,- ..- _.... _-.....

'''-'-""" ...... -----., . , -',

. I. ~.-

Supports fixed

dz max - dz min - 3130 nm

Scale of Isocontours : delta dz = 100 nm

Figure 28: Mirror surface z-displacements isocontour plot. 10% force variation in support ring 6. - 83 -

~- / ..... :-::-:~)

dZmin.

Supports fixed

dz max - dz m1n - 421 nm Scale of Isocontours : delta dz - 10 nm

Figure 29: Mirror surface z-displacements isocontour plot. 10\ force variation in 3 supports as indicated. - 84 -

Deformations Support Reactions

dz max - 595 nm Fx max - 291 dN dz rms - 94 nm Fx min - 71 dN

Scale of Isocontours : delta dz - 10 nm·

Figure 30: Mirror surface z-displacements isocontour plot. Radial gravity loading (-x). - 85 -

Maximum peak-to-valley displacement 595 nm RMS value 94 nm

Although large, the distortion of the mirror in the vertical position could probably be corrected by the axial active system. The remaining alternative is to increase the number of radial supports.

The analysis of the mirror in vertical position was performed disregarding the real location of the radial support within the structure of the mirror. In order to determine the influence of this effect on the deformation of the mirror surface a more detailed model of the radial support load introduction area is being prepared. d) Manufacturing

This has already been discussed in chapter I.3.4. The preferred manufacturing process for the moment is the -build-up welding-. It is not clear whether it is simpler to first manufacture a solid block and sUbsequently machine it to attain the desired lightweighting or to built directly a structure as shown in fig. 4. Both approaches appear feasible and facilities for making a solid block of the size of the VLT mirror already exist. In any case, a subsequent fine machining of the cavities appears necessary. This may in fact be the most expensive part of the process. Either quasi-conventional milling or electro-erosion are possibilities. One problem is the difficulty of obtaining a back-plate that covers a large fraction of the mirror surface. It is very likely that the back plate will have rather large apertures to allow machining. All these operations will necessarily be done on a computer controlled machine. e) Thermal aspects and control of a structured steel mirror

Both internal and external thermal equilibrium are important. The latter has mainly an influence on thermal turbulences above the mirror which can considerably degrade the optical performance. The internal equilibrium is important to avoid figure distortions and print-through of the internal blank structure to the mirror surface.

The equilibrium of the mirror temperature with the ambient air is important when the telescope is embedded into a dome. It has been shown by Wolf (70) and Gielingham (71) that 1°C of temperature difference of the mirror with respect to the air may cause about 0,5 arcsecond image degradation. with a telescope in the open air, where it is swept by an air stream, differences of a few degrees will probably remain unnoticed. This has been shown by LOwne (72) and is also clearly demonstrated by Fig. 31 which illustrates an experiment performed at ESO on an aluminium mirror. Though heated up to 30° above ambient, the original wavefront is nearly retrieved when the surface is slightly ventilatea (about 0,2 m/sec). This also demonstrates the relative thermal insensitivity of a highly conductive material. More serious with a structured mirror and a material such as steel whose thermal conductivity though better than glass, is not as good as for aluminium, is the internal equilibrium, particularly with respect to print-through which, unlike warping, cannot be corrected with active optics. (Xl (j\

0 0 a) Ambient temperature b) + 30 e c) + 30 e + ventilation (0.2 m/sec)

Figure 31: Effect of ventilation on air turbulences above a mirror. A 500 mm aluminium mirror was heated up to 30°C above ambient. A slight ventilation almost suffices to eliminate the effect of st~gnatingunstable layers of air. - 87 -

In order to estimate the thermal sensitivity of the structured steel blank, a detailed hexagonal structure model was used to determine the surface deformations due to axial and lateral thermal gradients. The very first results indicated that local thermal gradients i.e. at a scale of one hexagonal cell, should certainly be maintained below O.loC in order to prevent discernable print-through effects.

Internal gradients are dependent on the internal conductivity. The conductivity of a steel mirror can be increased in two ways:

i) Deposition on the cavity-walls of a thick layer of pure aluminium. A layer 1 mm thick would increase the conductivity of the ribs by a factor of 5 with a weight penalty of 10%. The aluminium deposited with a spraying technique would be slightly porous and should not affect the mirror mechanical stiffness nor induce serious bi-metallic effects.

ii) Internal ventilation. Since the purpose of the ventilation will be more to eliminate the internal gradients than to control the actual mirror temperature, a relatively small turbulent air flow would be sufficient. The same ventilation outfit may be used during day-time to cool the mirror, with a circulation of refrigerated air, down to the expected midnight temperature. Approximately one hour before observing the active refrigeration will be stopped to allow the high internal conductivity to eliminate gradients when observing.

111.2.2.3 Mirror Supports and Mirror Cell

a) Axial supports

It is clear from the numerical analysis of the steel mirror that the number of axial supports so far considered for the preliminary analysis has been overestimated. It is also desirable to reduce the complexity of the control system. This can be achieved either by reducing the absolute number of supports, or by coupling several supports together in whiffle-trees. using whiffle-trees with 2 or 3 pads it is possible to reduce the number of active axial supports to about 200.

The active axial supports should compensate for gravity deflections (astatic effect), mirror figure correction (manufacturing low order effects, thermal effects) and wind loading. The basic idea is that the active optics scheme based on a variable distribution of forces should be extended so as to compensate also for rapidly varying effects such as the wind buffetting.

The force is measured directly from a load-cell located between the back of the mirror and the support. position sensing from the mirror cell is not considered because the cell is likely to be more flexible than the mirror and therefore does not provide an adequate reference. Nevertheless, it would be useful to couple the mirror to the cell to reduce the amplitude of high frequency disturbances mostly caused by wind buffetting. This approach differs from the classical flotation principle using astatic levers. - 88 -

In order to increase the range of the active correction it seems desirable to adopt a push-pull system. This is only useful, however, to compensate quasi-permanent errors such as manufacturing errors and warping. The actual range of correction which includes the variation of gravity load with the telescope position has been set to 150% of the nominal load. For the high frequency wind load correction one has considered that the system should be capable of modifying the load by 10% at a frequency of 3HZ, although the real requirements are considerably less.

At least two types of support seem to be of interest. Figure (32) shows the principle of a system based on a spring whose compression is driven by a DC linear actuator. This solution has the advantage of being simple and reliable, and the spring stiffness can be easily adapted to the need. It may however be limited in frequency response.

Figure 33 shows the principle of a sealed hydraulic system (73) driven in two steps by a linear translator and by a piezo-electric actuator. The linear translator permits the positioning and the correction of the mirror over a large range, whereas the piezo-electric actuator permits a high frequency correction over a smaller range. The variation of volume of the chambers is obtained by the axial deformation of metallic bellows having high longitudinal compliance and high radial stiffness. The system has practically no friction. It is also possible to connect the supports to a counterweight-driven central piston which will then work as an astatic lever. The force exerted on the mirror depends on the amplification factor, i.e. the ratio of the active surfaces in both chambers, of the displacement of the actuators and of the spring constant of the system (its expansion under pressure). Dynamically, the system is equivalent to a very stiff spring and a strong coupling between the cell and the mirror can be anticipated. The heat dissipation due to the piezo-elements is very small. The power supplies and control electronics can be concentrated in a temperature controlled assembly preferably located in a lower structure.

prototypes of both systems are going to be built and tested. b) Radial supports

About 50 radial supports distributed over the surface of gravity of the mirror are necessary. These supports will be totally passive, since any remaining effect can be corrected by the axial active system. Coupling axial and radial supports into one unit is a possiblity but if mass saving is important, it seems simpler and safer to separate the two functions.

The radial supports could be totally conventional and use a single astatic lever or they could use an astatic lever coupled to an hydraulic amplifier similar to that of the axial supports. The advantage of the latter is its lightness and its mechanical simplicity. An hydraulic amplification of the applied force by a factor of 20 can be easily achieved. A mechanical lever with an arm ratio of 3 would permit to support radially the mirror with 5 kg counterweights. - 89 -

---".....;:"..."'--'>...... ;:"..."'--->o.r"-'...... :.....:>.....:>.....:>...T'.....;:"...>....>..."'--'>.....>.....>.~ MIRROR TO CONTROL ELECTRONICS

1 MOTORIZED LINEAR 3 TRANSLATOR 2 LINEAR BEARING 3 COMPRESSION SPRING 4 LOAD CELL

~ca~aL~~W MIRROR CELL FROM CONTROL ELECTRONICS CD

Figure 32: principle of a push-pull mechanical active support.

TO CONTROL MIRROR ELECTRONICS

1 PIEZO-ELECTRIC ACTUATORS 2 BELLOWS-AREA S1 3 MOTORIZED LINEAR TRANSLATOR 4 ACTIVE BELLOWS-AREA S2 5 LINEAR BEARING 6 LOAD CELL MIRROR CELL

Figure 33: principle of a push-pull hydraulic active support. - 90 -

SECONDARY MIRROR (M2)

TIP- TILT MIRROR (MC 1)

ACTIVE PRIMARY OFF-AXIS BEAM MIRROR WAVE­ SELECTOR OR (M1) FRONT --- I BEAM SPUTTER SENSOR I TO INSTRUMENT ON NASMYTH PLATFORM/IN SCHEMA TlCAL COUDE LAB DIAGRAM OF THE coma, OR ACTlVE/ ADAPTIVE fine TO INTERFEROMETRIC tracking COMBINATlON OPTICAL SYSTEM focu.~---JL....- .., tilt CONTROL SYSTEM figure higher order control aberrations of the atmosphere

Figure 34: General scheme of the active/adaptive correction system for the VLT unit telescopes. - 91 -

c) Mirror cell

Although not yet defined, the mirror cell will be made - if only for weight reasons - of a space frame lightweight structure. If dynamically rigid supports are preferred, it may be desirable to have a rather rigid cell which will help in keeping the mirror figure for high frequency buffetting. Most probably it will not be necessary to make use of carbon fiber elements.

The mirror cell should necessarily include a provision for an easy washing of the mirror. This has been proved to be the best way to clean telescope mirrors and extend the lifetime of the coatings.

The mirror cover could be made of a high strength plastic fabric rolled on one or two sides of the cell. correctly stretched, such a cover would provide an efficient mirror protection, and would not significantly increase the weight and the wind cross-section.

III.2.3 Active correction

During the past five years, a system of active control has been developed at ESO for the NTT for optimising the form of monolithic primaries and for the alignment of the primary with the other optical elements of the telescope. Using this system, the ESO NTT has a specification for telescope errors (low spatial frequency) of 80% geometrical optical energy in 0.15 arcsec, which effectively means diffraction limited performance at visible wavelengths at all times.

The technique of active optics is very closely related to adaptive optics (see chapter I.5.3.l). Figure 34 shows the schematics of an active and adaptive compensation system. The two correction loops are indicated. The typical frequency range for the active part is DC to 2HZ and 1 to 100 HZ for the adaptive system. Figure 35 gives the requirements for the active and adaptive systems, the type of correction performed by the individual element (active primary, secondary, etc.), and the frequency range of some possible actuators.

AS mentioned above the active correction of the VLT primary is intended to correct for all disturbances below a cut-off frequency of 1 to 2 HZ. The sampling of the wavefront must therefore be achieved at a frequency of about 5 HZ. An extrapolition from the Shack-Hartmann sensor designed for the NTT as well as of commercially available sensors of the same principle to a 8 m telescope gives a limiting magnitude for the active correction of approximately 14.5. These systems assume an optical input flux of 100 to 1000 photons per sUbaperture and per sample for a typical maximum error in the reconstructed wavefront of A/IS. The above calculated value for the limiting magnitude assumes the worst case of 1000 photons per measurem~nt. The recent developments indicate that it might be possible to 'work down to 100 photons/subaperture/ sample which will then be equivalent to a limiting magnitude of 17. The star population in the VLT field of view for a magnitude of 14.5 is even at the galactic poles dense enough for an unrestricted operation over the whole sky (see figure 36). - 92 -

Re~uirements for an active/adaptive system

I I I I I I I FIGURE CONTROL (THERMAL, I GRAVIT ATIONAL, WIND EFFECTS) I __tRACKING FOCUSING _ IMECH.VJBR.) SEEING CORRECTION I I lATMOSPHERIC TURBULENCE) I 1 WAVEFRONT CORRECTION: 1 !-=::::::::::::======::::::=:======---M1 -'--- I --t ADAPT. MIRROR MC2----- TlLT CORRECTION: I ~======---TRACKING+ M2 ------:------TILT MIRROR MC1------FOCUS: I F:=::::::::===------M2 --+1 ---. ... I I ACTUATORS: I PIEZO ELECTRIC -----4------;:------­ ELECTRO MECHANICAL ------:-1 -- -- HYDRAULIC ------'------i - .- I [Hz] temporal frequency

Figure 35: Amplitude and frequency requirements for the active and adaptive optical systems. The range of correction for the active elements and possible types of actuators are indicated. - 93 -

1000 ~~~~~!::.-~~!..-.-I 200

QI QI 100 L- to­ C\ ...J QI "C QI> .&: .... QI QI 100 20 L- .&: ", .E"'" ::J 50 10 tT 1: .... VI o 0 :;: L- :. QI I1l QI Cl. 10 2 :; .:; Cl. I: 0 .... 0 5 1 Cl. 0 :;: I1l L- "C :; 2Gi Cl. VI;;: 0 ~_------4 0.2 Cl. 1 L- 0.5 . ------1 ....I1l VI 0.1 10 15 20

magnitude Figure 36: The diagram shows the number and magnitude of stars per square d~gree and for the full fi~ld of view of the VLT betw~en the two extreme galactic latitudes b (shaded area between b=Oo and b=90 0 ). The spotted area indicates the s~nsitivity range of a Shack-Hartmann s~nsor. The cross section is the operation range of the active correction system.

+ REFLECTIVITY OF COATINGS lE~ CO) -}

•..

""FlVE:L.E:NGTH CNM)

Figure 37: Reflectivity of silver and aluminium undercoated with copper and of silver protected with aluminiumoxid in comparison with standard aluminium coatings (test coatings on glass substrate). - 94 -

III.2.4 coatings

New coatings for the telescope optics are under investigation. Figure 37 shows results for the reflectivities of silver and aluminium undercoated with copper and silver protected with aluminiumoxide in comparison with a standard aluminium coating. The gain of reflectivity of silver is obvious, although there is a trade-off at blue and UV wavelengths. The protected silver coating is more efficient than aluminium for wavelength greater than 450 nm. Samples of these coatings are exposed to real environmental conditions at La Silla, to test long timescale aging processes. These investigations are just beginning and will be extended also to non-vacuum deposition techniques. Especially the conductivity of metal mirrors opens new possibilities in this direction; electroplating is in particular regarded as promising. AS far as vacuum deposition is concerned it seems that an interesting alternative to the classical thermal evaporation of metal is sputtering at a short distance with a linear planar magnetron source arranged along a radius of the mirror. The mirror rotates during the coating process for a homogeneous deposition of the material. In this case the vacuum tank is small, with an overall height of about 3 meters, compared to about 6 meters with standard evaporation technique.

III.3 Mechanical Design of the 8 m unit Telescopes

III.3.1 General Approach and Specifications

The conceptual design of the 8 meter unit telescope of the ESO VLT has been developed within the frame of four study contracts, two for the conceptual design of the tube and two for the design of the fork. The purpose of the design studies was to compare different approaches and to assess the achievable performance. Generally the conceptual design of the 8 meter unit telescope was oriented towards the goal of developing a telescope with

low cost lightweight structure low thermal inertia small wind attack cross-section very great stiffness high reliability

The design studies were based on the working concept for the ESO VLT project, a domeless telescope on an alt~zimuth mounting. Due to the domeless telescope concept, wind loading on the one hand was one of the main drives for the structural design and on the other hand the advantage-of natural ventilation of the structure could be considered in the thermal design.

The two main structures of the alt-azimuth 8 meter telescope are the tUbe and the fork together with the interface support ring to the concrete pillar.

The tube of the telescope serves as an optical support structure and its task is to maintain a zero-pointing error condition while the telescope is moving in the gravity field. In the conceptual design - 95 -

of the tUbe structure the optical elements, i.e. the primary mirror cell, the secondary and tertiary mirror units, were regarded as Rblack boxesR with geometrical interfaces and masses as given below (see also figure 38)

++ primary mirror unit Ml: - diameter of cell 10 meter - total weight 30.000 kg (including the mirror)

++ Secondary mirror unit M2 :

- diameter of unit 1.4 meter - diameter of mirror 1.5 meter - height of unit 1 meter - total weight 2000 kg ++ Tertiary mirror unit M3 :

- diameter of unit 1.3 meter

- diameter of mirror 1.4 meter (projected) - height of unit for spider attachment (appr.) 1 meter - total weight 2500 kg

The design of the tUbe followed the principle of stiffness and weight optimization in order to minimize the static elastic deflections due to gravity loading and consequently the hysteresis effects, which cannot easily be corrected by a pointing model. The design goals for the tUbe conceptual design can be summarized as follows. When moving the telescope from zenith to horizon, the tube structure has to keep the mirrors at position within tolerances or allowable static deflections which are given in the following table:

Mirror Deflection Tilt

Ml 2 mm 20 arcsec M2 2 mm 1 arcmin M3 1 mm 1 arcmin

The deflections, which are perpendicular to the optical axis, are assumed to compensate so that the resulting net deflection between Ml and M2 is less than 1 mm. This can be further reduced during final optimization. The specified tilt angles are about the altitude axis. The static deflections and rotations of the optical elements specified above are not critical, because active correction of the primary and secondary mirror is foreseen in the design of these units. Concerning the dynamic behaviour of the telescope tUbe, the first structural eigenfrequencies should be as high as possible, in order to maximize the frequency response of the telescope drives. The design goal for the tUbe structure was set to:

- first eigenfrequency 10 Hertz

- weight 30.000 kg - 96 -

1100

ID 1$0

l.__"11111.__I . __' j ~

r INlEAfACl WIl" ([HUAl PAAI IT 11'

z

Figure 38: Dimensional specifications used for the conceptual design of the telescope. - 97 -

In addition to the 34.5 tons for the optical units, a generous provision of 10 tons for the auxiliary equipment (cabling, electronics, mirror cover etc.) was considered. The design target for the complete tUbe is therefore 75 tons.

In order to minimize the effect of wind loading on the structure the design was concentrated on space frame type structures instead of closed box type structures wherever possible. This was especially the case in the design of the centerpiece where much effort has been spent to develop a truss structure design concept which fulfills the above mentioned requirements. Besides the better performance with respect to wind loading the truss design concept offers the advantage of a thermally fast structure, i.e. that the thermal time constant for temperature equilibrium between the telescope structure and the environment is much lower than in a closed box structure.

During the design of the structure the possibility of exchanging top units was not considered as mandatory, but the conceptual design described in the next sections allows without major alterations the exchange of top units and/or complete top ring assemblies if necessary. In order to provide observation on two Nasmyth stations the possibility of rotating the M3 mirror 180 degrees about the optical axis was considered in the design of the M3 spiders. The necessary free beam path diameter in the altitude bearing shafts was 1.2 meter, which defines the minimum size of the altitude bearings.

The fork serves as a support structure for the tube. Its main functions are:

to carry the telescope tUbe to transfer the load from the altitude to the azimuth bearings to carry the Nasmyth platforms and transfer the load of the Nasmyth instruments to the azimuth bearing to provide the attachment for the coude tubes

The design goal for the fork structure was to optimize the stiffness to weight ratio in order to minimize the elastic deflections and consequently the hysteresis effects, which are related to the deflections. The structure has to be sufficiently stiff to guarantee a proper tracking stability under all loading conditions. As in the case for the telescope tUbe the fork should be designed as a truss structure in order to minimize the reactions to wind loading and to provide a thermally fast structure. The design target for the first frequency, not taking into account the stiffness of the bearings and drives, was set to

- first eigenfrequency 7 Hertz

- weight 120.000 kg (not including the bearings, drives, electronic equipment, Nasmyth platforms and instruments, secondary structure like stairs, catwalks etc.)

The design must allow for a rotation of the telescope about the azimuth axis of +/- 270 degrees. The two Nasmyth platforms attached to the fork have to carry instruments with a weight of approx. - 98 -

4 tons per platform. The interface support ring to the concrete pillar of the VLT building has to provide the support for the track of the azimuth axial bearing.

The bearings for altitude and azimuth axes have been provisionally prescribed as follows: rolling contact bearings with a diameter of approx. 2500 mm for the altitude axis and hydrostatic pads for the axial azimuth bearing. The radial centering bearing for the azimuth axis should be a rolling contact bearing.

Tube Structure

During the conceptual design phase of the telescope tube structure, two firms, i.e. MAN and MATRA independently produced a structure design for the tube of the ESQ-VLT.

MAN design

The MAN tube design (74) is based on a classical design with Serrurier struts for the upper tube and flexion bars for the attachment of the primary mirror cell. Starting from this, additional stiffness-enhancing structural components are provided and other appropriate changes are made.

The following structural components were added: connecting struts between front ring and center piece, additional connecting points from the center piece to the Ml cell frame. These stiffening measures cancel the classical decoupling effect of the Serrurier design principle between center piece, front ring and mirror frame deformation.

The center piece is an open truss design, a solid-web design being heavier and less favourable from the thermal point of view. The M2 spiders are arranged so that two spiders are connected in one point at the M2 unit. This design provides the highest torsional stiffness. For reasons of weight minimization the spiders will be of a welded hollow construction. The top ring is a welded circular hollow box with 4 or 8 bolted field joints for dismantling into transportable units.

For the gear rim of the altitude drive of the telescope, MAN has studied 2 different solutions. The first solution consists of a precision gear rim with hob-milled teeth and a maximum diameter of 6 meter. The gear units are rigidly flange-mounted on the azimuth part. The necessary adjustment of meshing between gear rim and gear requires.an alignment system in the fixing flange of the gears.

The second solution, which is preferable from the point of view of erection and transportation, is a gear rim girder of a light welded construction with a toothing of profile-cut tooth segments. The tooth segments are fixed to the gear rim girder via bolted connections. In that case the diameter of the gear rim can be adapted to the center piece geometry (9.4 meter diameter). In both cases, the moment transmission from the gear rim to the tube requires additional load transfer points on the center piece. - 99 -

The MAN tube design is shown in figure 39. The structural calculation of the tube structure was carried out by MAN with the finite element program STRUDL. The entire structure was calculated as a frame with resistance to bending except the spiders which have been modelled by triangular membrane elements. The mirror units M2 and M3 have been assumed as rigid. This also applies in the case of some components of the supporting structure which represent the members connecting the hollow shaft with the center piece frame. The Ml cell, which will be an open truss structure, has been integrated in the finite element model as a level grating. In this case representative stiffness values can be calculated from the overall height and weight of the cell.

The boundary conditions at the altitude bearings have been chosen separately for static and dynamic calculations. In the static calculations the displacements of the tube itself are of interest. Therefore the bearings have been assumed to be infinitely stiff. For the dynamic calculations, elastic spring elements have been introduced into the structure model in order to represent the stiffness of the yoke. The mass distribution in the structure was described by condensed masses acting on the physical nodes of the model. The results of the static analysis of the MAN tube structure can be summarized as follows.

In the gravity load case - zenith position - the deformation of the mirror units essentially constitutes a lowering of all three units in the direction of the optical axis of the tube. This load case only influences the focusing. In the horizontal position the gravity load case gives a lowering of the mirror units within the tube structure perpendicular to the optical axis. Since the tube is balanced, there is practically no rotation of the center piece. This load case results in a deflection of the mirrors that only have an effect on the pointing of the telescope but not on the defocusing.

The maximum deformation at mirror M2 under gravity in horizontal position is 1.37 mm. Its movement in the case of a wind speed of 100 km/h is 0.41 mm.

In the dynamic calculation the first six eigenfrequencies and corresponding modes have been determined. The first eigenmode of the supported tube is a horizontal movement of the tube together with the yoke arms in the direction of the altitude axis. This vibration is essentially determined by the mass inertia of the tube and the horizontal stiffness of the yoke arms in the direction of the altitude axis. Internally the tube structure is very stiff and moves as a rigid body in this vibration mode. Since the vibration is in the direction of the altitude axis, it is decoupled from the altitude drives and cannot be influenced by the latter. The first eigenmode of the free tUbe structure, with a frequency of 16.3 HZ, is a widening of Serrurier bracings and rotation of front ring and Ml cell around the optical axis. The mass breakdown of the MAN tube structure is shown in the following table: - 100 -

SECTION A-A '!G00 A-+j I

." le ~~~~~~~~~-==.~

\ c c S, i 1. '__+-+-' ---;- ~

SECTION B-B

TOP VIEW TUBE

----::: -::;:::; ;:::::::=- "=-1:=;-==-= ..::::- ~ r~--:'" '~-.~\ \

_-J"\

'"

:!'~ -- -- -~ """''-+-+ _,'_, __ L :;' - -L..,., ! I

Figure 39: MAN conceptual design for the tube structure. - 101 -

ITEM WEIGHT [kg]

centre piece 25050 Flexion bars 2100 Serrurier struts 7240 Spider M2 860 Spider M3 860 Top ring 4300 Hollow shafts 11630 Gear wheels 12000 Data wheels 3600

SUbtotal 67640

Ml unit 30000 M2 unit 2000 M3 unit 2500 Additional equipment 10000

SUbtotal 44500

GRAND TOTAL 112140

MATRA design

The MATRA tube design (75) is based on the choice of the hexagonal truss concept for the upper tube. The hexagonal truss concept for the upper tUbe provides a stiff solution with short members connected with each other with wide angles which raise the first eigenfrequency of the structure. The upper truss consists of two halves. The upper half is similar to a Serrurier truss but with 6 isosceles triangles instead of 4. The lower half of the upper truss consists of 6 quadrilaterals with diagonal cross bracings. The hexagonal tUbe is connected to a hexagonal top ring and center piece.

An alternative solution, a quadrupod structure for the fixation of M2, replacing the upper truss of the tube, has been rejected, because of its much higher obstruction compared to the classical spider design for the secondary mirror unit support. Also, a quadrupod may p~esent a problem for the IR because it is not possible to get an image of it in focus where its radiation could be eliminated with a proper baffling. For the center piece MATRA developed and optimized a framework structure which fits to the hexagonal truss concept of the upper truss of the tube. The center piece cross-section is formed by 7 main beams running along the circumference of the center piece and thus defining the static moments of inertia. This primary structure is connected and stiffened by a secondary structure of vertical, horizontal and oblique bars.

With a detailed structural model, the mechanical properties of the center piece have been calculated. These properties were used to determined the characteristics of an equivalent beam, which was introduced in the overall tUbe model. - 102 -

u UP~U RWG ·\JPPfA TRUSS

-1 f z• ~I l .,,~.. __ J I

I

x

Figure 40: MATRA conceptual design for the tUbe structure. - 103 -

During the optimization of the center piece, the influence of the shear factor as well as the characteristics of the load transfer from the altitude axis shaft into the center piece have been studied in detail.

For the support of the M3 unit, two different solutions, i.e. a four legs solution connected to the center piece and a direct attachment of M3 to the primary mirror cell have been studied. For the conceptual design of the tUbe a six legs spider for M2 and the 4 legs spider for the M3 unit have finally been selected. The attachment of the primary mirror cell to the center piece is provided at 12 connection points. Due to the limited space left between the center piece and the Ml cell a direct connection of the cell to the center piece is also possible. The MATRA tube design is shown in figure 40.

The structural calculation of the tube design has been carried out with the MSC/NASTRAN program for the analysis of the static as well as the dynamic behaviour. The static analysis showed a maximum deformation under gravity in horizontal position at mirror M2 of 1.6 mm. For the wind load case a wind speed of 14.4 m/s perpendicular to the altitude axis with the tUbe in zenith position has been assumed. The resulting force at the M2 unit was calculated to 282 daN assuming a drag coefficient of 1.3. The maximum displacement at mirror M2 under this wind load is 0.15 mm.

The first eigenmode is a local torsional vibration mode of the secondary mirror unit about the optical axis and has no influence on the optical performance of the telescope. Its frequency is 6.8 HZ, if necessary this first resonance frequency could be raised by stiffening the top ring and the spiders. The second mode of the tube structure with a frequency of 10.2 HZ is a bending vibration of the upper truss structure. The mass breakdown of the MATRA tube structure is given in the following table.

ITEM WEIGHT [kg)

centre piece 23241 LOwer truss 0 upper truss 5894 Spider M2 242 Spider M3 500 Top ring 837 Altitude boxes 5928 Altitude axis 6000 Wheels 6000

Subtotal 48642

Ml unit 30000 M2 unit 2000 M3 unit 2500 Additional equipment 10000

Subtotal 44500

GRAND TOTAL 93142 - 104 -

A comparison of the main performance data of the two different tube concepts shows that the two solutions more or less converge. The MAN design concept with a weight of 112 tons in total gives a first eigenfrequency of the structure of about 16 HZ and a maximum static deflection at the top unit of 1.4 mm when the tube is in the horizontal position. The MATRA tube design has a total weight of 93 tons and a first eigenfrequency of 10.2 HZ for the tube structure, which corresponds to the ESO specification. The maximum deflection of the top unit in horizontal position is 1.6 mm.

The weight estimate of MAN is nearly 10 tons higher, because two separate wheels for drives and encoders have been assumed. The most important weight difference in the tUbe structure itself lies in the top ring, where the MAN design weighs 4.3 tons and the MATRA design 0.84 tons. This explains the low first local frequency for the torsional rotation of the secondary mirror unit in the MATRA design.

111.3.3. Fork structure

AMOS fork design

During the conceptual design phase of the telescope fork structure, two firms AMOS and NEYRTEC independently developed a fork structure and support ring design.

The AM OS design concept (76) is shown in figure 41. In this concept the restriction of having an azimuth axial bearing consisting of one track only was abandoned. Without this restriction the fork could be designed in such a way that the load from the altitude bearings could be directly transfered to the axial azimuth bearing, which was designed as a hydrostatic bearing with pads fixed to the fork and moving on two concentric rails. That means that the fork consists of two individual pedestals, which are self-supported on the hydrostatic pads. The two pedestals are interconnected by a stiffened box structure, which provides the torsional stiffness of the fork. With this design the distance between the altitude axis and the plane of axial azimuth hydrostatic bearings is minimized, which results in a high bending stiffness and eigenfrequency.

The calculation of the static and dynamic performance was performed with the finite element code SAPL1S. FOr the modelization of the structure some assumptions have been made. The tUbe structure has been considered as a simple mass-spring system, i.e. the tube is modelled by two concentrated masses in the center points of the altitude bearings which are linked by a spring. The altitude bearings are assumed to be unconstrained in all 3 rotational degrees of freedom (ball joint).

In the first approach the stiffness of the axial azimuth bearing is assumed to be infinite, i.e. the fork structure is rigidly supported in the axial direction. The radial degrees of freedom at the centering bearing of the azimuth axis are also blocked. The maximum displacements at the altitude bearings are 0.36 mm in the vertical direction and 0.19 mm in the direction of the altitude axis for the gravity load case. The first eigenfrequency is 7.5 HZ. - 105 -

______COU~ ~====::::::======:;--II~ ---...------1 I ~ ______a.+-_-.~_=_-.=-_=_---,_.,...__-~---=--M+----'_....._...... _...... ~ _ o

E~j1a--' ...... IV.'''. _A / --... / • COUPE C(:- "" .I / -­ '" / !/

8 I .--l

1------'...------,£.---1

Figure 41: AMOS conceptual design for the fork structure. - 106 -

_ VUE SUIVANT F_

El EYATION E;

:=

AZIMUTH C.ENTRAL BE."'RING

Figure 42: NEYRTEC conceptual design for the fork structure. - 107 -

The total mass of the AMOS fork structure is approx. 110 tons, with the following breakdown:

ITEM WEIGHT [Kg]

Fork structure (steel): 66360 AZ central bearing box (incl. bearing) 7000 Alt bearings 13200 Support for alt bearings 10020 AZ hydraulic pads 6800 Alt drive units 1000 Coude mirror supports 3000 Miscellaneous 3000

TOTAL 110380 + (instruments)

The NEYRTEC Fork Design

The NEYRTEC fork design concept (77) is shown in figure 42. The fork truss structure is constructed from standard profiles (HEB) which are connected by gusset systems. The housing of the altitude bearing is a box structure connected with an adjustable system to the truss structure. The connections between truss and support points (fixation points of hydrostatic pads) are also adjustable. The distance between the altitude bearings is 12 meter and the height of the altitude axis above level of the axial azimuth pads is 11 meter. The diameter of the azimuth bearing track rail is 16 meter.

The fork is supported on 10 hydrostatic pads. The calculation of the static and dynamic behaviour was performed with the MSC/NASTRAN finite element program.

For the two wind-load case calculations, i.e. tube in horizontal and zenith position respectively, the assumed wind speed is 14.4 m/s and the direction of the wind is parallel to the altitude axis. The resulting forces and moments acting on the fork structure at the altitude bearings is calculated from the loading on the tube, which is 900 daN acting at a distance of 10 meter from the altitude axis. In order to assess the effect of a misalignment of the axial azimuth bearing rails, the rail surface was assumed to be set between two horizontal planes 0.2 mm apart, which means a tolerance of +/- 0.1 mm with respect to the reference plane.

In a first calculation one pad of the axial azimuth bearing was assumed to be free to move in the y-direction, while all others are in contact with the rail. The elastic deformation due to dead load of the structure on that pad was 0.6 mm, which shows that the pads will always stay in contact with the rail for a rail misalignment of +/- 0.1 mm. In a second calculation step one pad was displaced to 0.1 mm upwards to simulate the situation where a pad is located at a hump of the rail. - 108 -

All other pads are assumed to be fixed in the reference plane. The results of the calculation showed a change in the load distribution on the pads but no separaton of adjacent pads from the rail. In the dynamic analysis the first three eigenmodes of vibration have been calculated with the finite element model. The structural performance of the fork structure is summarized in the following table:

Structure:

Weight 181000 kg Azimuth bearing diameter 16 m No. of pads 10

Static performance:

Max. displacement at alt bearings dx O.OOmm dy 0.72 mm dz 0.01 mm

LOad distribution on hydrostatic pads 1 : 19000 daN (symmetrical) 2: 50000 daN 3: 26000 daN 4: 26000 daN 5: 50000 daN

Sensitivity: (displacement at altitude bearings)

Wind loading Hor. / zen. dx 0.02 / 0.00 mm dy o•01 / O. 0 4 mm dz o•0 5 / O. 32 mm

Rail misalignment dx -/+ 0.06 mm (+/- 0.1 mm) dy + 0.04/ - 0.03 mm dz -/+ 0.04 mm

Dynamic performance:

Eigenfrequencies: fl 6.20 HZ f2 6.70 HZ f3 9.45 HZ

In the following table the mass breakdown of fork and support ring structure is given:

ITEM WEIGHT [Kg 1

Fork structure (steel) 181000 Central tUbe (incl. az guide bearing) 20100 Alt bearings 13200 Support for alto bearings 11600 - 109 -

Az. hydraulic pads 8500 Alt. motor units 1000 Coude mirror supports 3000 Miscellaneous 3100

TOTAL: 241500 (+ instruments)

The NEYRTEC design, when compared to that of AMOS gives a similar but slightly inferior first eigenfrequency with a higher weight. However it uses only one track for the hydrostatic pads instead of 2. Taking account of this, the cost of construction for either solution appears comparable.

III.3.4. Support Structure

The design and the dimensions of the support structure depends on the concepts described above. The support ring structure is the interface between the moving part of the telescope, and the concrete pillar of the telescope building. Since both fork concepts have a hydrostatic axial bearing system for the azimuth axis the support ring structure has to provide the attachment devices for the bearing rails.

The support ring design in the case of the NEYRTEC fork is shown in figure 43. Details of the adjustment system are given in the following section with the description of the bearings.

In the case of the AMOS design, two concentrical circular rails with 11.2 and 18 meter diameter are required. The design of the AMOS support ring is shown in figure 44.

111.3.5. Bearings

Since the design of the bearings was not one of the main subjects of the conceptual design phase, which was concentrated on the structural design aspects, only the preliminary work to assess the provisional feasibility of a proper solution has been done. The detailed design of the bearings as well as the drives will be subject of the next phase of mechanical design.

Some remarks should be made on the accuracy requirements for the bearings. The tilt accuracy of the altitude and azimuth axis depend on the following parameters:

- mislevel - elastic deformation - wobble

Mislevel means a slanting of the axes caused by manufacturing imperfections. Those errors are reproducible and can be reduced by adjustment. - 110 -

Figure 43: Support ring for the NEYRTEC fork design.

------·------.. '.000------...., r------ltlllOO------,1111l1lO------l r----~4!l1D =-:lJ~ rjIQQIHCQ WHHb

Figure 44: Support ring for the AMOS fork design. - III -

Elastic deformations are caused by mechanical imperfections, thermal and wind loading, leading to eccentric rotary movements. Elastic deformations caused by mechanical imperfections are reproducible but not adjustable. They can be effectively eliminated by application of a mathematical pointing correction model. Thermoelastic deformations induced by temperature gradients during operation of the telescope are low frequency effects. They can be eliminated by an active tracking of the telescope providing absolute pointing errors are not greater than 1-2 arcsec. Elastic deformations due to wind loading depend on the stiffness of the bearings and must be minimized.

Wobble is the result of manufacturing inaccuracies and wear out. This high frequency kind of inaccuracies are neither reproducible nor readjustable and the resulting misalignment of the optical axis should be kept under 0.05 arcsec.

For the altitude axis it has been decided at the beginning of the conceptual design phase to investigate only the rolling contact bearing solution. TWO concepts have been proposed by different suppliers. One concept from SKF uses double-row cylindrical roller bearings and the other concept proposed by FAG is the scaled up solution from an NTT design proposal with two centered oblique ball bearings slightly pretensioned in axial direction and creating a pivot in the center of the bearings on the altitude axis on both sides of the tube.

With both solutions, which are shown in figure 45, a combined axial radial bearing is provided. The housings of the bearings are welded steel box structures which have adjusting devices at the connections to the truss structures of the fork. In general one can state that the feasiblity of a rolling contact bearing for the altitude axis is not questionable, but a hydrostatic solution for the altitude bearings is not completely out of consideration. cost and performance for both solutions will have to be determined and compared during the next phase of detailed mechanical design studies.

For the bearings of the azimuth axis it has been decided at the beginning of the conceptual design to separate the axial and radial bearings. The radial centering bearing of the azimuth axis should be located as close as possible at the center of the fork to minimize its size. It does not carry high loads but provides the accurate centering of the telescope. Either a cylinder roller bearing or a preloaded shoulder ball bearing can be used giving almost the same overall performance data. The bearing is sUbjected to wind load shear forces only. A misalignment of the rails causes virtually no rotation at the central bearing. From the dimensional standpoint, such a bearing is quite conventional.

The axial azimuth suport in the NEYRTEC fork design is provided by 10 hydrostatic pads. The forces on the various pads depend on their location in the fork structure and varies from 19000 to 50000 daN. In the presence of a rail misalignment of 0.2 mm between two parallel planes, the force variation on the pads is in the order of magnitude of +/- 15%. Wind induced force variations on the pads are also in the range of +/- 15%. To achieve a maximum bearing - 112 -

711 us s60

'60.. •to .to '" I 1 ~ \ I;; I 11­ \ ..le I o I \ I

Altitude Bearing Design (FAG)

I ----t I I

Altitude Bearing Design (SKF)

Figure 45: Altitude bearing designs (FAG and SKF). - 113 -

----_.-._-

I -~~

I

VUE A

~ COUPE CC. ClllPE BB

Bydrostatio Pad Design (FAG/AKOS)

A4Ju~T""C.NT ~lf~T~M,"'­ SOIJlIC

NOLLIN~ rlfllCI. IC'AIl.. ('D ..,l),.l"tlf,t:> SteA"'*'6 ::tV:.TlM rollr 4J,,~T

Bydrostatio Pad Design (S~F/NEYRTEC)

Axial AZimuth Bearing (NEYRTEC) Figure 46: AZimuth bearing. - 114 -

stiffness, the pads have to be fed at a constant flow. A force variation of 15% causes a film thickness variation of 2.5%. If the film thickness under normal operating condition is 100 microns, a 15% force variation in the pad would reduce the film thickness by 2.5 microns. The stiffness of a pad loaded with 40.000 daN accordingly would be around 2.4 * 10E6 daN/mm. The stiffness of the fork structure itself has been calculated to be 6 * 10E4 daN/mm. The film stiffness is 40 times higher than the structural stiffness of the fork. Consequently one can state that the dynamic behaviour of the fork structure would not be affected with respect to the first eigenfrequency, if the hydrostatic pads were included in the mathematical model of the fork structure.

In figure 46 the axial pad design together with the azimuth rail adjustment system is shown. The pads are flanged to the structure via a fine adjustment system. A swivel system, also hydrostatic, enables each pad to fit perfectly to the rail. This pad concept was developed by SKF.

The design of the hydraulic pads for the AMOS fork concept is shown in figure 46. The fork is supported on 8 pads moving on 2 concentric rail tracks. The alternative hydrostatic pad design was developed by FAG.

For the hydraulic power supply unit the following solution is proposed: a unit comprising a number of pumps for the oil supply in view of the different support reactions. The pumps will be attached to the non rotating part of the telescope structure. The oil supply to the pads will be provided by local flexible pipes and a winding reel. Oil flow controllers are located on panels at various places in the structure. The oil feed unit itself can be provided with two pumps with an automatic switching capability between the two so that one is on standby while the other operates.

111.4 Beam combination

The VLT linear array concept allows a versatile use of the unit telescopes. The individual telescopes could be used either independently or in various combination schemes. The major beam combination modes are (see figure 49):

Incoherent beam combination (Combined Coude FOCUS) coherent beam combination (Interferometry)

The incoherent combination with a combined coude focus offers the light collecting power of a 16 m equivalent single dish telescope. The cOherent combination opens long baseline interferometry with a resolution span of 0.5 mi11iarcsec in the blue to 30 milliarcsec at 20~m wavelength in case a 150 m baseline (distance between the two extreme telescopes) is selected.

The optical design of the beam combination paths is mainly determined by the desired field of view of the combined image and the distance between the single telescopes. Both should be as large as possible, a large field of view and a long baseline for high resolution imaging in the - 115 -

1/)9000

Figure 47: complete telescope structure design (MAN-AMOS concepts). - 116 -

Figure 48: complete telescope structure design (MATRA-NEYRTEC concepts)...... -.J ......

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coherent combination mode. However, the field of view and the baseline influence directly the dimension and the number of the optical elements in the combination path and therefore the cost and efficiency of the sjstem. Many technical aspects are valid for the incoherent as well as for the coherent combination modes, although the interferometric combination has much more difficult technical requirements, since very high optical stability is required. It seems to be appropriate to start the interferometric combination in the IR range and then gradually extend it towards the visible.

The optical efficiency of the combined beam modes depends strongly on the availability of high reflectivity coatings. Because of the increased number of additional combining mirrors it might be necessary to provide two or three sets of mirrors, optimized for different wavelength bands. A possible combination includes two dielectric mirrors for the visible and Uv range and one silver mirror for red and the IR. They could be mounted on a motorized turret to allow a fast change of wavelength range. In the IR the emissivity of the mirrors might be a limitation. While it might be necessary to cool the mirrors, the problem might also be overcome by improved coatings.

In any case it is of utmost importance to keep the mirrors absolutely clean. Due to the relatively large number of mirrors and their various reflection angles polarization sensitive measurements are probably excluded.

The beam path in both combination modes has to be protected against air turbulences, which as laser transmission experiments have shown (78) are critical in the horizontal sections and against other environmental effects like dust, humidity, etc. which ruin the imaging quality and accelerate the ageing of the mirrors. Experiments to study the effects of turbulence on horizontal near ground optical propagation are planned. Operating the whole combination path under vacuum is a possible solution. This would prevent all the above mentioned effects and allows cooling of the mirrors to low temperatures (e.g. with refrigerator systems, liquid nitrogen, or closed circuit cryostats). A mercury filled cylindrical tank or magnetic liquids could be used as low-friction rotatable sealing between the moving and stationary part. But a major disadvantage of a vacuum system is the need for an entrance window close to the Nasmyth focus. LOcated close to the focal plane the windows would not exceed 50 mm in diameter, a size in which all typical IR materials are available. To cover the whole spectral range a set of different windows could be mounted in a special selector to allow a fast exchange without filling and again evacuating the vacuum system. In any case protection tubes should be considered. The cost of vacuum operation or of cooling the mirrors must be studied, together with the technical problems associated with these solutions.

As shown in Fig. 49 there are a number of options for the beam paths of the two combination modes. Solutions using free standing mirrors, as well as on 3 sets of mirrors of which one will be cooled and operated under vacuum are not now being considered. Remaining realistic options are: - 119 -

Option 1: Simple insulated tube to prevent convection and one set of mirrors with silver coating (A 450 nm).

Option 2: Same as option 1 but 3 sets of mirrors (UV, blue-visible, red-IR).

Option 3: Vacuum system and one set of mirrors with silver coating.

Option 4: Same as option 3 but 3 sets of mirrors.

Option 5: Vacuum system and one set of cooled mirrors with silver coating.

The optimum combination with respect to performance and cost seems to be the option 3 or 4. With option 3, the combined focus will not be usable for UV and blue (wavelength 450 nm) observations. In that case, one could equip one of the 8 m unit telescope with a coating optimized for the blue and UV range and the three other telescopes with silver coatings. The combined focus could benefit from a very high throughput for wavelengths longer than about 450 nm.

A final decision strongly depends on the coatings available when the VLT goes into operation and more studies are necessary. ~ mentioned above an experimental analysis of horizontal beam propagation close to the ground will begin soon. The use of fiber optics is discussed later in this chapter.

111.4.1 Combined coude Focus

The beam combination in an array of individual alt-az telescopes requires a minimum of 4 to 5 additional mirrors. For long transfer distances a parallel beam is not good because the mirrors become very large. An intermediate image on a pupil relay mirror is a way to overcome this problem but this solution increases the number of mirrors. It is necessary to introduce additional mirrors to avoid extreme off-axis elements and to ensure a high pupil imaging quality which is particularly important for IR operation.

Figure 50 shows a possible optical design for the combination path. The largest optical element has a diameter of 700 mm for a distance of 75 m between the azimuth axis of the telescope and the combined focus. In this case a total unvignetted field of view of 1 arcminute is achievable. The requirement for IR chopping at the combined focus is probably the determining constraint for the minimum field of view. The sensitivity of the proposed beam combination path with respect to pointing and focusing is given in the following table. The stability and alignment requirements are not very difficult and can be easily met.

As described in chapter 1.5.3.1 the use of adaptive optics could provide a diffraction limited Coude image for wavelength greater than 4 ~m. MC2 could be a deformable mirror and MCl a tip-tilt mirror controlled by the same adaptive system. MCl corrects tilt errors and MC2 all higher aberrations up to the limitation given by the number of sUbapertures (or actuators). COMBINEDCOUDEFOCUS SPOT DIAGRAMS

A21MUTH AXIS •Y :.:~]~~:~'.:.y' = 34,5 mm I .-,6;i iji\;.-,.. (30 arcsed I

I Z MC1 ELEV~O!!.._ X-0-.. _ ! :W • AXIS i (tb=3001 y' = 24,1 mm %jii~W- I F/15 NnIIyth Focus "*'I (21 arcsed I I I I I y' = 0 IMC4• Itb=440) (CROSS 0,2 arcsec) IT + ~ \--~(~:~~O) I-' COMBINED COUDEFOCUS o'"

'---2500 __~,,'''I''';~".. Itb=700) MC7. I MC7. Itb=700) I ::::>"11 I rJ-=:::::: I( I I

'''u.~It ~ Eit.ilI ~ =---117I cl j4I MC8. (tb=140) .. r~ MC9 .- ~ == -==-- MC8, Itb=140)

Itb=500) MC7. MC7, Itb=300) 1-23068 I f-1825 MC5. Itb=460, pupil relay mIrror)

Figure 50: Optical design of the incoherent beam combination. The figure shows the combination path for telescope 0 (identical to telescope A). The only difference in the 4 combination paths is the magnification and size of the off-axis telescopes consisting of the mirrors MC? and MCa. The spot diagram shows the beam quality for the 1 arcmin field of view.

~. - ~---

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t~- ~ .. .. '; ~ ~i~ ..~_~'I : i . I :::.::=-J-HJ 1~_I-"'.. __ " .."!L, -.:-11'_) _ - -- (I .... , ,,,,.._J

Figure 51: Possible vacuum system for the combining optics (as proposed by Leybold-Heraeus, FRG). The 2 pumping stations per section are not shown. Each of these stations consists of rotary and roots pumps for a fast evacuation and rotary and turbo-molecular pumps for maintaining the pressure. - 122 -

It is foreseen that the beam combination will be equipped with built-in stabilization and alignment systems. The mirror cells will be supported by motor driven actuators. Though all mirrors will be remotely adjustable, only one will remain adjustable after the initial alignment is performed. Therefore the sole purpose of the remote adjustment is to facilitate this initial tuning of the optical system. For the mirrors being not sUbjected to gravity variations, it is hoped that the optical train will not need any real-time adjustment. Nevertheless a laser beam may be very useful for checking the alignment and if necessary could provide a continuous stabi1isation.

MIRROR TILT (arcsecs) DISPLACEMENT (microns)

a b dyn dxn dzn

dy dx dy dx dx dy dz

Ml - .50 - .50 + .0042 + .08 - .08 M2 + 3.11 + 3.11 - .0046 - .09 + .09 M3 - 7.50 - 8.85 + .64 + .13 - .64 - .13 MCl -23.8 -26.5 - .55 + .13 + .55 - .13 MC2 +15.0 +15.0 - 2.88 - .20 - .52 + .52 MC3 -20.9 -24.6 - 5.75 + .52 MC4 +36.3 +43.1 + 5.75 + 5.75 MC5 700 700 12.0 - 5.75 MC6 -345 -345 - 5.75 + 5.75 MC7 + 6.51 + 6.51 + .19 + .19 + 1.0 MC8 -32.8 -32.8 - .24 - .23

Table: The values of the table give the tilt or displacement of a mirror which result in translation of the Coude image (F/30) of 1 arcsec = 1150 ~ m (dx, dy) or a defocus by 1 mm (dz). The coordinate system is indicated in figure 50. Tilt a is around the x-axis, b around the Z-axis.

A vacuum system as proposed in option 3 is shown in figure 51 for one of the beam path. The pressure could be selected in the range of 1 to 10 mbar in order to avoid all turbulence effects or below 10-4 mbar if cooling of the mirrors is required. The major difference will be in the type of pumps for reaching the final pressure within reasonable time (few hours). In any case, after reaching· the final pressure, the system will switch to smaller pumps for maintaining the vacuum. Typical service intervals will be 5000 to 10000 hours. An alternative would consist of using Helium instead of vacuum. Although this will not be adequate for cooling the mirrors, it is a cheap solution for limiting the influence of thermal turbulences (1.4.2).

The present concept with telescopes in alt-az mount produces both image and pupil rotation. This drawback can be overcome by the use of an image de-rotator or by synchroneously rotating the instruments. - 123 -

The size of the instruments likely to be used at the combined focus will probably preclude rotating the instrument. An optical de rotator will necessarily introduce undesirable additional optical surfaces. It is likely that finally the combined focus will exclusively be used for the spectroscopy of single objects as is the case with nearly all existing Coude facilities. It is therefore questionable whether a field of view larger than, say 10 to 20 arcseconds, is effectively necessary. A reduction of the F.O.V. would increase the efficiency by reducing the number of mirrors. As mentioned previously, the need for IR chopping may require a larger field.

The concept of flexible scheduling has also a strong impact on the optical design. A fast switch-over between the Coude instruments has to be provided. In the present design mirror MC9 is foreseen to feed the combined image to the current instrument. A whole set of instruments will be stationarily arranged around MC9.

The Coude laboratory could be equipped with a special protection of the detectors against cosmic radiation. The recent progress in detector performance indicates that within the next few years the CCD arrays will have such a low noise level that cosmic radiation may determine the signal-to-noise ratio of the measurements. An increased thickness (50 to 100 cm) of the Coude laboratory ceiling made of special low radiation concrete would provide good shielding. The advantage of this global shielding is, that it need not be integrated into the instrument itself, where it would create severe weight problems.

III.4.2 Fiber optics for beam combination

The application of fiber optics for the beam combination is a very attractive approach, because of their lightness, flexibility and simplicity. This technique could save quite a number of optical elements in the beam combination train, which have additionally the requirement to be very stable. It would reduce the complexity and specifications on the mechanics and optics, and, thus, drastically reduce costs.

ESO has experience in the application of fiber optics mainly in the visible wavelength range for instrumentation (79). Here, fiber optics have proved to be extremely useful. An extrapolation of this experience towards a fiber optical beam combiner is very critical, because of the increased length of the fibers (approx. 80 meters) and the limited wavelength coverage. The application of fiber optics depends mainly on the future developments. promising achievements have been made in the development of infrared fibers (80), the prospective materials, and fabrication methods.

The application of fiber optics for the VLT is under investigation, including optical design for a complete fiber combination, and the partial use of fibers only in the moving parts of the telescopes. Figure 52 shows a possible scheme to use fiber optics between the telescope focus and a fixed collimator in the telescope base. This way the length of the fibre can be reduced to a few 10s of meter. - 124 -

TELESCOPE image of the Nasmyth R=1.0mm telescope pupil focus

.. " ,' ...

II/'(.IC ..."hng.. 3.0 mm : 8.17 mm 150/Llll Fiber

COUDE LABORATORY TELESCOPE BASE Parabolic mirror Parabolic mirror fl-'~~ 11 9 Fibers F=600 mm from F=600mm other 75m ~~scopes

""'''0'"' -l F/3 beam Figure 52: Optical train for the coude beam combination with optical fibers. For the long path between the telescope bases and the eoude laboratory a two mirror relay has been selected, in order to optimize the transmission of the combining system.

Pupil relay mirror • = 80mm vith adjustable curvature

.. / 8S00

Combined coude focus axis ---7"-~------

Interferometer axis Image 01 the pupil on the table

Colli.ator •• 620lllm Figure 53: Schematic view on the interferometric beam combination path. A key element in this design is the variable pupil relay mirror with a focal length range from 17 m to infinity. For the combination of the light from two telescopes a F/6.2 telescope with 600 mm diameter moving on an air cushion table is foreseen. - 125 -

111.4.3 Interferometry

'rhe present VLT concept allows the use of pairs of telescopes as long baseline interferometers. A non-redundant distribution (for instance with separation of 25, 75, and 50 m) would provide up to 6 baselines. The concept however does not exclude the interference of the 4 telescopes simultaneously. A realistic target for the interferometric operation is the IR range from 5 to 20 ~m and later a gradual expansion to shorter wavelengths.

The gain of the 8 m single apertures are only effective if adaptive optics (see chapter 1.5.3.1) is applied for a real-time partial or full correction of the degradation due to atmospheric turbu1ences (82)(83).

With the independently mounted telescopes, the entrance pupils are not cop1anar as the telescope points off the zenith angle. In order to guarantee phasing of the separate beams, the Lagrange invariants for the individual telescopes in the array must be equal and additionally the overall Lagrange invariant of the array must be conserved. This requires optical path length and pupil corrections. FUrther investigations have to show the tolerances allowed for the pupil corrections in case of 8 m apertures. The experience with the CERGA interferometer indicates that with moderate apertures a pupil correction seem not to be necessary. It is questionable Whether this could be extrapolated.

By setting the VLT to the interferometric mode the beams from two telescopes can be ~ombined with an equal path length in the interferometric laboratory. There are at least two options for the path length correction:

1) use of a trombone as an optical delay line in the combining beams of the individual telescopes and a stationary combining system or

2) use of a moving combining system to ensure equal path length.

The optical elements which have to be translated could be mounted on tables supported with air or magnetic bearings to ensure a smooth motion with low pitch and yaw errors, and high vertical and horizontal straightness. Linear brush1ess DC motors which are available in the necessary length would meet the specifications for a homogeneous slip stick free motion.

Pointing off the zenith angle with two telescopes results in a pupil foreshortening which decreases the synthetic pupil diameter. TO maintain the geometrical scaling of the lateral pupil geometry the pupil separation at the combining optics must be compensated.

Due to the change of the relative location of the optical elements in the system the longitudinal pupil position must also be compensated.A spherical continuously deformable mirror with a focal length range from -17 m to infinity could be used to maintain the pupils at the same position. - 126 -

The interferometric operation mode requires a vacuum system even mor~ than the incoherent beam combination does. To avoid vacuum tubes with variable length, the most reasonable solution seems to be to have the optical delay lines or the whole combining system inside the vacuum chamber. Unfortunately this will exclude the use of air bearings for the mobile parts. comparable systems have been built for large laser interferometers (30 m, Max-planck Institute, Garching) and Fourier transform spectrographs (15 m translation, France), and even larger systems are being planned.

The operation of the VLT in the interferometric mode requires a continuous optical path correction. To achieve the necessary stability the coherent beam combination path has to be actively stabilized. Techniques developed for laser beam stabilization systems could be applied. A practical implementation could use three separate servo control systems: a baseline control to maintain the approximate equality of the mean optical path length in the two arms of the interferometer while tracking the star, the adaptive optics to compensate wavefront distortions due to atmospheric turbulences, and a path length compensation system using a real-time fringe tracker which compensates large-scale atmospheric and mechanical instabilities in the optical path length to within a fraction of a wavelength. Instead of fringe tracking it is also possible to stabilize the path length with a laser ranging system, a solution that will work with even quite faint sources. The operation and performance of the interferometer depends on the ability of measuring the fringe contrast in spite of the changes in fringe phase and the presence of contrast degradation effects caused by the atmospheric and the instrumental perturbations. Measuring the squared modulus of the mutual degree of coherence of the two interfering beams (81) will give the fringe contrast

y2 = [2(AC - CC)/(AC + CC)], where AC is the number of auto correlation events on CC the number of cross correlation events measured with a photon counting detector in an amplitude interferometer. This possible technique allows a simpler detection scheme than the classic Michelson interferometer. In the original linear array concept of the VLT as described in April '84, it was proposed to achieve the coherent combination in a laboratory separated from the normal coude laboratory for mainly two reasons: i) Rapid switch-over from normal operation to the interferometric mode: ii) possibility to use mobile 2 m class auxilIary telescopes with the same set-up. This combination provides several important advantages such as a fUll-time use of the interferometer with an optimum telescope size and complementarily a continuous coverage of the spatial frequency plane. - 127 -

III.5 Building concept

III.5.1 Optimization of cost, seeing and wind load

Classical oversized domes, which used to represent about half the total cost of the eXisting telescopes, while still far from optimal from the seeing point of view, are hardly acceptable for very large telescopes. Already in most recent telescopes they have given way to light-weight box-like bUildings rotating with the telescope and largely open to the wind during observation times.

The preliminary analysis of Chapter I.5.1.2 indicates that, if the classical dome concept is abandoned, a configuration with the telescope fully exposed to the open environment during observation is in some respect more favourable than the other building arrangements chosen for the latest telescope projects.

Still the technological step from the present state of the art to designing telescope and mirror structures that are able to sustain the full wind loading should not be understated. Therefore, in order to establish an optimum trade-off between cost and performance, it appears advisable to set a tentative limit on the maximum wind velocity incident on the telescope. A wind screen could then be used to extend the operational range above this limit.

Since the likely sites for the VLT have a predominant wind direction from the north, the simplest solution is to have a semi-permeable wind screen in front of the telescopes, which then could be placed perpendicular to the main wind along an east-west line.

Then, not to lose the favourable effects of wind flow on seeing, the wind screen will be made such that it becomes effective only when the wind has a mean velocity higher than the value acceptable for stable telescope operation.

A preliminary analysis of wind conditions and their effect on the telescopes (25) led to the proposition that this value be set at 9 m/so The maximum mean velocity for telescope operation will be of the order of 18 m/s which is the present limit at La Silla for telescope operation. The wind screen will be effective when the wind velocity is between 9 and 18 m/s and will have a permeability of 50%. With the sites presently considered for the VLT winds between 9 and 18 m/sec occur about 30-32% of the night time. Almost 90% of this time the wind comes from a direction within a 60 deg sector around the predominant direction. Winds below 9 m/sec correspond to about 65% of the night time with also the same predominant direction.

A building concept, based on the previous considerations, has been developed, and the technical solutions evaluated. The validity of the concept is ultimately dependent on the site characteristics.

It is mostly conceived in view of the typical sites in Chile (La Silla or paranal, see Chapter III.8) where the operating conditions (rain, snow, freezing temperatures etc) are not extreme. A high elevation site above - 128 -

4000 m would probably require the concept to be reviewed. Also, it is not certain at this stage of the study whether a wind screen is an absolute necessity. For the time being, the wind-screen has been retained and included into the building concept.

The general arrangement is illustrated in figure 54.

It consists of the four massive telescope pillars and all the laboratories and control rooms required for the observations. A surrounding independent structure reaches the level of the telescopes so as to provide easy access to support the telescope shelters and the handling equipment, and to insulate the telescopes from the effects of the structures underneath.

Upwind, a removable wind screen provides protection for the telescope against high wind loads. For winds below 9m/sec, the wind screen is either removed and set horizontally or, if a fixed frame is used, its panels are tilted to make as little drag as possible.

Day-time protection is achieved either by inflatable domes which, when open, disappear in the support structure or, as an alternative, by movable shelters with two tilting doors hinged on the support platform which will let the air flow through when the telescopes are in operation, so that turbulence created by the presence of the shelter is minimized. A gantry crane can serve all the telescopes during the construction as well as later for maintenance.

In figure 54 the distances between telescopes is about 25-75-50 meters, a configuration found interesting for interferometry, and consequently called the 1-3-2 configuration.

111.5.2 pillars, laboratories and auxiliary buildings

General

The final choice of the telescopes configuration, which is dependent upon the optimal configuration for interferometry, will have an impact on the actual configuration of the VLT complex and also on the cost but will not affect the basic layout concept.

The bases or pillars supporting the telescope are concrete structures of cylindrical shape. The diameter, as required for supporting the telescope, is about 16 meters.

A mirror assembly for beam recombination is placed inside the base. Thus from strictly functional requirements the minimal height of the base would be about 4 meters. In fact the altitude axis of the telescope may have to be placed up to about 20 meters above the ground so that the optical performance is not affected by the higher turbulence normally present close to the ground. In this case the height of the pillar, considering the likely size of the telescope yoke (9-11 meters) should be about 10-12 meters. An analysis of ground induced seeing and near ground wind turbulence will ultimately fix this distance which could possibly be reduced. I-' N \0 OOCOO 0

Figure 54: General arrangement of the VLT building. - 130 -

Since two different types of observations (incoherent beam combination and interferometry) are foreseen with the VLT, we are led to a principle configuration which consists of the following elements (figure 55).

Telescope pillars

A circular base 16 m in diameter is required for the telescopes. Three options are envisaged for the reinforced concrete structure of the telescope pillars.

1) Standard cylindrical shell of thickness 30 cm. 2) Open frame, which lets the interior of the pillar open, with possible advantages with respect to cooling of equipment.

3) TWelve-sided cylinder made of pre-fabricated elements.

Coude laboratory

This building has internal dimensions of 20 x 20 m and is articulated in 4 spaces (10 x 10 x 5 m) around a central hole (diameter 3 m, depth 3 m) which, like the floor, is independent of the main structure. corridors allow one to reach the 4 instrument rooms. Thermal control and stable conditions are achieved by a cooled floor. In the present solution, the main walls, slabs and beams are pre-fabricated elements in reinforced concrete. AS an option the structure may be reinforced so that 1 m thick concrete blocks can be placed on the roof to shield instruments from cosmic rays. control room

The control room is located beside the coude lab, either along the array or transversally passing above the interferometric lab. Surface is approximately 200 m2•

In the present solution, the main walls, slabs and beams are pre-fabricated elements in reinforced concrete. Access is possible from the Coude lab catwalks by staircase and elevator.

TWo 100 m2 storage and service rooms are situated at the same level as the control room. Another storage room is situated underneath at the reference level.

Interferometric lab

The interferometric lab consists of a long building situated parallel to the array. Its actual length will depend on the total distance between the four telescopes. For the 1-3-2 configuration it is about 185 meters.

Main walls, slabs and beams are pre-fabricated elements in reinforced concrete with good insulating qualities.

The ground slab consists of three parts: two working spaces (respectively 2 and 3 meters wide) separated by a ground slab laying on cork aggregate for optimal damping receiving the interferometric table. A cooled floor will ensure thermal control and a stable environment. - 131 -

U!QUT 0' MClLItI! "'lI!!

Figure 55: Telescope bases, OOude laboratory and control room. PATH OF TRAVELLING

{ Z

'I t:::: ~[ I ~Ir; ~~~~~!H~T i~~ II ,:J\I I

WATER EVACUATION

I-' W 12 12 10 10 8 l\.J

.-~

Figure 56: Mirror maintenance building. - 133 -

Mirror maintenance and service building

A bUilding for mirror maintenance will be part of the VLT complex, being, however, relatively independent from the other elements its location will mainly depend on the ground configuration. The useful dimensions of this building are 52 x 14 x 6 m and it is divided into 4 sections: storage, cleaning, coating, services. It is foreseen that the same building may shelter several telescope facilities such as pumps, air conditioning, compressors etc.

In the solution presently retained, main walls, slabs and beams are pre-fabricated elements in reinforced concrete.

A travelling crane (SWL 25 tons) will allow the handling and the transport of the 8-meter mirrors from the telescope to the coating section. The mirrors will be in horizontal position, face upwards in all phases.

It is presently envisaged to coat the VLT primary mirrors with the sputtering process which requires a relatively low aluminisation tank. As noted above, other techniques may be possible if metal mirrors are used.

The general sequence of operations will be as follows:

1. The mirror, still with its cell, is placed in the storage room. 2. Separation of the mirror from the cell; the cell stays in the storage room. 3. The mirror is carried in the adjacent cleaning room and laid down on a fixed support. 4. cleaning of the mirror from a cleaning module, moving above the mirror, which includes cleaning products, the required equipment (pumps) and space for one or two operators and is attached to the crane. 5. The mirror is raised and then placed in the bottom of the coating tank, which can move on rails under the mirror. 6. coating of the mirror. 7. Opening of the tank and transport of the mirror to the storage room for reintegration into the cell.

IIl.5.3 Telescope enclosure and associated platform

A service platform around the telescope gives a solution to various requirements: ease of access to the telescopes, support for a gantry crane, support and closing floor for the movable shelters, separation and insulation from all thermal sources in the laboratories underneath. This platform is supported by columns only, letting part of the wind flow pass underneath.

1) The air layers nearer to the ground which have the largest turbulence values and the largest velocity and temperature gradients, are captured under the platform. - 134 -

2) The presence of the platform shields the atmospheric environment of the telescopes from all thermal and aerodynamic effects of the ground and other buildings in the vicinity. These effects are potentially damaging for the seeing and often difficult to clearly identify and control.

The corollary is that aerodynamic and thermal considerations are themselves important in the platform design, as this is (with the wind screen, in case of strong winds) the only structure which may affect the wind flow on the telescope. Wind tunnel tests will be required to analysp. these effects.

With respect to thermal considerations, the main requirements are a low thermal constant for the surface and, in thermal equilibrium conditions, minimal temperature differences with ambient air both during the day and during the night.

From the structural point of view, the actual definition of the platform is dictated by the support requirements for the telescope shelters and the gantry crane.

Movable roll-off shelters are a convenient solution to protect the telescopes during day and down-time and from precipitations.

In order to minimize the cost of these structures, each telescope should be sheltered with the tube in horizontal position. If this were not acceptable, for telescope maintenance requirements for instance, larger shelters are envisaged, with an internal volume sufficient for telescope movement. These larger shelters are, however, heavier and more expensive because of the larger wind loading. They would also require a heavier support structure. Therefore a careful trade-off will be required to verify whether the advantages of a larger volume pay for the additional cost. A faster primary mirror would allow the telescope to be moved inside the shelter without increasing its size prohibitively. (F/1.5 to F/1.? would be adequate).

AS a low-cost and light-weight alternative to movable shelters, inflatable domes are proposed. Such a dome may be composed of a double wall fabric hemisphere supported by rigid hoops that open and close in two symmetrical parts.

Inflatable shelters would be cheaper than the movable ones. AS they are also much lighter (45 - 60 tons against 200-400) and they are storp.d around the telescope when in open position savings can also be made in the supporting structure. Although the design of the inflatable shelters can profit from the considerable experience that exists already with large antenna facilities, which have similar requirements, it must be noted that their application to telescopes, where frequent openings are required, is a novel concept which will require some further study and tests before they can be confidently proposed as the shelter of choice for the VLT. - 135 -

111.5.3.1 Thermal aspects

In order to preserve optimal seeing conditions the temperature of the structures surrounding the telescope must be kept as close as possible to that of the ambient air. To estimate the problem let us look at the orders of magnitude of the different components affecting the heat balance of exposed surfaces.

A first term is the convective heat transfer with ambient air. Its rate depends on wind velocity and surface shape: see figure 57. TO fix ideas let us assume a flat surface, a wind of 5 m/s and that the delta T with air is effectively kept within 2 K: the maximum heat transfer is of the order of 50 w/m 2•

If the surface is exposed to daylight it will receive a solar flux averaging during peak hours 800-1000 w/m 2 times the solar absorptivity.

The surface will then exchange heat in radiative mode with space and the atmosphere in the amount:

where e is the surface emissivity, aT4 is 390 w/m 2 at 15°C and (aeA TA~) should be about 70 w/m 2 at low humidity sites like La Silla. Thus during the night an exposed surface will lose about 320.ew/m2•

This trivial evaluation of orders of magnitude suggests a strategy for thermal control in as much as the net radiative heat flux should be minimized by selecting suitable coatings, leaving the convective flow for the -fine tuning- of thermal equilibrium. In this way the net heat f1uxes into the structures will be minimized and the analysis made simpler and more reliable.

For this however, one must distinguish what happens during the day and night.

During the day, one should limit the temperature increase of the exposed surface with a coating of low solar absorptivity and high emissivity such as a white paint (a = 0.25, e = 0.9). The incoming solar heat will be nearly totally re-irradiated. In fact the net radiative heat flux [qs ­ ea (T~ - eA TA-)] will be close to zero for a /e = 0.3. During the night, however, this white surface will still radiate 290 W/m2 and may cool as much as 11 K with respect to ambient air (with a 5 m/s wind). Thus the need to remove the telescope shelter well out of view, to avoid bubbles of colder air being carried by the wind in the telescope line of sight.

During the night, there is no solar input and minimising the radiative transfer simply requires the lowest emissivity possible. Unfortunately, low emissivity surfaces have all absorptivity/emissivity ratio much greater than 0.3 and will therefore tend to heat up during the day. The consequences are not necessarily bad, for instance if the surface is even - 136 -

Heat transfer coefficient

160

140

120

100 • O.16m 80

60

40 plane surface Wind 20 velocity V 0 .. 0 5 10 15 20 25 [m/se~

Figure 57: convective heat transfer. Note the dependence upon wind sp~~d. EVen low wind velocities greatly increase the heat transfer from free convection. - 137 -

moderately insulated it should lose its excess temperature rapidly after sunset but if significant conductive fluxes start to appear in the structures the conditions become more complex and more difficult to evaluate reliably.

Let us consider the suggested inflatable enclosure of figure 58. During the day the dome will be closed: thus its external surface must be coated white so that its temperature will not exceed that of ambient air. The internal surface, however, may be aluminised Mylar or something equivalent with very low emissivity (0.02-0.04) so that it does not radiate to the inside. The telescope structure should be wherever possible treated for a low emissivity (either a paint such as aluminised silicon: e = 0.20 or Ebanol treatment: e = 0.10). If it is made of tubular members of small diameter (day 15 cm) with a heat transfer coefficient of about 55 W/m2/K with a 5 m/s wind the night cooling will not exceed 1.2 K for e = 0.20 and 0.6 K for e = 0.10 with respect to ambient air. The service floor inside the shelter will be likewise treated for low emissivity. In this way, during the day, almost no heat is transmitted by radiation between the shelter and the inside structures and the only source of heat to the telescope may come from the inside which can be effectively controlled.

For the platform outside the shelter the best compromise would be an aluminised resin paint (a = 0.27, e = 0.20). During the day heating would be limited to 3°K above air temperature with the same assumptions. To further decrease the risk of affecting the telescope seeing, one would place the platform level slightly below the main mirror so that this one is out of the local boundary layer, which has a thickness of the order of a few tens of centimeters.

III.5.3.2 Telescope shelters

The feasibility study for the movable shelters was performed by the firm DANALITH, (84), for the inflatable shelters by SODETEG, (85).

Movable roll-off shelters

These shelters are made of a movable steel structure with a rectangular plan view. The support structure consists of transverse frames constructed as lattice work. LOngitudinally the frames are interconnected by bracing. All the frames are supported on wheels and there must be motors on all wheels to ensure a sufficient safety margin at high wind loads. The maximum velocity will be about 0.28 m/s so that the shelters can be moved to or from the telescopes in less than 3 minutes.

The roof and wall cladding are either a sandwich construction assembled on the site or factory assembled composite units.

Each shelter has two large doors hinged on the platform at both front and back ends.

In closed position the doors are set vertically and seal the telescope inside, also effectively locking the shelter during excessive wind loading. During observation the doors lay on the platform and the shelter is removed with both ends open so that the wind flow can pass through as undisturbed as possible. TelesCODe Textile inflatable roofin Gantry crane Gantry crane runway

-~- I-' LV --- 00

r ~~ ./

" C<>r'crete Tower Service platform

Figure 58: principle of an inflatable enclosure. - 139 -

Here follows the main dimensions and data (the small shelter is assumed), see also figure 59.

Length 26 m Width 28 m external, 24 m free internal Height 17.5 m external, 15.5 free internal Main frames 6, for 12 wheels diameter 0.4 m Roof shape arch, radius: 19 m Access doors 2, tilting, hinged on the platform Weight 168 tons, without doors Door weight 17 tons per unit Power required peak 70 KW, can be reduced if a slower movement (1 shelter) is accepted

Inflatable shelters

Each inflatable shelter consists of a double wall fabric hemisphere supported by rigid hoops that open and close in two symmetrical parts.

Three types of hoop are used for supporting the fabric, figure 60:

TWo main hoops that take up most of the loading, especially during maneuvering. Four secondary hoops that maintain the overall shelter shape when there is no internal pressure. TWO auxiliary hoops that provide two intermediate support points for the other hoops guiding them and therefore decrease the loading during opening and closing. These auxiliary hoops are themselves hinged and stored on the platform when the shelter is open.

The internal pressure must vary according to the wind speed, the fundamental principle being that the internal pressure compensates for the average wind pressure. Figure 61 shows the pressure distribution for several load cases. Best average conditions are obtained when the internal pressure is equal or slightly superior to the average wind pressure. It varies in practice between 3 and 30 mbar.

Each of the two sections of the double-wall cover consists of nine inflatable ribs of lenticular cross-section (fig. 59).

This double-wall solution gives a high reliability to the shelter: if the main blower fails with an ensuing drop in internal pressure, the inflated ribs keep the fabric tensioned. If the external fabric is torn or leaks, the internal fabric is forced against the external one and vice versa, thereby keeping the external shape, which is very important to resist high wind loadings.

The hoop hinges will be set close to the platform level, so that when open and folded, the shelter is completely stowable within the platform, thereby minimising aerodynamic perturbations on the telescope. - 140 -

~Il ~ - . ~ (I) .lJ ...-4 (I) ..c: (/j \l.I \l.I 9 O'l c: •..4 ...-4 ...-4 0 p:: I ~~ I 1 I I .. j 0\ I III (I) ~ ::;j O'l •..4 lZ.o 1 I F - 141 -

Figure 60: support structure for the inflatable shelters.

:i -0,27 I unit I

Pi er- O•GS ,unit I

3 elq •• I unit l I

Figure 61: pressure distributions around a half spherical structure: the significant factor is the ratio pi/q of the internal pressure to the wind dynamic pressure. - 142 -

\ \ \ \ '~, \ ~'

______-.------jr-,..j . -~------.-

Figure 62 principle of the double-wall cover for the inflatable shelters. - 143 -

It must be noted also that, as the hoop axes are necessarily slightly shifted with respect to one another and in order to ensure an optimum stowage of the cover, the plan view space required by the stowed shelter is slightly elliptic, with the hoop hinges on the shorter axis. From the point of view of wind loading during opening/closing operations, it should be more convenient to set the hoop axis normally to the main wind direction. In case of strong wind the upwind half can be raised first, protecting then the operation of the second half.

Main dimensions and data:

Diameter 30 m Height 15 m Space required ellipse area with axes approx. 30 and 36 m, small shelter Power required peak 70 KW, average 20 KW, (blowers, lightening, miscellaneous) plus eventual air conditioning 30 KW Weight 45-60 tons

III.5.3.3 Service Platform

TWO alternative solutions for the service platform around the telescopes are presented here. The first one was conceived by the firm DELTA MARINE, (86) for the configuration with movable shelters. The second one, by the firm SODETEG, is designed more specifically for the inflatable type of shelter.

For the movable shelters, the presently preferred construction consists of a framework of steel beams (I profiles up to 1 meter high) supported by cylindrical 1 m diameter columns also in steel. The main set of beams is specifically designed to carry the load of the movable shelter and the gantry crane respectively, while a number of secondary beams carry the load of the tilting doors and provide support for the slab elements for service areas around the telescope.

According to the preliminary design about 740 tons of steel beams are required. The quantity of steel required for columns depends on the platform height: for 15 meters, this would require 260 tons.

Several alternatives have been investigated for the slab elements and the present choice is for steel profile sheeting which can carry a load of about 200 Kg/m2 with a maximum span of the order of 6-7 meters. Service paths for heavier loads can be arranged along the beams.

For supporting the lighter inflatable shelters a lattice steel structure is proposed both for beams and columns. As covering slab for service areas, corrugated steel sheeting or grating can be suitable.

A schematic plan view for one unit is shown in figure 63 below. The inflatable shelter is stowed in a recess formed by a peripheral structure all around the platform central hole and a floor, supported by short cantilever beams towards the inside of the hole. This floor is designed to be easily air-tightened. •en lE' '" i~ 0" -~ ." CO" ." CO" ~ t I - _._- I

.. ... ~---- • ----I I-' ~ ~

ptan view at top chord of main trusses Plan view ~at. bottom chord fI lAin trusses

GENERALPLAN VIEWSFOR 1 UNIT

Figure 63: Schematic plan-view for I unit of the lightweight platform for inflatable shelters. - 145 -

It may also be noted that the lattice structure will also be more favourable from the thermal point of view than the beam and pillars one, which is, however, cheaper.

111.5.4 Wind screen

A semi-permeable wind screen is included in the present proposal of the VLT facility.

Available experimental data indicate that the best way to halve the mean wind velocity is with a screen having approximately 50% permeability placed upwind from the telescope at a distance at least 1.5 times its height and slightly higher (by 20-25%) than the telescope to be protected. In the VLT case this means a structure rising about 24 meters from the level of the platform surrounding the telescopes and placed about 36 meters to the north of them.

The screen elements are supported by columns spaced about 20 meters.

TWO concepts have been investigated in a feasibility study performed by the firm NEYRPIC, (88):

1) A fixed frame with louvers so that a variable permeability can be obtained.

2) A series of removable screens.

This second solution, which is presently the preferred one, is obtained by fixed elements that are set in position or removed horizontally on the platform level, according to the principle shown in figure 64.

The screen permeability cannot be varied but this solution has the advantages that low winds are left undisturbed, since the screen can be removed. Also it is not exposed to storms, hence saving structural weight and cost. Each screen element is made of a frame which integrates panels made of 1 m wide plates set on 2 m centers.

Numerical simulation of air flow through the VLT arrangement were performed at EPFL, Lausanne, by means of a fluid flow finite element model (89). The model, in two dimensions, represented a cross-section of the VLT enclosure in open position with the service platform, the wind screen and the movable shelters in removed position. Different configurations were analysed, and the main parameters were adjusted.

The objectives of the study were to compute the profiles of air velocity, turbulence and temperature in the telescope region. A typical result is shown in figure 65. The finite element model, in two dimensions, represents a cross-section of the VLT enclosure in open position, with the service platform. Different configurations were analysed, with different main geometric parameters.

In this analysis, a wind screen rising 24 meters from the platform level, itself 12 meters above ground level, and placed 36 meters in front of the telescopes was considered. Note the -quiet- region behind the screen, which could have a favourable effect on the wind load of the primary mirror. - 146 -

UPRIGHT SCREEN / OBLIQUE GUIDE WAY I \ I

HORIZONTAL GUIDE WAY

TELESCOPE PLATfORH

Figure 64: principle of a removable wind-screen.

WIND VECTORS

------i-

------~- - ---_....._------+-- ---=~_

-______~;;r.....=,_------\.-_ - §- - .­------::.:-~- - -

Figure 65: Wind flow patterns on the VLT, computed by a 2-d finite element flow model. - 147 -

Figure 66a presents the profiles of mean horizontal wind velocity in the telescope region, computed respectively for the cases of a free mountain ridge, a platform only and a platform with 50% permeability windscreen. The reduction of velocity behind the screen computed is only about 1/3 but it should be considered that a 2-dimensional model tends to underevaluate the screen efficiency.

Figure 66b shows similar profiles for the turbulent velocity. This quantity is related to the gradient of horizontal mean velocity. Therefore an optimum design of the wind screen will try to achieve a minimum velocity gradient in the upper region of the telescope which is the most sensitive to wind turbulence.

These simulations showed that a movable shelter located downwind from the telescope does not affect significantly the flow conditions near the telescope. The height of the service platform appears to have some effect on wind screen efficiency: the higher the platform, the more effective the wind screen is in reducing wind velocities and also the turbulence is reduced in most of the region near the telescope.

The results of the model also showed a strong dependency on the slope of the input velocity profile and on the amplitude of the vertical velocity component. It will be then important to get measurements of these parameters on sites before drawing more definite conclusions from the finite element model.

111.5.5 Handling equipment

A gantry crane is foreseen for the installation and maintenance of the telescopes.

Its main requirements are:

a) Safe working load 40 tons. This in the hypothesis that assembling of the telescope will take place on the site.

b) Span 32 m. However, if inflatable shelters are assumed the span must be increased because of the stowage space for the hoops and the folded cover.

c) Clearance under hook 3 m above top of telescope.

After completion of the telescope the main purpose of the crane will be the handling of the primary mirrors for maintenance operations. The concept presently envisaged foresees removal of the mirror and its cell with the telescope at zenith position. The mirror would be lowered to a guided carriage inserted in the fork. Then it can be picked up by the crane and transfered in front of the maintenance bUilding where another carriage will move it inside. It is also possible to move the mirror onto a railway facility. - 148 -

MEAN VELOCITY 130,....------, A TOP OF MOUNTAIN (without any structureI + PLATFORM ONLY 90 • WIND SCREEN AND PLATFORM

j; 50 N a .IIII " 10 ~ I j o = 1-- """'-0=-:....-=-== -=-=-:-/-....- -- --_ platform level ground level

o 8 16 24 32 U (M/SI

profil.. of horizonhl wind .elocity computed with a f,nill element fluid model.

N.B. the tel..cope will be found between Z.O and Z.20 H

TURBULENT VELOCITY

80,....------__ o MOUNTAIN A PLATFORM + WIND-SCREEN

60

b j; 40 N

20

0 platform level

ground level

-20 0 2 4 6 8 10

TURBULENT VELOCITY (M/S)

Figure 66a: Profiles of mean horizontal wind velocity in the telescope region, computed respectively for the cases of a free mountain ridge, a platform only and a platform associated to a 50\ permeability wind screen.

Figure 66b: Similar profiles for the turbulent velocity. This quantity is related to the gradient of horizontal mean velocity. An optimum design of the wind screen will try to achieve a minimum velocity gradient in the upper region of the telescope which is the most sensitive to wind turbulence. - 149 -

For transporting personnel and small loads up to the telescope and Nasmyth platforms level, a number of lifts will be installed near each telescope.

111.6 control system

Already eXisting telescopes are technically rather complex and technical down times including instrument exchange in the order of 5% are rather co,nmon. The complexity of the VLT can be estimated to be at least 5 times higher (4 complete telescopes plus beam combination) than existing modern telescopes. Therefore special care must be taken to improve the reliability of the control system.

Experience has shown that most of the technical problems at the telescopes are with the cabling and the electrical connections, especially after an instrument exchange. Faulty electronic components are rather rare.

Therefore two main ways of increasing the reliability will be the reduction of cabling and the reduction of instrument exchanges.

TO reduce the cabling, the dedicated electronics (drive electronics and microprocessors) will be incorporated locally into the electromechanical units. (See figure 67).

The number of instrument exchanges can be reduced by leaving all instruments always connected to the computer and powerline. By switching mirrors the light beam can be sent to the requested instrument.

111.6.1 Distributed intelligence

The microprocessors are distributed in such a way that the cabling between motors, tachos, encoders and their dedicated electronics is minimized in length and in the number of interconnections. Control electronics will mostly be incorporated into the electro-mechanical units. The only interconnections which are then needed will be one cable for the data transmission and one for the power supply.

Incorporating the electronics into the electro-mechanical units has also the advantage that after dismounting they can easily be tested on a bench.

The data transmission cable will be a LOcal Area Network (LAN) connection which starts at the telescope computer and connects one microprocessor after the other. The type of LAN will probably be an Ethernet which is an international standardized communication network between various computer systems. The microprocessors and I'flCRO- PROCESSORIf! WfE~

.01 • ON/OFF OR/Ves

. 1 __ - r---- I ------, ---- ~ ACTIVE OPTICS I ACTIve OPT/CS-j

: --~DR;:eS OR/vts r-- 'I I I: t I I .. Am:~·...... J·• TIVC SUPPORT CABU LOOP 7D fuel CAlIU LOOP TOTUfIC ------,

. L_I

ADAPTER ADAPTER DRives DRIVes

ALTITUDe ALTITUDE VI DRIVe ~

AZI1fUTH I I t; ~ ~ AZ"urH DRIVf t i!Ot I DRive I ~ ... r------~ ------, I I I I I I I I ..... I I I I I U1 I I I I TCLCSCDPC TeLCSCDPf o I I I I I I CONTROL CONTROl I I I I I I C()IfPfITER I I COHPUTfR I I L J :!S~E!~ I NASlfYTH INSTRI#fCNT I No. , No. , L ~A~'fYJ'!

.~~+ '",,~'­

\,\~1-• ~o

~ ~::,:t':£':~S::; RCHOTf CDNrJlOl LINK

METCCRJLOG1CALDA TA DISCS CENTRAL

COMPUTER HAG. TAPES

(""INEO FOCUS /NSTRUHENT

USER {~ IHA(j{ STATUS If'KJ((SS/NG OISPUY· D/SPUY Cf7tlPfANOS Figure 67: Diagram of control system. - 151 -

the main part of the electronics will be housed in VME chassis. VME is an international standardized and commercially available system for industrial process control. The telescope computer and the distributed microprocessors will work on the basis of a master-slave principle. Normally microprocessors will not communicate directly with each other. Each microprocessor will pass information first to the telescope computer which will process the data and redistribute it as needed.

Another advantage of the principle of distributed microprocessors is that all real time problems like position control will be handled by the microprocessors. This keeps the telescope computers free for fast data acquisition and image processing. The principle of distributed electronics and the LOcal Area Network connection will be tested already at the NTT.

111.2.6.2 Main Servos

The precalculated maximum windtorques for the VLT will be 14 times higher than at the NTT. Due to the size of the telescopes the reduction factor in a one stage spur gear can be 2 to 3 times higher than at the NTT. TOrque motors with 5 or 7 times the torque of the NTT motors are available. The main part of costs for the drives will not be the price of the motors but the mechanical construction to cool the motor coils efficiently to avoid warm spots on the telescope. A solution comparable to that of the NTT drives should be considered.

AS the telescope will be exposed to nearly full wind load special care has to be taken to push the resonance frequencies (locked rotor and free rotor) and the eigenfrequencies of the structure as high as possible to improve the tracking performance under wind load.

AS the diameter of the light beam through the altitude axis is much bigger than for the NTT a different solution for the encoding has to be used. For the VLT it is therefore proposed to use incremental strip encoders for the positioning of the telescopes combined with friction roller coupled incremental encoders for the tracking.

111.6.3 Image analysis for autoguiding, active optics and adaptive optics

In principle it should be possible to use one single intensified CCD detector to derive all the necessary information for autoguiding, active optics and adaptive optics. But the differences in resolution, required sensitivity and real time performance may require two different guide probes and two detectors. The required real time performance of the adaptive optics will probably be the first case where a special hardware connection between the CCD detector microprocessor and the processor for the control of the adaptive optics will be needed (thus not following the master-slave principle, see diagram). The adaptive optics is controlled by each telescope computer while the optical beam combination is controlled by the central computer. - 152 -

111.6.4 Instrument control and data acquisition

The control of the instrument (that means the control of the electromechanical units) will be done via the same LAN as for telescope control. As the data acquisition of the detectors always needs very high data transmission rates, a special parallel link to the telescope computer or to the central computer in case of beam combination will be needed.

The proposed control system will allow optical as well as electronic beam combination.

111.6.5 User's end and remote control

In a certain sense the control of telescopes and instruments from the central control room is already a type of remote control as the user will get all the information via the central computer and will not have any visual contact with the telescopes during observation. In this context Wremote control Wmeans that the observer will stay in Europe and that he should have the same observing facilities as on the mountain. He will have a duplicate of the control computer and its peripherals. Such a computer-to-computer communication helps in addition to minimize the data transfer. Technically the remote control will not create enormous difficulties. The main problem at the moment is to get for the remote link a reasonable bandwidth for a reasonable price. The multiplication of elements at the VLT compared with a conventional telescope implies also improvements at the user end. Therefore a complete image display system with maximum graphic possibilities and colour screen to enhance any sort of visual presentation will be used in a similar way as on large-size process control systems.

By the time the VLT goes into operation, wide bandwidth channels will certainly be available at quite affordable costs. It is not certain however whether a complete remote control as described above, i.e. the remote observer performs the same task he would normally do at the telescope, will still be the most efficient observing mode in the future.

If flexible scheduling is largely applied, the responsibility for selecting a program will in practice depend on a local authoritj which should preferably be an experienced astronomer. The observing will then be almost entirely carried out by the local operators with little, if any, intervention from the original researcher. In this case the requirements for the link are limited to the final data transfer. The remote wobserverw will not necessarily have to travel to a central European facility but will communicate and receive his data through a local terminal at his home institute.

Indeed special instruments and debugging of new ones will require astronomers and engineers to work at the site in a classical way. This mode of operation should not represent a very large fraction of the observing time, however. - 153 -

111.7 Site requirement and site testing

111.7.1 Particularities of an ideal VLT site

The qualities required for astronomical observatories have been often described and the ideal site is seen as an isolated summit standing well above the inversion layer with relatively low wind velocity and diurnal temperature variations. It should be far from any light pollution source but close to a major town. The atmosphere should have a low water vapour content and the flow above the ground layer should be as laminar as possible; high altitude clouds (cirrus) would appear only occasionally, etc.

The present VLT concept first of all requires a summit large enough to layout a 150 m or longer baseline. The interferometric beam combination mode needs a roughly East-West orientation to achieve a satisfactory image reconstruction but a - possibly more limited ­ North-South baseline is also necessary. This North-South baseline may be used either by one of the large telescopes made mobile or by a smaller auxiliary telescope.

The building is designed not to introduce any major additional thermal turbulence. Yet it does not have to filter adequately transverse wind effects because most sites in Chile encounter strong winds (faster than 10 m/s) from North and only occasionally from the South.

From the optical point of view, image motion and scintillation are very low because of aperture filtering. The dominant seeing effect is the image spread or speckle structure for short exposures. Among the various atmospheric turbulence models, the -lucky observer­ model of Barletti et al (90) combines the best conditions ever experimentally measured. It allows at best nearly diffraction limited images at 10 microns, which corresponds to more than 3000 elementary speckles per image in the visible. This can be considered as the ultimate limit.

That is why in order to observe under such conditions on a routine basis, one of the new features of the VLT is the introduction of adaptive optics to correct real time phase errors due to atmosphere.

The performance of such a system is strongly dependent on amplitude disturbances and anisoplanicity which are entirely produced by turbulence occurring at high altitude. Thus, regarding adaptive optics, the quality of a site depends more on what happens above the atmospheric boundary layer than under it.

Moreover, the elected site has to be investigated thoroughly so as to make predictions reliable and flexible scheduling possible, i.e. to be able to choose each night the instrument and the program adapted to atmospheric conditions rather than relying on the chance to get a good seeing during the few nights a year allowed to an astronomer. - 154 -

The efficiency of the VLT will of course depend on its technical performance but also on its adaptation to the site for long term (buildings, life conditions), middle term (flexible scheduling), and short term (adaptive optics) considerations. Added to standard parameters comparison (cloudiness, water vapour, light pollution••• ), this will draw the guidelines of the cost benefit analysis between a possible new site and an existing one (La Silla).

111.7.2 The eXisting site: La Silla

The ESO observatory of La Silla and two other US observatories have been located twenty years ago at around 30 degrees south latitude at the southern end of one of most deserted areas in the world. The whole area up to 20 degrees South benefits from the stable sUbtropical anticyclone of the south-east pacific as well as from its cold surface waters which produce a well defined inversion layer preventing humidity to raise above 800 m altitude inland. Annual rainfall does not exceed 50 mm and diurnal temperature variation on the 2400 m summit of La Silla is low (average 5°C, peak = 20°C, but only 2 degrees centigrade during photometric nights). The wind velocity at 20 m height is fairly low (yearly average = 4m/s) but some snow storms may occasionally occur (45m/s peak reported by B.E. Westerlund in July 1971). The winds are mainly from North, North-East or South, and high winds come only from N-NE at night (figure 68). Statistical distribution of wind velocity shows that 90% of the wind velocity at night is lower than 10 m/sec. (figure 69).

Urban sky glow is very low though some astronomers have noticed sodium lines from the 100.000 inhabitants La Serena-coquimbo area (100 km South-West of La Silla) on some of their recent spectra.

Over 60 percent of nights can be considered as photometric and the water vapor content is one of the lowest among eXisting observatories.

The high atmosphere is perturbed by a strong W-E jet stream whose core (80 to 100 m/s winds) oscillates between 25 and 35 degrees south. Seeing on large telescopes has so far often been limited by the thermal conditions inside the domes. The recent installation of the 2.2 m telescope which benefits from an improved building design, as well as improvements carried out inside the 3.6 meter telescope building show that sub arcsec seeing is not exceptional at La silla. Reliable statistics would have to be obtained however with an independent seeing monitor (111.7.4).

The installation of the present VLT concept would be possible either on the existing site or on a close summit, which could provide a platform about 300 x 80 meters. The orientation of the ridge is roughly East-West, like La Silla. Installation of the VLT on this summit would not be costly and would not perturb the operation of the eXisting telescopes.

111.7.3 possible other sites

Future telescopes will largely be used in the IR. consequently water vapour content of the atmosphere becomes an important criterion for - 155 -

'rERR 1985

TABLE , HD3B5SlL o o i I I i I I 1 o 1 ",

o o o o

o - 0 ID <..> • - U1 Z x

o o o o

o o o U1 1 -10.00 -5.00 0.00 5.00 YNIGHT

Figure 68: Bi-dimensional histogram of night wind direction (percentage of time per angular sector). The shaded area corresponds to a wind velocity greater than 10 m/sec.

I~,2B55IL o TABLE o o o ... 0 ,.."'­

o o o ....U1

.... I <">0 -0 ZO -l • ::>0 :l:U1 :::l u

o o o U1 N

o o o / o 0.00 5.00 10.00 15.00 SPEED AVERAGE MI5 Figure 69: Wind velocity distribution at La Silla in 1985. The integrated percentage of winds above 10 m/sec was about 10%. - 156 -

site selection. As it was suspected that North Chile would offer sites with water content even lower than at La Silla, the area between 25°S and 2l o s latitude has been examined since 1983 both near the coast (2500-3000 m) and in the Andes (4500-6999 m). AS it was necessary to get regular measurements over a long period of time, a 2700 m summit named Cerro Paranal (24°40'S, 70 0 25'W, 12 km from the coast) has been monitored permanently since September 1983 (91). Preliminary results show low integrated water vapour as well as an increase of the number of photometric nights when compared to La Silla.

The nearest major city, Antofagasta (250.000 inhabitants) 150 km away by road, lies in the shadow of other mountains which protect the site from direct urban light. The summit orientation with respect to main wind is similar to La Silla, but its size precludes any further extension and would hardly accept a baseline longer than 150 m. A nearby summit named Cerro Armazoni (3064 m, 25km from coast) would probably be equally graded.

The high atmosphere is less turbulent for these Northern sites than over La Silla. The Jet Stream average velocity is reduced by 30% during Summer but comparable for the remaining 2/3 of the year.

The study of the statistics of the wind distribution over the Southern Hemisphere indicates that the situation improves when moving further North and latitudes between 15 0 and 20 0 South show a 50% reduction of wind velocity at 200 mbars. This would suggest better conditions exist near the border area between Chile and Peru or, in other longitudes, for the island of La Reunion. However recent detailed analysis of moisture data from the Tiros-N/NOAA satellites confirms the expected increase of water vapour towards the equator. The analysis of the 700-300 mbars (H 3000 m) integrated content during the months of May to August 1979 gives the following averages for the driest period of the year (D.L. Cadet 83-85):

15 0 South - 75 0 West, (coastal southern Peru): 9.2 mm H20 20 0 South - 70 0 West, (1quique area, Chile): 5.5 mm H20

This may be compared to an approximate average of 2.5 mm H20 measured from May to August 1984 above Cerro Paranal. For still southerly latitudes, the decrease of moisture in the upper atmosphere is compensated in Chile by an increase at ground level as the climate becomes less desert. The optimum for 3000 m summits seems to lie in the Paranal area and a site in the 1quique area would have to approach 4000 m to match it.

111.7.4 present investigations

Data collection on sites from various data bases (world charts, satellites, radiosondes) will continue. Trimestrial meteorological reports are issued on a regular basis for summits equipped with automatic meteorological stations. - 157 -

Figure 70: ESO seeing monitor. The telescope has an aperture of 350 mm and is designed to work in the open air. A CCD measures the differential image motion between 2 apertures located on the telescope pupils.

Figure 71: Acoustic echo sounder (SODAR) in operation at La Silla. In the background is a 30 m high meteorologic tower. - 158 -

...... ~.5~Om: ...... ' "" ' ,. • i"'"

I' '..... " I' .. '. ~

Figure 72: SODAR fac-simile. The degree of turbulence is proportional to the intensity of the trace. This example shows the decrease of local turbulence at sunset.

At the same time, dedicated instrumentation is being developed and tested at La Silla:

Microthermal sensors (platinum wires) are used with a microcomputer to deliver in real time the temperature structure parameter of the ground layer at different levels up to 30 m.

An acoustic sounder that detects turbulences in the first 800 m above the site (figure 71) has been installed in 1985. The backscattered acoustic signal is doppler processed and a facsimile deliveres the time history of inversion layers evolution (figure 72). The device is permanently calibrated at its first measuring level (30 m) with the set of microthermal turbulence sensors. An equivalent image spread can then be computed.

A portable seeing monitor (Fig. 70) now under development in Europe will allow to measure rO values up to 350 mm in direct imaging mode when monitoring the turbulence induced relative motion of images produced by two distinct sub-apertures. Simultaneous measurements of the index of scintillation will allow to distinguish high altitude turbulence from its orographic counterpart. The telescope is a lightweight 350 mm clear aperature cassegrain on an alt-alt mount. It has been designed for open air operation to suppress any dome effect. A 2D intensified CCD camera is used with a desktop computer for image acquisition and centroid determination. Short exposure images are processed in real time and seeing statistics continuously updated. - 159 -

It is clear that a new site will have to prove far superior to La Silla to be finally selected. It is nevertheless necessary to acquire objective data on which a decision would be based. specific reports on site analysis will be issued regularly. A discussion of existing preliminary results would altogether exceed the scope of this report and would be premature.

11.8 Summary of the VLT Characteristics and Performance Goals

Concept

16m equivalent light collecting power Linear arrangement of 4 x 8m alt-az telescopes

Observing Modes

Combined Coude focus Combined observation at unit telescopes with distributed instrumentation Single telescope independent observation Interferometry between pairs of telescopes

Telescope Description

Optical characteristics of the 8m unit telescopes

Ritchey-Chretien optical system 2 Nasmyth focii: F/151 0.5 degree unvignetted field-of-view primary mirror: F-ratio 21 weight 15 T candidate materials: - Zerodur - steel - silica - aluminium - borosilicate glass - composites possible forms: - thin solid meniscus (for Zerodur, silica, composites) - cellular structure (for Zerodur, steel) On-line active correction of the primary mirror based on wavefront sensing and active supports (wavefront sensing 5 HZ1 mirror correction: 2 HZ1 limiting magnitude of reference star: 14.5) Active axial supports: push-pull systems using hydraulic or electromechanical actuators Atmospheric dispersion compensation at one of the Nasmyth focii Field rotation compensation by rotating the instruments Active tracking with auto-guider: accuracy 0.05 arcsec1 correction using the telescope drives up to 1 Hz and the secondary mirror up to 10 Hz pointing: 1 arcsec RMS; 0.5 degree blind offsets: +/- 0.05 arcsec Unbaffled secondary mirror; central obscuration: 3%1 active alignment of baffles foreseen inside the instruments - 160 -

Mechanical characteristics of the Bm unit telescopes

Designed for open air operation structure characteristics: - lightweight framework - low thermal inertia - low wind drag - high stiffness Structure parameters: Tube: - total weight 100 T - first eigenfrequency > 10 Hz -maximum static wind deformation at secondary mirror 0.4 mm at 100 km/h -windspeed (28m/sec) -maximum obscuration of spiders 1.5% Fork: -total weight 120 T - first eigenfrequency > 7 Hz - maximum load on Nasmyth platform 4 T Bearings: elevation axis: roller bearings azimuth axis: hydrostatic bearings Drives: Dc-motors~ large diameter gear wheel Encoders: large diameter absolute strip encoder and incremental high resolution encoder for fine tracking and pointing Maximum rotation angle: elevation: -5 to +90 degree azimuth: +/- 270 degree

Beam Combination

Optical train using 3 sets of mirrors with optimized high efficiency coatings: UV: 300 to 470 nm (multi-dielectric) R > 98% Visible: 380 to 700 nm (multi-dielectric) R > 98.5% IR: > 700 nm (silver) R > 99% Beam protection: vacuum or helium filled tubes Field-of-view: 20 to 60 arcsec Number of mirrors: 5 to 9 (depending on field-of-view) Built-in laser aligning system with tip-tilt mirror Alternative fiber optics scheme to replace 4 mirrors

Building

Designed for open air operation of the telescopes during the night and environmental and thermal protection during day-time Telescope enclosure: roll-on/roll-off or inflatable shelters Combined platform and removable windscreen to optimize wind load and local seeing conditions Central building independent of telescopes including: Control room Coude laboratory Separate building for interferometric recombination Separate mirror maintenance facility Gantry crane for construction and maintenance - 161 -

Interferometry

Combination of one or several pairs of telescope beams in a separate laboratory Path length compensation by delay lines or moving beam combiners Atmospheric turbulence compensation with adaptive optics (phasing of the apertures) Active stabilization of beam and phase Optimization for the IR spectral range from 3 to la microns Baseline configuration: compact (redundant): 100 m baseline or extended (non-redundant) 150 m baseline Auxiliary telescopes: a pair of specialized 2 m movable telescopes could provide full u-v plane coverage in two dimensions as well as full-tine ~se of the interferometer independently of the large telescopes.

Astronomical Operation

Flexible scheduling: fast change of observing modes and instruments (less than 5 minutes); selection of observing program according to prevailing atmospheric conditions Remote observing through permanent data and communication link between on-site telescope operators and remote observers IR optimization: - minimization of obstruction to <5% - optimized pupil imaging for cooled baffling inside the instruments - low emissivity coatings Minimization of down-time: elimination of change-over with single-focus, fixed configuration telescopes and stationary instrumentation polarimetry: at one telescope, possibility of a fold-back cassegrain focus with an auxiliary mirror in front of the Nasmyth mirror; field 1 arcmin Operating conditions for optimum performance: without windscreen: 9 m/s average wind speed with windscreen: 18 m/s average wind speed I-' 0" IV

Figure 73: photograph of a VLT model based on rolling-off shelters. The two doors are hinged on the platform and are moved by hydraulic actuators. The array is facing the prevailing wind and a wind screen may be used to reduce the wind speed by a factor of 2. The distances between telescopes are in a ratio 1-3-2. This arrangement has been found convenient for interferometry. The 4th telescope could possibly be made mobile along a direction perpendicular to the array. - 163 -

Performance goals

Single Telescope Combined Coude Nasmyth focus Focus

Field-of-view 30 arcmin 1 arcmin

Image scale 1.72 arcsec/mm 0.87 arcsec/mm

Instrinsic Image 0.15 arcsec 0.20 arcsec Quality with Active Optics (80% light concentration)

Efficiency visible 65% 55% infrared 85% 77%

Emissivity at 10 micron required 10% 20% desired 7% 15% pointing 1 arcsec RMS

Tracking 0.05 arcsec

Maximum wind speed for optimum performance 18 m/s for operation 24 m/s

Fig. 73 and 74 are photographs of 2 models in which the various mechanical and building concepts discussed in chapter III have been distributed. •• .'. 'la• .'. J l• 1 ..11".• ' " ,11 • Ira. ~.~'"' 'la. '11 .. I fI ._ J n ]

I-' '"~

Figure 74: photograph of a VLT model based on inflatable shelters. Because of the relative low weight of the shelters, the support structure can be much lighter than in the previous solution. The telescopes are set there at equal distances. - 165 -

Ill. 9 PROVISIONAL COST ANALYSIS

The following approach to cost analysis has been adopted:

i) The main technical aspects have been the sUbject of detailed feasibility studies in which a cost estimate was included. Those studies have mostly been performed by industrial firms having an established reputation in their respective domain. Whenever possible, 2 parallel studies have been carried out. This allows a great deal of confidence to be placed in the cost figures when (as was mostly the case) the results were similar. When comparative estimates differed by large amounts, it has been possible to point the origin of the discrepancy and then to correct for major errors.

ii) Other items have been compared to similar constructions such as the 3.6 m telescope for which detailed cost data are available.

iii) Cost is being independently evaluated with the help of a computer model, PRICE-H. This model is based on the statistical analysis of several thousands of projects of various kinds. An analysis of the construction of the 3.6 m telescope has confirmed the validity of the model for optical telescopes. The simulation is for the time being limited to mechanics and electronics. Its application to optics is being investigated. The NTT will also be used later on for calibration.The final result of this cross-analysis is not yet available and this report is based solely on the results of feasibility studies and of internal estimates.

We have considered that the project management, conceptual design and electronics development will be ESO's direct responsibility. The estimates include the detailed engineering (development), the manufacturing, pre-assembly, transport and assembly in Chile (the latter only whenever it cannot be assumed by ESO directly). A pre-assembly in Europe of the complete telescope has not been envisaged. The cost of this operation would probably be unrealistically high considering that large mechanical elements would have to be transported across Europe.

Definition of base-line solution

Considering the strong impact of the mirror technology on the project cost, it would be dangerous to rely on too optimistic assumptions. The scenario which has been supposed for the purpose of the cost analysis can be considered to be a worst case. Because Zerodur blanks would not, according to the available schedule, be ready on time at least for the first telescope, we have assumed that one metal mirror will be manufactured and will equip the first - 166 - telescope. A total of 4 Zerodur mirrors would subsequently be produced, the last one replacing the metal mirror in the first telescope. This scenario assumes that Zerodur mirrors will be of a better quality than metal ones. It may however turn out that metal mirrors will be fully satisfactory, thus leading to a considerable saving.

The beam combination is based on option 4 (see chapter III.4), which includes 3 sets of mirrors with high efficiency selective coatings. The building is based on the inflatable shelter solution and the site has been assumed to be La Silla or a close-by mountain. Instrumentation has not been included.

Results

The following table gives the cost of the various telescope items. It corresponds to a total of 4 unit-telescopes, manufactured at short intervals. As explained in the notes, the cost estimate figures for several of the listed items are not sufficiently reliable at the present time. These lead to an overall budget uncertainty of +/- 15% on the total project cost. More accurate estimate should become available as the project studies advance. - 167 -

Sub-Total Total ~ /sub-system Notes

PRIMARY MIRRORS 126200

Metal blank (Ix) 5500 (1) Glass ceramic blanks (4x) 56000 Optical figuring (5x) 27200 Handling/Transport 3500 Cell+active supports (4x) 14000 Participation to investment ( 2) for the production of primary mirrors 20000

SECONDARY MIRRORS (4x) 5700

Mir rors + cells 3500 Top units 2200

NASMYTH MIRRORS (4x) 4900

Mirrors + cells 3300 Mech. units 1600

TELESCOPES (4x) 61900

Tubes 15800 Yokes + bearings 27000 Other tel. functions 4100 Drives + encoders 9800 Computers + controls 5200

AUXILIARY FUNCTIONS 67800

Beam combination 13500 Adapters+wavefront sensors 7800 Atmosph.dispersion comp. 3700 Coating plant 12800 Adaptive optics 12000 Interferometry 18000

BUILDING AND SITE 41800

Tel. bases and labs 9900 Tel. shelters + platform 20000 Wind screen 6900 Infrastructure (La Silla) 5000 (3 )

TOTAL PROJECT COST 308300

TABLE: PROVISIONAL COST BREAKDOWN (KDM 1985) Notes

(1) Subsequent units are evaluated at 4 MDM. Should the metal option be finally selected, a total saving of 40 MDM would be realised even if 5 mirrors are produced. (2) Tentative figure. Would be the subject of detailed negotiations. (3) Should a site remote from La Silla be decided upon, this item would increase by about 25 MDM. - 168 -

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