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.