The Jodrell Bank Radio Telescope

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Paper No. 6270 THEJODRELI, BANK RADIOTELESCOPE bY Henry Charles Husband, B.Eng., M.I.C.E., M.1.Mech.E. Consulting Engineer

For discussion at an Ordinary Meeting on Tuesday, 18 February, 1958, at 5.30 pm., and for subsequent written discussion.

SYNOPSIS The Paper deals with the civil engineering problems associated with the design and construction of the 250-ft-dia. fully steerable alt-azimuth radio telescope recently com- pletedat Manchester University’s Research Station, Jodrell Bank, Cheshire. The term “civil engineering” is here used inits broadest sense because the whole design has required close correlation of structural, mechanical,and electrical techniques. The science of radio astronomy has been developed in very recent years, and the Jodrell Bank telescope is the only instrument of its size that has hitherto been built. Because of this, emphasis has been laid upon the reasons for adopting the various features incorporated in the telescope instead of giving a more detailed description of the individual parts, which are, however, illustrated. The wind-tunnel experiments, which had a considerable influence on the structural and mechanical design, are described, together with the conclusions therefrom. Com- parisons are made between alt-azimuth and polar-axis mountings. A cost analysis is given.

INTRODUCTION THE250-ft-dia. fully steerableradio telescope is located at Jodrell Bank Research Station in Cheshire, 4 milesnorth-east of Holmes Chapel. The Research Station began as a branch of the Physics Department of Manchester University, and is now under the charge of Professor A. C. B. Lovell, F.R.S., the first occupant of the Chair of Radio Astronomy. 2. Up to 1948 a 218-ft-dia. fixed reflector, formed of wires stretched on a framework of tubular scaffolding, had produced extremely useful results, a!- though the fieldof its observations was limitedto the narrow track of the heavens which it commanded in conformity to the earth’s rotation. Subsequently, by providing a tilting aerialtower, it waspossible to studyradiations from a zenithal strip within 2 20“ in width. Fig. 1, Plate 1, shows the general design and dimensions of this early large radio telescope, the aluminium-alloy tilting mast being controlled by a single electric winch. This comparatively rough- and-ready displacement of the beam was not suitable for use on the shorter wavelengths required in radio astronomy, and the whole telescopehad thegrave disadvantage of never remaining fixedin direction on any point in space. 3. Soon the great advantages of a large fully steerable radio telescope were appreciated by Professor Lovell, and early in 1950 the Author was consulted 7+ 65 Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. 66 HUSBAND ON THE JODRELL BANK RADIO TELESCOPE regarding the engineering problems likely to be involved. In 1951 a feasibility study wasmade together with a preliminary design for alarge alt-azimuth instrument. From the outset the advantages in the simplicity of control of a polar-axis mounting werefully appreciated, but it isdifficult and extremely expensive to accommodate a very large reflector on any variation of the normal types of astronomical mountingif a reasonably full sky coverageto be is provided. For any purpose other than the making of astronomical observations the alt- azimuth type of mounting with a reflector capable of a complete transit from horizon to horizon has many advantages. 4. By 1952 the general requirements for the Jodrell Bank proposal had been arrived at, after a number of conferences with ProfessorLove11 and his colleagues, by a process of reconciling the acceptable limitationsof the scientificoperational requirements withan engineering design whichcould be carried out at a rational cost. At that time the largest existing radio telescopes had diameters of about 50 ft, and in fact the largest completed fully steerable radio telescope to date, apart from the Jodrell Bank instrument, has a diameter of only82 ft. 5. It was considered that a 250-ft-dia. paraboloid reflector was the minimum size necessary to make possible significant advances in all branches of radio astronomy. In 1951 the general considerations which formed the basis of the fist detaileddesign for the Jodrell Banktelescope were agreed. The paraboloid was requiredto work on a frequency range of approximately30-300 Mc/s, that is,wavelengths from 10 to 1 m. From both the radio and engineering standpoints, the four most important design parameters, after settling the diameter of the reflector, were the focal length, the type of reflecting surfaceto be used, the dimensional limits ofthe reflecting system, and the speed and angular accuracy of the directional control system. These four parametersare discussed in the following paragraphs.

FOCAL LENGTH 6. Mainly in order to reduce the proportion of unwanted radiations received from directions other than that of the beam, it was decided to adopt a relatively deep paraboloid, havinga shortfocal length, withthe focus lying in the plane of the aperture. From purely structural considerations the choice of focal length is of little importance, although the use of ashort focal lengthhas the advantage of requiring a shorter mounting for the aerial and a comparatively stiff and stumpy aerial tower, whereas a reflector of long focal length creates a slight structural complication by requiring the aerial to be mounted a considerable distance outside the aperture in an almost inaccessible position. 7. Theweight and sizeof the aerial array and other apparatus which it might be necessary to install at the focus were matters requiring early consideration. Generally speaking, with mirror a of greater focal length the effective area of the array and its weight would be increased; and a combination of the factors mentioned above appeared to provide a sound argument for the shortfocal-length mirror in the form of a focal-plane paraboloid. Ready access to the focal region is a matter of prime importancein any general-purpose radio telescope. After considering several alternatives it was decided that the most convenient way of changing or adjusting the aerial apparatus was to arrange for the reflector to be capable of complete inversion,so that the aerial carrier could be lowered to a platform near groundlevel. This decision had a great influence

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. HUSBANDON THE JODRELL BANK RADIO TELESCOPE 67 on the design ofthe telescope as a whole, and the Jodrell Bank instrument isthe only known large radio telescope in which such provision is made.

THE CHOICE OF REFLECTING SURFACE 8. The leakage of radiation through the surface of the mirror should be minimum to permit maximum efficiency in the radio design, particularly at the higherfrequencies. A solidsurface having uniform electrical conductivity provides the ideal reflector and also the best screen for unwanted radiation in directions other than the telescope’s beam; the good screening of the Jodrell Bank telescope has already been proved in operation. 9. On the other hand,with a structure of this size, windage was a vitalfactor in the structural and mechanical design. A 30-ft-dia. alt-azimuth instrument had been constructed at Jodrell Bank prior to the design of the 250-ft telescope, and in some ways this served as a small-scale prototype. Its original reflecting surface was a mesh of chicken wire, which was subsequently replaced by ex- pandedmetal having diamond-shaped openings approximately Q in. X + in. Other aerial designs of radio telescopes which have been completedin Holland, America, and Germany have also used open-mesh reflecting membranes. 10. It was originally hoped that the amount of radiation leakage on wavelengths of 1 m or more would be tolerable with a square mesh of 10-gauge wire at 4-in.centres. Subsequent investigations showed that anythingmore open than a 2-in. square mesh would producetoo much leakagein the upper frequency ranges. A comprehensive series of experiments wascarried out in the Tedding- ton wind tunnel of the National Physical Laboratory to determine wind resistances on wire meshes for a considerable range of dimensions and angles of attack, In the British climate icingconditions occur fairly frequently and large masses of snowand ice can be deposited remarkably quickly, evenon thin wires. These icing effects were simulated in the wind tunnel by using round bars of various dimensions in place of the wires ofthe meshes under investigation. The conclusiondrawn from winterobservations was that completeicing up or blockage by snow of a 4-in. square mesh was unlikely, whereas a 2-in. square mesh would quickly become impervious to wind under conditions of icing and snow blizzards which might be expected in Great Britain. A further danger in the case of a large-diameter-mesh reflector was the possibility that one half of the reflector, probably the upper half, might become snowedup in a blizzard much more quickly than the lower half. That would worsen an already serious turning moment due to differential wind pressures affecting both the structural and mechanical design of the telescope. Some experimental data were already available on the effect of windon parabolicsurfaces and they were supplemented, again using the wind tunnel at Teddington, by a comprehensive investigationon a scale model of the final designadopted for the bowl structure.

DIMENSIONAL LIMITS 11.While these aspects of structural and mechanicaldesign were under consideration, rapid developments were taking place in the science of radio astronomy. A much shorter wavelength than 1 m became of vital importance; the study of 21-cm radiation from various sources in space had become an essential branch of radio astronomy.

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. 68 HUSBAND ON THE JODRELL BANK RADIO TELESCOPE 12. The dimensional tolerances originally agreed for the Jodrell Bank 250-ft reflector, on the basis of the minimum wavelength of 1 m, were as fo~1ows:- (a) Under conditions of no wind, the shape of the paraboloid reflector (for local irregularities) was to be within & 5 in. of the true shape relative to the focus. General overall distortion up to & 8 in.could be allowed, these limits being maintained for all positions of the bowl. (b) Atwind speeds of 30 m.p.h. a doublingof the abovedimensional tolerances could be allowed. (c) In gale conditions it was not essential that the instrument be usable; but it was to be safe up to 100 m.p.h. and to be controllable under such conditions. 13. Although, because of the possible incidence of snow and ice, a 2-in. mesh had to be considered as a surface sometimes impenetrable by wind, the dimensional tolerances didnot impose particularly difficultstructural conditions, even allowing for differential wind pressures, varying gravitational forces, and possible backlash on the driving system. 14. The first bowl structure to be designed was on the basis of a heavy cradle girder from which the main cantilever armsradiated, and these were assistedby the hoop tension in circumferential members lying immediately behindthe surface of the paraboloid. Many alternative structural arrangements were considered, some of whichhad obvious advantages fromthe standpoint of pure stress analysis. Apart from any question of accurate stress determination in a structure of this magnitude, the feasibility and reasonable ease of erection was a matter of major importance. 15. It will be remembered that any design adopted for the bowl-supporting structure had to allow for complete inversion in order to provide a method of easyaccess to the aerial at the focus.This consideration precluded,in an instrument of this size, the use of a supportingring girder having a diameter of about two-thirds the reflector diameter, which wasone of the earliest methods of suspension to be considered. It was soon found thatthe mounting of trunnion shafts on extended cantilevers gave riseto awkward deflexionsand high torsional stresses due to differential wind pressures, which became very severe at certain angles of elevation. The salient results of the wind-pressure investigations are given in an Appendix which accompanied the MS. 16. A further factor of vital importance in the design of such a large reflector structure, and indeed of the telescope as a whole, was the assessment of the natural frequencyof the bowlframework and its supporting towers. By keeping the towers completely clear of the bowl structure these could be pro- portioned so as to be inherently stiffand to have such a high natural frequency as to cause no concern on this account under the worst wind conditions. On the other hand, the possibility of a structural flutter developing on the reflector bowl structure did call for keen attention. Apart from the risk of structural damage, any seriousvibrations set up by wind forces would have highly undesir- able effects on the driving system and also on the automatic control apparatus. 17. A full consideration of all these matters, coupled with the great importance, by 1954, of making the telescope usefulas a general-purpose instrumenton wavelengths considerably below 1 m, caused a complete redesign of the bowl structure to be undertaken. 18. By this time the 50-ft-dia. alt-azimuth instrument at the Naval Research

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. HUSBAND ON THE JODRELL BANK RADIO TELESCOPE 69 Laboratory at Washington had been completed. This telescope has a reflecting surfaceformed of cast aluminiumsectors machined to comparativelyfine limits. The use of aluminiumas a structural material aswell as for the reflecting membrane had been carefully considered from the outset of the Jodrell Bank design, but had been rejected for three reasons:- (U) High cost. (b) The relatively low modulus of elasticity was a grave disadvantage in limiting deflexions. (c) The fairiy high coefficient of thermal expansion of all the lower-priced aluminiumalloys would create temperature distortions of a more serious naturethan wouldbe obtained in a correspondingsteel structure. 19. The use of a composite design was not ruled out, but apart from the obvious advantages during erection at high level there was no real gain from the lighter weight of aluminium. In fact, if an aluminium structure had been used for the Jodrell Bank radio telescope, ballasting or some more complicated form of wind anchorage would have been necessary to create the necessary moment of stability against overturningby wind. 20. The final design adopted for the reflector consisted of a continuously weldedmembrane of 14-gauge mild-steel sheet. Under normal dead-load conditions, with the reflector facing the zenith, the membrane is not appreciably stressed.Under heavy wind loads the membrane,which is continuously welded to the supporting framework, acts as a "stressed skin" and is useful in reducing deflexions. It will be seen, therefore, that the final choice of the re- flectingsurface was made by consideringjointly the radio and structural desiderata. 21. Theprecise physical dimensions and focal aberration ofthe Jodrell Bank reflector are now being determinedby a survey which is taking placeunder various wind conditions and different angles of elevation at such times as the telescope can be sparedfor thisinteresting structural investigation.Three alternative methods of surveying a large bowl of this nature are possible; by light-reflexion methods, by ultra-short-wave radio-reflexion techniques, and by direct physical measurements using normal surveying instrumentsand stretched wires.Using simple arrangements of stretched wires fitted with extension recorders, measurements to an accuracy of about 6 in. can be made across the bowl showing the distortions due to either transiting or wind pressures. It is doubtful if the much more expensive optical or radar techniques could achieve much more accurate measurement. The exposure to bad weather experienced in the bowl, particularly when it is tilted, makes precise surveying uncomfortable, and safety precautions had to be devised. 22. Preliminary radio investigations have already commenced, and there is every reason to believe that the power gain, particularly on the shorter wavelengths, and the freedom from interference from behind the membrane,will fully justify the redesign of the reflector.

SPEEDAND ANGULAR ACCURACY OF THE DIRECTIONAL-CONTROL SYSTEM 23. In the original specification it was suggested by the radio astronomers that the speed in both azimuth and elevation should be continuously variable from 2"/hour to 36"/min, and an automatic control accuracy of t 12 minutes of arc

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. 70 HUSBAND ON THE JODRELL BANK RADIO TELESCOPE was to be maintained up to speeds of 4"/min in both azimuth and elevation. These were arbitrary requirements which were considered at the time to be suitable for generalradio-astronomical purposes. Rotation both in azimuth and elevation was to be a full circle, plus an overlap of about 30". 24. The whole instrument was to be remotely controlled to comply with the following basic conditions:- (U) The whole telescope had to be capable of being locked at any given azimuth and elevation. (b) The telescope was to be capable of continuous movement within the speed limits and angular tolerances stated above: (i) in azimuth at fixed elevation; (ii) in elevation at fixed azimuth; and (iii) at any predetermined differentrates in azimuth andelevation. (c) In addition to the basic movements in azimuth and elevation the paraboloid was required to perform automatically severalother operations, such as following a given star or other heavenly body with full com- pensation for the motions of the earth, scanning a specified area of sky round a star, and scanning predetermined "rectangular" areas of sky. 25. The speeds for following a star vary greatly with its position in the sky and may range fromabout lO"/hour for a star near thesouthern horizonto infinity for a star precisely at the zenith. Fortunately the high following speed is re- stricted to a very small area of the sky around the zenith, and the maximum desired speed for all practical purposes was fixed by the need to rotate or set up the instrument in a reasonable time, say 10 min, for a complete rotation in

azimuth or elevation. ' 26. On the azimuth driving circle,this maximum angular speed corresponded almostexactly to 1 m.p.h., and, provided that sufficienttractive effort was available to overcome wind forces and friction, there seemed to be no major driving problem at this end of the speed scale. On the other hand the slowest speed at which the astronomers would like to rotate the telescope was approximately 1/1,OOO of the maximum, corresponding to approximately + in/min on the azimuth driving circle. There was no information available at the time as to whether such nicety ofcontrol would be feasible byadopting existing engineering techniques of reasonable cost, and applying these to an instrument whose moving parts would certainly weigh more than 1,000 tons. 27. The Admiralty Gunnery Establishment at Teddington were very helpful with practical advice on the mechanical limitations associated withthe driving of heavy gun turrets, both by electrical and hydraulic means, and on methods of automatic angular control used in naval gunnery, which provided a close geo- metric analogy to the movements required for the large alt-azimuth telescope. 28. It will be realized that duringthe whole period of the design of the radio telescope, the structural, mechanical, and electrical problems could seldom be examinedseparately. In fact the onlyrelatively straightforward decisions which had to be made were in connexion with the foundations, and even here the preliminary site investigations by boring showed that it would be necessary to take piles down 70 or 80 ft to the red marl, in order to be sure of a platform which would remain level under the heavy rotating load of the telescope and the variations in water content of the ground near the surface.

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. HUSBAND ON THE JODRELL BANK RADIO TELESCOPE 71 29. The Authorwas so impressed with the mechanical perfectionthe of driving mechanism used to rotate 15-in. naval gun turrets that he agreed at an early stage of the design procedure to earmark two complete 27-ft-dia.internal racks and their drivingpinions from hydraulically operated 15-in.gun turrets on H.M.S. Royal Sovereign and H.M.S. Revenge, which were shortly becoming available as thosebattleships were being broken up. An inspection of the racks was made at Inverkeithing; they were found to be in perfect conditionand were subsequently purchased at a nominal price, although present-day costs would probably have precluded the manufacture of comparable racks for the elevation drive of the reflector. The great attraction of these large full-circle racks was that they fitted in with the original conception of inverting the bowl to give easy access to the apparatus at the focus. It was a happy coincidence that the strength of the teeth on the racks corresponded, with a reasonable factor of safety, to the anticipated maximum torque caused by wind loading on the reflector, which at that time was being considered as a close copper mesh completely iced up. 30. Consideration had been given to an alternative method of driving the reflector in elevation, by using a large roller chain or a series of such chains, either to form a large-diameter rack system behind the bowl, or as chain drives witha tensioning device near the trunnions. While the reassemblyof the segmental battleship racks within 34-ft-dia. circular stiffening girders (Fig. 5) presented considerable problems both in the machine shop and, more particularly, on the site, a high degree of accuracy was obtained inthe reassembly, and during the actual operation of ihe telescope there has so far been no reason to regret the incorporation of thishigh-class second-hand material in the driving system. 3 1. Although the tooth form of the racks and pinions had been designed to eliminate backlash as far as possible, and two pinions are in constant mesh on each side of the bowl, it was considered prudent, with a view to avoiding shock on the teeth, and advantageous towards achieving a steady motion of the bowl during gusty wind conditions, to apply a controllable resistance to the rotation in elevation. This was arranged by forming a roller track of approximately 270 ft 6 in. dia. behind the centre of the bowl, which is constantly in contact (except whenthe bowl is inverted) with two groups offour large pneumatic tires mounted onwheels connectedto a hydraulic braking system, which is adjustable for retardation and interlocked with the driving system (Fig. 3, Plate 1). The pressure of the pneumatic tireson the track is also adjustableby a screw-jacking system, and the wheelsthemselves are mounted on heavyleaf springs. The pressure on the track can be estimated fairly closely by the deflexion of the leaf springs, and by the distortion of the pneumatic tires under a knownair pressure. The maximum tangential controlling force is approximately24 tons, but a much lower resistance to rotation than this has been found adequate to prevent any appreciable oscillation of the reflector during starting, stopping, or rotation. Visitors to the telescope are impressed by the fact that, when standing within the reflector (the rim of which provides the only horizon), it is impossible to detect stopping, starting, or movement. 32.While the mechanicaldesign of the elevationmotion was relatively determinate, assuming the reasonable accuracy of the wind-tunnel experiments and the adequacy of the structural stiffness of the bowl structure to obviate

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. 72 HUSBAND ON THE JODRELL BANK RADIO TELESCOPE flutter, together with an almost negligible friction on the trunnion bearings, the motion in azimuthprovided a somewhat more difficult problem. 33. The total weight ofthe moving parts of the telescope as finally constructed is about 2,000 tons, although it had been hoped originally to build the more flexible design witha weight limit of about 1,250 tons. The reasonable distribution of thisdead weight, coupled with considerablevariations in bearing pressure due to heavy wind loads, required about a hundred points of contact between the rollers and the curved track. 34. For three reasons it was not considered feasible to carry any substantial proportion of the weight of the instrument onthe centre pivot :- (a) A particularly large foundation would have been necessary to preclude any possibility of differential settlement betweenthe centre pivot and the main azimuth circle. (b) The cost of a thrust race to take 500 tons or more would have been excessive. (c) The structural form of the telescope made it far easier to transmit both dead loads and wind loads from the reflector trunnions directly to the 300-ft-dia.azimuth circle. It wasdecided, therefore, to design the centre pivot (Fig. 4, Plate 1) to be capable of taking the whole of the horizontal wind forces on the telescope in any direction and a nominal vertical loading of about 200 tons. Thisallowed for the centre pivot taking a reasonable share of the dead weight of the diametral girder, the weight of the electrical machinery located over the centre pivot, and forces transmittedfrom the elevationdamping device. 35. In spite of its inherent stiffness the main framework of the telescope is sufficiently large to be capable of slight distortion by hydraulic jacks used at speciallyprovided jacking points near eachbogie. A reasonably accurate distribution of load between the bogies is possible, subject to the level of the track remaining constant. It was decided to take approximately half the dead load of the telescope on the four driving bogies, whichare located directly under the bearing towers. This load is amply sufficient to provide all the track ad- hesionnecessary to preventwheel slip. The remainder of the dead load is shared between eight bogies locatedat the foot of the eight mainraking members, which ensurethe stability ofthe instrument against overturning under maximum loads. This arrangement leaves a margin of load-bearing capacity on the eight “wind” bogies to compensate for the transfer of load from one set to another, depending on the direction of the wind reactions. No provision is made on the bogies to withstand uplift under normal working conditions, but under the maximum designed wind loading (which would normally be avoided by taking simplesafety precautions) the wholeof the loadof the telescopewould be carried by eightbogies, that is,the four drivingbogies and the four wind bogies on the leeward side. 36. The considerableweight of the windwardbogies would contribute to stability under such abnormal conditions, but the main raking members on the windward side of the towers would then be in tension. Each bogie is provided with powerfulanchors designed to take uplift, but these should onlybe necessary under completely abnormal conditions, such as blast from nearby explosions. The centre pivot (Fig. 4, Plate 1) is also designed so that it could take, in an

FIG. ON ONE OF THE FOUR MAIN AZIMUTH DRIVING BOGIES

! I

=lv,-~ .-T ~

FIG. 1 I .-ONE OF THE WELDED FRAMESCARRYING THE MAINPINION BEARINGS

FIG. 12.-Two OF THE MAIN WIND BRACES

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. HUSBAND ON TJB JODRELL BANK RADIO TELESCOPE 73 emergency, an uplift of about 500 tons on a safety ring. This, however, would only come into action if the whole telescope started to lift off its tracks.

MOTlVE POWER 37. The choice of motive power for rotating the telescope in azimuth and elevation lay betweena combined electrical and hydraulic system or some form of infinitely variable electric drive. A study was made of the hydraulic drives which had been used with complete success on the 15-in. battleship gun turrets previously referred to. Quite clearly a hydraulic drive of this nature could be adapted for the big telescope, and this might have presented a way of avoiding any electrical machinery on the telescope other than the radio receivers and transmitters, although electricalenergy would be essential to provide the hydraulic pressure and for the remote and automaticspeed controls. However, it was soon apparent thathydraulic machinery of the power necessary to handle the telescope in gale conditions would prove extremely costly,and at a comparatively early stage ofthe design two independentWard-Leonard rotary converters were adopted; they feed infinitely variable direct current to slow-speed shunt- wound motors standardizedfor each of the two main motions ofthe telescope. 35. In determining the power of the drivingmotors, the wind resistanceof the bowl was the major factor in the case of the elevation motion. Even the out- of-balance effectsdue to accumulations of snowor ice were comparatively small compared with the probable torque due to the maximum wind speed of 100 m.p.h., under which it might be requiredto rotate the telescope in a case of dire emergency. The frictional resistances of the two main trunnion bearings were altogether negligible and the wind-damping device, previously referred to, was adjustable. 39. In the case of the azimuth motion the Author had some difficulty in obtaining reliable information concerning track friction at such high loadings and slow speeds; furthermore it was not possible to be sure at the design stage how accurately the tracks could be laid and maintained within reasonable limits of expenditure. For design purposes a coefficient of track friction (starting) of 0.032 was adopted after considerationof published informationon British and foreign railway practice. At the operating speeds finally adopted, which will be dealt with later in the Paper in relation to the control system, it was calculated that approximately 100 h.p. was required under the worst wind conditions to operate the telescope on each of its axes.

THE AZIMUTH TURN-TABLE 40. The heaviest readily availablerail section was the lOPlb/yd flat-bottomed rail. While heavier rails would have been preferred it was decided to adopt four rings of this section on which to rotate the whole telescope in azimuth. The rings were arranged in two pairs and each roller has two contact points. The rails were curved and laid on continuous steel bed plates accurately levelled on folding steelwedges and packed witha carefully prepared fine granite concrete having a crushing strength at 28 days of more than 10,000 Ib/sq. in. The rolling mills were extremely helpfulin providing a set of rails produced from a new set of rolls. The dimensional accuracy ofthe rails from these rolls was surprisingly good and the overall depth of the rails varied less than 0.02 in. All the rai1 joints were scarfed and staggered. Each pair of rails was packed in relation to

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. 74 HUSBAND ON THE JODRELL BANK RADIO TELESCOPE each other to match the tapers provided on the rollers. In addition to tapering the rollers (the apex of the cone being at the centre of rotation of the telescope), they were all set on spindles accurately radial. By these means it was hoped to reduce track friction to a minimum. All the tracks are fixed by holding-down bolts and can readily be replaced or adjusted for level by unscrewing exposed nuts. To date, no adjustment has been necessary; the whole turn-table appears to be remaining true to level, and no difficulty due to track expansion or con- traction has been experienced. A surface-grinding machine was designed for truing the rail heads, but has notyet had to be used; it is gratifyingto know that the coefficientof track friction obtained in practice is as low as 0.003. It follows that to rotate the telescope at a normal operational speed, when there is no serious wind on the bowl, only about 3 h.p. is required in azimuth. This corresponds to an actual current consumption by the four azimuthdriving motors (in series) underfavourable conditions of only 30 A at 10 V. This means that the 2,000-ton telescope could be rotated for quite a considerable time from an ordinary motor-car battery. A cross-sectionof the track construction is shown in Fig. 2, Plate 1, and the general design of one ofthe four main azimuth driving bogies is shown in Fig. 6, facing p. 72, and Fig. 7, Plate 1.

MOVEMENTIN ELEVATION 41. Considerably less power than in azimuth is needed to tilt the bowl in elevation, and in practice the only appreciable resistanceapart fromwind effects is the artificial braking caused by the pneumatic-tired damping bogies. 42. The arrangement of the elevationwind-damping bogies is shown in Fig. 3, Plate 1. It will be appreciated that the production of 270-ft-dia. accurate circular track for the purpose of this damping device would have been extremely costly. By adopting pneumatic tires bearing on curved chequer plates a very satisfactory coefficient of friction has been obtained, and the balanced bogies with the axles mounted on heavy leaf springs adequately compensate for slight irregularities from the true circle. The ends ofthe large arc formed by the roller track are slightly reduced in radius (where they intersect the circumference of the reflector) so that when the bowl is returning from its completely inverted position the track leads comfortably on to the pneumatic tires without any adjustment to the bogie settings being necessary.

POWER SUPPLY 43. Having estimated the total power necessary to drive the telescope under adverse wind conditions, a small extension to the existing Jodrell Bank Diesel- electric power station was provided so that the electricity for the telescope is, at all times, obtainable from a single Diesel-driven alternator plant of 250 kW capacity. In case of breakdown ofthis generating plant the other Diesel-electric sets in the power station have ample capacityto work the telescope, and switching arrangements weredesigned to facilitate a rapid change-over. A voltage- stabilized supplyfor radio purposes is provided on a separate circuit to the driving supply, which is subject to appreciable voltage fluctuations. 44. During the wholeof the construction period the two20-ton derrick cranes, mounted on high gabbards giving a clear lift of 250 ft, were fed with power from the Diesel alternator set provided for the running of the telescope. Electric welding on the site was also fed from the same source.

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. HUSBAND ON THE JODRELL BANK RADIO TELESCOPE 75 45. Only a small fraction of the total power which had to be provided for rotating the telescope in heavy winds would be expended during normal usage. Therefore an electric thermal-storage system was adopted which could be used in the winter for heating the control and administration buildings associated with the telescope. This provides at all times a small steady load on the Diesel generator set, which is conduciveto easy maintenance ofthe Diesel engine.

STRUCTURAL DESIGN 46. It will be seen from the tabulated cost of the radio telescope given in Appendix I, that although the structural framework was the most impressive feature in the general appearance of the telescope, it absorbed less than half of the total expenditure. An alternative design for a comparable radio telescope on an equatorial mounting having the same degree of rigidityas the alt-azimuth design required almost twice as much structural steel. This seems to be a major factor which must be borne in mind when considering any future large radio telescopes for astronomical purposes. Admittedly, if angular-control tolerances of the order of 10 seconds of arc are required, the cost of reliable-equipment, whether electronic, mechanical, or a combination of both, for converting continuously celestial right ascension and declination to terrestrial azimuth and elevationmay be considerable;however, the structural, mechanical, and economic advantages ofan alt-azimuth mount for carrying 250-ft-dia. reflectors are likely to outweigh the simplicity of control on a polar axis. 47. Fig. 8, between pp. 72,73, illustrates models whichweremadeinconnexion with alternative designs usingequatorial mountings. One of the models gives full sky coverage and the other a limited field towards the northern horizon. In developing these designs some financial advantage appearedto be possible if the reflector framework was constructed in a light alloy, and the remainder of the instrument in steel. Reasonable solutions were found for carrying out the roller tracks to the necessary degree of accuracy, but it was estimated that the total cost of a safe telescope of this nature would have been about twice that of the alt-azimuth telescope adopted at Jodrell Bank. 48. In the case of the Jodrell Bank telescope the basis of the structural design was a compromise which tended to vary while the details were being prepared, and even after the work had started on the site, A rational interpretation of the wind-tunnel experiments, particularly with regardto the forces on the paraboloid itself at different anglesto the wind, enabled a reasonable assessmentto be made of the total loads to be sustained or rotated. 49. The deflexion tolerances originally specified for a reflector suitable for wavelengths above 1 m were not particularly difficult to achieve with respect to dead loads and windloads. There was, however, a danger of someflutter developing at 100 m.p.h. if the natural frequency of the structure was lower than about*c/s. A stiffer structure not onlyprovided smaller deflexions, which would increase the usefulness of the instrument on lower wavelengths, particu- Iarly the 21-cm band, but also raised the natural frequency of the bowl framework to a figure which would eliminate dangerous flutter, even at wind speeds much greater than the 100 m.p.h. originally assumed as the maximum gale to which the reflector might be subjected. 50. It was finally calculated that the natural frequency of the telescope bowl, approximately 3 c[s, wouldcoincide with the frequencyof the oscillations

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. 76 HUSBAND ON THE JODRELL BANK RADIO TELESCOPE induced when the wind velocity was 167 m.p.h. and the deflexion at the edge of the bowl, due to wind, was 3+ in. 51. Deflexions at the edge due to wind moments were considered and the elastic stiffness:

where M denotes the wind moment in lb.-ft, d the corresponding edge deflexion in inches, and R the radius of the bowl. 52. The natural oscillatory frequency of the bowl, assumed to be rigid in itself, is then given by:

53. These calculations did not take into account the effect of the braking rim, which is certain to add considerably to the factor of safety against flutter. The tests of the aerodynamic stability of the bowl were carried out by Mr L. Woodgate, under the direction of Mr C. Scruton, whosework on dynamic considerations in suspension-bridge design iswell known. Brief details of these tests are given in Appendix 11. 54. Meteorological observations including wind speeds at heights of 10 m and 150 ft are taken regularly at Jodrell Bank, and itis consideredthat adequate provision has been made for this matter of wind gradient with height, which, in the Author's opinion, is one of the mostimportant factors to take into account in the structural design of a large steerable radio telescope. 55. A preliminary assessment ofthe dead weight ofthe structure was particularly difficult at the outset; there was no precedent for a framework of this size mounted on trunnions andsubjected to such stress variations on account of the rotation on the horizontal axis and the extremely variable nature of the wind loadings. 56. The firstdesign, which was worked out in detail,envisaged a heavy cradle-shapedgirder between the trunnions, with radial cantilevermembers connected with a circumferential secondary system carryingthe reflecting membrane or wire mesh. An erection scheme was workedout in detail by which the cradle girder was to be built on comparatively low scaffolding fromthe diametral girder joining the two trunnion towers, and then the cradle was to be inverted through 180" so that the radial cantilevers couldbe conveniently handled by the erection cranes. The whole bowl structure was to be completed in the form of an umbrella, the only additional scaffolding being that necessary to steady the structure against rotation. 57. As the stressanalysis proceeded it became apparentthat while the deflexions at any angle of elevation would be withii those orginally specified for l-m radio reception, the structure did not lend itself to providing the increased stiffness which was highly desirable if the telescope was to work effi- ciently on the 21-cmwaveband. Stiffening up the cantileversadded weight, which was not a very serious matter when the bowl was directed at the zenith, but which produced troublesome torsional effects in the cradle girder when the bowl faced the horizon. 58. These deflexion and dead-weight difficulties led the Author to take more interest in the geometry of the paraboloid, and particularly of the nature of its

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. HUSBAND ON THE JODRELL BANK RADIO TELESCOPE 77 deflexions during rotation on the elevation axis. When directed towards the horizon orat low angles of elevationthe parts of the reflector near the upper and lower ends of a diameter at right angles to the axis of rotation tended to slump under the effect of gravity, with a corresponding displacementthe of focus, which would upset the directional control of the radio beam. On the other hand, if a form of construction could be devised by which the periphery of the reflector could be kept reasonably in one plane and circular, inevitable deflexion distortions of the central portion of the paraboloid, due to the changing angle of gravitational forces during rotation, would mainly cause a small variation in focal length and a much smallerangular displacement of the radio beam. With a “rigid” periphery the reflecting surface, supported by main trusses arranged radially, even after deflexion, would still closely approximate to a paraboloid; small variations in focal length,if necessary when workingon short wavelengths, would be compensated for by a relatively simple servo-mechanism. 59. With this general arrangement the bowl is deepest and the focal length minimum when the instrument is directed at the zenith, and shallowest when directed towards the horizon. 60. These considerations encouraged the adoption of a “space-frame’’ form of construction with a very deep circumferential girder. Sixteen main radial trusses were used to connect the panels of the circumferential girder to a central hub. The calculation of stresses in the framework was tedious but reasonably determinate, bearing in mind that the whole structure was supported at two points only. The most difficult parts of the framework to design were the members connecting the trunnions to the circular structure. The shear forces in the framework due to dead weight and wind forces acting at all directions through 360” were troublesome to deal with, and required a much greater weight of steel than hadbeen allowed for in the preliminary structural estimates. Fig. 11, facing p. 73, shows the assembly of a trunnion, also the framework connecting this heavy trunnion assembly to the main structure of the bowl. 61. The final design theof bowl structure was extremely tedious because of the large number of loading conditions which had to be considered, and because every alteration affected the position of the centre of gravity. It was obviously highly desirable that the centre of gravity of the whole of the bowl structure should be closely on the axis of rotation, and the necessary arithmetical calculations fully occupied an electric calculating machinefor nearly a year. 62. It was decided to carry out the erection of the framework according to this second design with the bowl facing the zenith. Bearing this in mind, all the radial members on the lower side of the framework were arranged in the same plane as the bottom chord of the peripheral girder. Towers of tubular scaffolding, approximately 130 ft high, were built under each main panel point of the rim girder, and also a central tower on which the structural hub was first erected and carefully centred and levelled. The general sequence of operations was:

(U) the main panels of the ring girderat each trunnion were erected; (6) the two panels of ringgirder adjacent to the trunnions were erected and connected across by four of the radial trusses; and (C) construction proceeded outwards from the trunnions, a main panel of the ring girder being erected and connected to its associated radial truss.

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. 78 HUSBAND ON THE JODRELL BANK RADIO TELESCOPE This procedure was followed with both sides of the bowl kept in balance until the whole ring was completed. 63. During the whole of the erection process the bowl was supported on a combination of screw and hydraulic jacks so that appropriatecambers could be maintained. 64. It was originally intended that the whole of the bowl structure should be supported on the temporary towers until the reflecting membrane had been completed. The area of steel in a radial section of the membrane, which is continuously welded to forty-eight circumferential purlins,is roughly 180 sq. in., and as a “stressed skin” it adds appreciably to the structural strength of the whole reflector framework. Unfortunately, the great mass of scaffolding in the towers and the bracing between the towers were seriously impeding other work on the installation of the machinery and the completion of the telescope. To save construction time the scaffolding towers were actually taken down before fabricating the steelmembrane, and this can now be consideredonly as a contribution to resisting wind loads. 65. All the more important members, both in the tower structures and in the bowl framework, have been equipped with recording strain gauges, and observations have so far confirmed the reasonable accuracy of the original stress calculations. 66. The final general arrangement ofthe telescope is shownin Fig. 9, between pp. 72, 73, and Fig. 10, Plate 2. A recording tensioned-wire measuring system was adopted for checking the dimensions of theparaboloid andthe distortion of the paraboloid as itrotates in elevation. The displacement of the focus from its true position in relation to the axis ofrotation, as the direction ofthe beam moves from the zenith to either horizon, is approximately 2 in. This error includes the deflexionof the aerial towerwhich is designed to act as a horizontal cantilever when the bowl is facing the horizon. A servo-mechanism has been designed to correct the position of a short-wave aerial, to compensate for this deflexion, but its use has not been found necessary to date.

THECONTROL SYSTEM 67. From the control desk one person can operate the telescope, having it in full view, floodlit at night, through a plate-glass window 25 ft X 11 ft. The control room is constructed over a basement which is directly connected by a 400-ft-long tunnel to an annular chamber surrounding the upper part of the foundation block carrying the centre pivot of the telescope. All the cables- radio, power, control, and general services-pass on racks along this tunnel and enter the revolving structure of the telescope through a device (Fig. 4, Plate 1) which enables themto be twisted without damagethrough a rotation of approximately 400”. Similar measures are taken with those cables which pass up the towers of the telescope and through the centre of the trunnion spindles. 68. The firststages of radio amplification take place either in apparatus mounted at the top of the aerial tower near to the focus, or more usually in a laboratory suspended on trunnions behind the centre of the bowl. This laboratory, which can be reached by walkways when the bowl is facing the zenith or completely inverted,is sufficiently largeto accommodate the apparatus and two or three scientists. An adjustable damping device is arranged to limit oscillations of the laboratory duringhigh winds.

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. HUSBAND ON THE JODRELL BANK RADIO TELESCOPE 79 General requirementsof the control system 69. The control system is required to move the radio telescope with reference to three different sets of co-ordinates; these are: (a) ceIestial right ascension and declination; (b) galactic latitude and longitude; and (c) terrestrial azimuth and elevation. 70. In order to work to (c), correction has to be made for the rotation of the earth about its axis. In addition, when observing the sun or planets, corrections must be made for the movement of the earth in its orbit. For lunar observations a further correction for parallax in altitude must be made. 71. The following movements are required of the telescope: (a) to move rapidly in azimuth and elevation in order to align itself on a required setting; (b) to adjust itself slowly to counteract the earth’s movement; and (c) to scan through predetermined arcs repeatedly to cover a required area of the celestial sphere. 72. Any one of the three sets of co-ordinates may be used to control the telescope. A computing device must be provided which converts: (a) galactic co-ordinates into celestial co-ordinates; and (b) celestial co-ordinates into terrestrial co-ordinates. 73. The relation between terrestrial and celestialco-ordinates can be expressed in terms of sphericaltrigonometry in variousequations. For the system of control suggested, different equations are more suitable in different areas of the celestial sphere. The control system provides facilities for automatic selection of the most suitable equation for use in converting one set of co-ordinates into another. 74. The actual method adopted to achieve the continuous solution of these equations depended on a standard article ofprecision electrical apparatus known as a “magslip resolver”. This is a development of the magslip repeater used to transmit electrically angular movements. The resolver has two-phase windings on both rotor and stator. If either the rotor or the stator windings are connected to an A.C. supply, a magnetic field is produced which induces voltages in the other member, the voltage in any given winding dependingon the strength of the field and the angular relation between the axis of this field and that particular winding. It can be arranged for the stator windings to yield alternating potentials proportional to the sine and cosine of the angle which the exciting winding of the rotor makes with the stator. 75. The accuracy of the instruments used at Jodrell Bank is of the order of one part in a thousand. It is probable, however, that apparatus of a similar nature, but on alarger scale, is now available which would givea greater degree of accuracy for the individual components of the control system, with a corre- spondingly greater accuracy theof automatic control of the telescope movements in azimuth and elevation. 76. Expenditure on the Jodrell Bank instrument has been strictly limited,and with the experience now gainedit would be possible, at greater cost, to produce a more accurate alt-azimuth control system than the one now in use. 77. Alternative electronic-computer methods for the automatic solution of the fundamental position equations are feasible, giving a much higher degree of

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. 80 HUSBAND ON THE JODRELL BANK RADIO TELESCOPE accuracy, and these could be used to superimpose correction ofangular position at short time intervals. Here again, the main difficulties are those of cost and reliability, bearingin mind that the telescope is requiredto operate continuously night and day. Another ingenious method of converting a polar-axis movement to azimuth andelevation is to use a mechanical analogue feeding extremely accurate magslip repeaters. While very great mechanical precision is necessary, the use of this method is one which the Author would favour at the present time. 78. More detailed description of thecontrol system is given in Appendix 111.

AUXILIARYMECHANICAL AND ELECTRICAL EQUIPMENT 79. In addition to the hydraulically operated brakes controlling the pneumatic-tired wind-damping device, there is a powerful hand-operated emergency clamp, using friction pads gripping a fin arranged centrally between the two tracks in contact with the pneumatic tires. These brakes can be applied by a hand wheel located in the motor-generator building situated immediately over the centre pivot of the telescope and revolving with the azimuth motion. This motor-generator roomis readily accessible from ground level. 80. An electric winch, remotely controlled, is providedfor lowering the aerial assembly to ground level when the bowlis inverted. A simple mechanicaldevice ensures the correct orientation of the aerial when the carrier tube is hoistedinto position and automatically locked into the aerial tower. Automatic rotation of the aerial assembly can be arranged. A 4-in.-dia. pressurized co-axial cable is provided from the base of one of the trunnion towers to the aerial tower. This can be coupled to powerful short-wave transmitting setswhich will be located in a house on the top of the diametral girder. It is expected that in due course signals will betransmitted andreceived back by reflexionfrom theplanets. 81. Remotely controlled aircraft-warning lights are provided on the highest points of the structure. These may be switched off for short periods of time if it is desired to takephotographs through optical telescopeswhich will be arranged to work in conjunction with the radio telescope.

LIGHTNINGPROTECTION 82. It is expected that the telescope structure will from time to time be struck by lightning. Apparatus is provided in the control room to give warnings of the approach of electrical storms or any unusual build-up of electrical potential in the atmosphere. The greatest danger of damage is to the main bearings of the telescope, should a heavy lightning current pass through them. The bearings are, therefore, short-circuited with heavy flexiblecopper tapes and the whole of the telescope, and all connected wiring, is protected by an extensive system of earthing plates and radial conductors surroundingthe centre pivot and the azimuth turn-table. The circular tracks on which the telescope rotates are earthed at close intervals by copper tapes.

DRAINAGEOF THE BOWL 83. Very considerable quantities of water can be collected in the bowl, and a drainage systemhas been arranged with outlets arranged along the diameter at right angles to the elevation axis. These outlets pick up the water at any angle of elevation greater than 45" above the horizon, and are led by fall pipes into a

30"from vertical- _- 7' Maxlmum inclination ofmast cowards winch. 30" from vertical'

..L~Mait i earthed

~ ... ~

END ELEVATION

FIG. I.-TILTING-MAST TRANSIT TELESCOPE Stiffening plates "X"

Scale: ;in.=l fr

l Packed track------/ 1- 1- <-rnominal cm 4-Vnork.l ctrr----4'-rnomirial ctrs-4

KEY ELEVATION SIDE ELEVATION SECTION AA PLAN FIG. 7.-ONE OF THE FOUR MAIN AZIMUTH DRIVING BOGIES FIG. 2.-TYPICAL CROSS-SECTION OF RAIL FlXlNGS FIG. 3.-GENERAL ARRANGEMENT OF THE PNEUMATIC-TIRED WIND-VIBRATION DAMPING DEVICE FIG. 4.-SECTlON THROUGH THE CENTRE PIVOT SHOWING THE HEMISPHERICAL PINTLE, THRUST RACES, AND CABLE FEED H. C. HUSBAND The Institution of Civil Engineers. Proceedings, january 1958. William Clower & Sons, Limited: London.

QI of main

CROSS-SECTION OF BOWL ON TRUNNION AXIS

Scale: l in. = 50 R Feet 100 50 0 loo R I*tIIIIIIII I

Datum level: 99.752 I B 'i of centre pivot 20'-3" ctrs FIG. 10.- GENERAL ARRANGEMENT jof chord-girder rtdes +--- 279:-S between centre-lines of cl:ord girders b.-___-__. 316'4j' between ctrs of bogie supports (intersections on centre-line of bottom boom) END ELEVATION SECTION CENTRE-LINE WITH BOWL FACING ZENITH .-~ ---_. ~_~_.. ~ ~ . *- SIDE ELEVATION WITH BOWL FACING ZENITH ON

HALF-PLAN ON LOWER BOOM HALF-PLAN ON TOP BOOM STEELWORK7 SUPPORTING THE BOWL BASE STRUCTURE TOP HALF SHOWING UPPER SURFACE OF STEELWORK BOTTOM HALF SHOWING LOWER SURFACEOF STEELWORK

TheInstitution of Civil Engineers. Proceedings, January 1958. H. C. HUSBAND William Clowes & Sons. Limited: London.

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. HUSBAND ON THE JODRELL BANK RADIO TELESCOPE 81 trough mounted on theinside of the 270-ft-dia.brake rim. This trough in turn discharges at its lowest point, and the water is led into a circular trough surrounding the central pivot, whence the rain-water is led away into a nearby pond. When the bowl is tilted at less than 45” above the horizon a serious cascading of water is prevented by hoppers arranged at each end of the “drainage” diameter and connected into the main drainage system.

CONTRACTUALARRANGEMENTS 84. No single contractor could be found suitable for undertaking the whole project, and, in view of the developments in radio astronomy and the improve- ments to the original design which were incorporatedfrom time to time, it was certainly an advantage not to have been bound from the outset to proposals prepared 5 yearsbefore. Thirty differentfirms were employed, each having direct contracts with Manchester University for constructing or supplying, and in somecases erecting, the variouscomponent parts of the project.While every effort was made to use conventional machinery, practically every part of the telescope had to be designed in detail for itsspecific purpose, and more than 1,OOO drawingswere required. It will be sometime before the telescope is fully calibrated, but the present indications are that the radio efficiency of the instrument is greaterthan was anticipated,and no serious mechanicalor electrical difficulties have yet been experienced.

ACKNOWLEDGEMENTS 85. The Author wouldlike to acknowledgethe great efforts made by all concerned in the design and construction of the telescope, and their happy co- operation. In particular he would mention the work of the Resident Engineer, Mr P. D. Goodall,B.Eng., A.M.I.C.E. His thanks are due to Manchester University for permission to present the Paper.

APPENDIX I ANALYSIS OF EXPENDITURE, SEPTEMBER 1952-AUGUST 1957 (excluding acquisitionof site, radio receiving and transmitting equipment, and professional fees) f. Exploratoryborings . . 370 Foundationsand cable tunnel . . . 47,000 Construction of generator house . . 6,000 Dieselset alternator . .. . 9,000 Pow er switchboard. Power . . 3,700 M ain power cables power Main . . 8,000 Cable-turning arrangementsCable-turning . . 1,000 M ain azimuth bogies and anchors and bogies azimuth Main . . 34,000 M ain elevation trunnion bearingstrunnion elevation Main . . 11,000 Centre pivot Centre . .. . 4,000 Service lifts and hoists and lifts Service .... . 6,500 Construction of controlbuilding, including services . . 33,000 C.F. €163,570 8

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. 82 HUSBAND ON THE JODRELL BANK RADIO TELESCOPE B.F. E163,570 Supply, erection, and painting of structural steelwork (1,800 tons), supply and erection of aluminium cladding for three-storey motor rooms and labora- tories in each tower, motor-generator house, and suspended laboratory . 270,000 Experimental works (wind tunnel, strain gauges, and membrane, electrical, and heat-reflecting experiments) ...... 2,000 Reflecting membrane . ... . 15,000 Control system instrumentation,co-ordinate converter, and control desk . 14,000 Circularazimuth rail tracks ...... 11,000 Electrical and mechanical drivhg equipment ...... 51,000 Pneumatic-tired wind-damping bogies . . .. . 3,000 Hydrauliccontrols for adjustable wind-damping brakk . . . 400 Elevation clamp ...... 1,000 Adjustable aerial carrier ...... 1,000 Bearings and wind-damping mechadsmfor suspended laboratory : . 700 Painting the reflecting membrane . ... . 5,000 Drainage system for telescope bowl ...... 2,500 Roads, drainage, and site works . 4,500 Supply and installation of electric cabling Ad &craft'-warr&g lights : . 15,000 f559,670

APPENDIX I1 86. The tests to investigate the possibility of self-induced oscillations being set up by wind were carried out on a model of the bowl 2 ft 6 in. in dia. In the experiments it was considered sufficient to deal with single degree-of-freedom pitching oscillations and the relatively rigid model bowl rotating about its horizontal axis. The rigidity in flexure of the trunnion towers was sufficientlyhigh to preclude any dangerous oscillations other than the pitching motion which might arise through backlash in thedriving mechanism, ortorsional displacements of the bearings carrying the driving pinions. Fig. 11, facing p. 73, shows the heavy inverted welded portal framework carrying each pair of driving pinions, and sufficient rigidity has been given to this to hold the bowl steady without the added safeguard of the damping device on the 270-ft-dia. rim. It will be recalled that the rim is not operative when the bowl is completely inverted, but it is not intended that the bowl should ever be inverted during severe weather conditions. 87. The wind-tunnel model was mounted on flat spring hinges carried on angle-iron supports at the appropriate height above the floor of the tunnel. Additional elastic stitrness was provided by two helical springs attached between a lever arm andstranded cables which passed over pulleys to the spindle of a reduction gear. This gear, which was driven by an electric motor, enabled the small changes in incidence of the reflector,

B-O'when reflector axis is a= O'when reflector air IS in line with wind direction in line with wind direction BODY AXES SIDEViEW PLAN VIEW

FIG. 13.-NOMENCLATURE USED IN DESCRIPTION OF WIND-TUNNEL TESTS

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. HUSBAND ON THE JODRELL BANK RADIO TELESCOPE 83 due tothe aerodynamic pitching moment, to be corrected from outside the tunnel. The lever am1 served as a visual pointer to indicate amplitude and, in addition, provision was made for recording the movement on a cathode-ray oscilloscope. Changes of incidence were made by rotating the model on face-plates attached to the ends of the spring hinges. The wind speed was gradually increased and spasmodic displacements of the model were found, while steady or growing oscillations were set up under some conditions of test. The critical speed for the onset of steady oscillations was measured.

FIG.14.-TURNING MOMENT DUE TO WIND AT DIFFERENT ANGLES OF ATTACK ON ~/~O~-SCALEMODEL WHEN SUBJECTED TO A WIND SPEED OF 41.6 M.P.H.

88. Scruton has shown' that the critical reduced velocity V, for maintained oscillations has the same value for model and full-scale structures, provided that: (a) the model is geometrically similar to the full-scale structure; and (6) the ratio ZS,/pD* is the same for model as for full-scale. Here V"= Vc/ND where V, denotes the critical wind speed in feet per second N ,, the frequency of oscillation in cycles per second D ,, a typical dimension, here taken as the diameter of the bowl Z ,, the mass moment of inertia of the bowl about its axis of rotation S, ,, the logarithmic decrement dueto structural damping, defined as the natural logarithm of the ratio of theamplitudes of successive cycles of oscillations p ,, air density The model was not scaled for inertia and the correlation between the model and the structure applied only to critical wind speeds and not to amplitudes. 89. The first series of tests was made with a=45" and the value of j was varied between 0 and 180". Critical conditions were found only within the range O< j<30"; the lowest critical speed occurred at P=Oo. The second series of tests were therefore made with P=O" and a was varied within the range 0 < a < 360". 90. The model inertial and damping conditions for all these tests were: I*= 10.9 lb.-ftz Smz = 0.0251

1 C. Scruton, "An experimental investigation of the aerodynamic stability of suspension bridges with special reference to the proposed Sevem Bridge". Proc. InStn civ. Engrs, Pt I, vol. 1, p. 189 (Mar. 1952).

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. 84 HUSBAND ON THE JODRELL BANK RADIO TELESCOPE 91. It was concluded that themost favourable attitude for stability was a=45" when fl=OD. For this setting V,=2.94 and hence the critical wind speed for sustained oscillations on the full-scale telescope, Vcp, is given by: Vcp= 2% X 250 N =734 N ft/sec =734 X 3 ft/sec= 167 m.p.h. 92. The full-scale amplitudes of the spasmodic oscillations referred to above were not predicted by the tests, but the tests did indicate that certain positions of the bowl in relation to the wind were much more favourable than others. Full-scale tests are now proceeding on the telescope itself in order to compare the wind-tunnel results with actual practice. 93. To summarize, the wind-tunnel experiments gave: (a) the necessary information about total pressures on the bowl at all angles to the wind; (b) information about torque on the axis of rotation of the bowl, in practice both vertical and horizontal; and (c) general information regarding the tendencies to oscillate, whichled to the adoption of the 270-ft-dia. damping device as a practical safeguard, particularly to the driving gears. 94. However, because of the great size of the reflector, a further factor had to be considered which produces a greater torque on the horizontal axis than would be directly deduced from the wind-tunnel experiments. It is well known that wind velocities increase with the height above ground level. The wind-gradient formula which was applied to the torque calculations was: -=-vz 1+ log102 v100 3 where Z denotes height in feet.

APPENDIX 111

GENERAL ARRANGEMENT OF THE CONTROL MECHANISM 95. The system consists essentially of eight driving shafts arranged in pairs which control co-ordinates for the right ascension and declination, latitude and longitude, azimuth and elevation, and for repeating back to the instrument-panel azimuth and elevation positions of the telescope. Each shaft is driven through a gear-box from a variable-speed motor generator and terminates through further gearing in sets of indicator dials which show the co-ordinates in hours, minutes, and seconds for the right ascension, and in degrees and minutes of arc for the remainder. Repeater indication 96. The azimuth and elevation repeater shafts are driven by motor generators controlled by signals repeated back from the magslips on the main azimuth and elevation bearings of the telescope. A system of warning lights operates when the required setting of the telescope shown on the indicators of the main azimuth and elevation shafts dif€ers (by a specified amount)from the corresponding valuesgiven on the repeater-shaft indicators. Sidereal time drive 97. Allowance for the earth's rotation is applied to the right-ascension value by means of a synchronous motor regulated by an oscillator. The oscillator is locked to the master sidereal clock by correction impulses at 30-sec intervals. The time shaft driven by this motor is provided with commutators giving signals at 2-, 4-,10-,and 30-sec intervals, and at I-, 5-, and 60-min intervals. 98. Magslip resolvers computing hour angle are driven through a differential gear jointly by the right-ascension shaft and the clock motor. It is the hour angle thus determined which is used in the conversion of right ascension and declination into azimuth and elevation.

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. HUSBAND ON THE JODRELL BANK RADIO TELESCOPE 85 Correction in right ascension and declination 99. The continuous correction of these co-ordinates, when applied to the sun or planets, is achieved by periodic signals released from the sidereal time system. Hand controls are pre-set to select the required correction per hour or per day. All corrections are effected in units of 1 sec of time in the case of right ascension and 1 minute of arc in the case of declination, (e.g., 300 seconds of correction per day in right ascension by individual corrections of 1 sec at 4-min-48-sec intervals of time). 100. The signal closes an A.C. mains circuit to a motor which does one revolution and re-sets itself before the next impulse. 101. This revolution adds or subtracts 1 sec (or 1 minute of arc) to or from the co- ordinate shaft under correction,driving through a differential. Driving co-ordinates 102. There are two computer circuits which convert latitude and longitude, and right ascension and declination into azimuth and elevation and vice versa. Any one pair of the three pairs of shafts associated with these co-ordinates may be used to control the telescope, the controlling pair being selected by a manual switch. Whichever set of co!ordinates may be selected, the final control of the telescope is taken from the azimuth and elevation shafts. Scanning 103. In order to scan a given area of the sky any one of the six co-ordinates can be selected by manual switch, while its complementary ordinate is varied in predetermined steps. Manualcontrol gives the choice of steps, controlled either by reversal of direction of the other co-ordinate, or by time or continuous adjustment. The angular value of the central position in therange of the scan is set up on a magslip. The angular range of the scan is set on a calibrated potentiometer. When the error signal from a transmitter magslip mounted on the scanning co-ordinate shaft equals in magnitude the e.m.f. from the setting on the potentiometer, a relay is operated to reverse the drive to the radio-telescope motion in question. At the same time a further relay causes a step adjustment in the other co-ordinate (if required) to be applied. The choice of angular steps is t,4, 1,2, or 4 degrees of arc. Infinite variation of the angularspeed of scanning is provided by manual selection.

Setting and tracking 104. To set the telescope to a required bearing the appropriate co-ordinates are set up ona pair of indicators on thecontrol desk. Operation of a push-button then causes the co-ordinate shafts automatically to adjust themselves to the required position. The adjusting motion is carried out at maximum speed, which can be 30"/min in azimuth and in elevation. When the co-ordinate shafts reach the required bearing the.high- speed setting drive is automatically cut out, and normal correction for sidereal tlme 1s applied. Normal tracking drive speeds may be catered for over the range 15"/hour to l"/min. Accuracy of position can be maintained to +f in azimuth and elevation at the maximum tracking speed of 4"/min. The scanning drive automatically cuts out when the error signal from the computer or the position error from the repeaters exceeds a pre-determined value.

Re-setting 105. For mechanical reasons the rotation of the 250-ft telescope is limited to about 420" in azimuth and 190" in elevation. Automatic limits are provided to prevent the telescope from exceeding these angular movements in any circumstances. Should one of the srops, be reached while the telescope is automatically tracking, the following process IS initiated:- (a) Tracking drive is stopped. (b) Full-speed re-setting drive in the reverse direction in azimuth is engaged until the telescope has turned through a complete circle and has regained the original bearing plus the appropriate adjustment for time in both azimuth and elevation. (c) Scanning or tracking is recommenced.

Downloaded by [ UNIVERSITY OF SHEFFIELD] on [12/06/16]. Copyright © ICE Publishing, all rights reserved. 86 HUSBAND ON THE JODRELL BANK RADIO TELESCOPE Drive control 106. Coarse and fine (ratio 1 :36) set transmitter magslips on the azimuth and elevation shafts of the control equipment dictate the bearing to be assumed by the radio telescope. These are linked to corresponding receiver magslips mounted at the main bearings of the telescope. Error signals in the form of e.m.fs proportional to theangu- lar discrepancy between the transmitting and receiving magshps are amplified and apphed to regulate the exciting voltage in the fieldwindings of two D.C. generators. The rotors of thelatter are driven at constant speed by an A.C. mains motor whenever the control switches are closed. The voltages induced in the rotor windings are thus proportional to the errorsignals from the magslips, and operate the fourdriving motors connected in series on each axis at speeds proportional to these voltages and developing a constanttorque (up to a specified speed of 1,000 r.p.m.). Motor speeds can be further increased to 1,500 r.p.m. (corresponding to 30 degrees of arc per minute) by automatically weakening the field and causing some loss of torque. Control can be maintained down to a motor speed of 10 r.p.m. (corresponding to an angular speed of 5"/hour).

Accuracy of control 107. Other incidental equipment in the control room includes an automatic recorder of the actual position of the telescope in azimuth and elevation at any time. These instruments are entirely independent of the automatic control gear, and manual operation of the telescope can be achieved with great precision. A movement of approximately 30 in. on the au'muth turn-table represents about 1". The slow-speed driving motors with the large gear reductions (1 :23,518 in azimuth and 1:22,240 in elevation) make a movement of the bogies of about 0.10 in. possible, that is, about 12 seconds of arc. This is well within the limitations imposed on such a large structure because of the differential temperature effects and unknown deflexions. 108. .An automatic recorder of theposition of thetelescope in azimuthand elevation at any time is being designed. It is proposed to keep this record at predetermined intervals of time from 10 sec to 1 min by punching the information on a continuous paper strip in a simple code. Standard equipment is available for transmitting automatically this code into printed schedules of time and angles, and the master strip will also be suitable for feeding into the Manchester University electronic computer for carrying out automatic analysis. 109. A large celestial hemisphere having a light spot indicating the beam direction of the telescope, directed by two suitably coupled magslips, will give a rough instan- taneous check on the part of the heavens under observation.

The Paper, whichwas received on 16 August, 1957, is accompanied by twenty-one photographs and seven sheets of drawings, fromsome of which the half-tone page plates and folding Plates1 and 2 have been prepared, and by three Appendices.

Written discussion on this Paper should be forwardedto reach the Institution before 15 March, 1958, and will be published in or after July 1958. Contribu- tions should not exceed 1,200 words.-SEC.