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 study radiations 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 a large alt-azimuth instrument. From the outset the advantages in the simplicity of control of a polar-axis mounting werefully appreciated, but it is difficult 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 para- boloid 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 dia- meter 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 con- sideration. 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 short- focal-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- panded metal 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 wave- lengths of 1 m or more would be tolerable with a square mesh of 10-gauge wire at 4-in.centres. Subsequent investigations showed that anything more 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 resist- ances 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