The new United Kingdom Infrared Telescope in Hawaii C. M. Humphries and T. E. Purkins

The recently completed 3.8 metre United Kingdom Infrared Telescope is the largest in the world designed specifically for use at wavelengths from the near infrared to submillimetres. It embodies important innovations in design and construction-applicable to the building of other large telescopes in the future-that have enabled weight and cost to be substantially reduced without loss of performance. Situated on Mauna Kea, Hawaii, at an altitude of 4200 metres it is providing British astronomers with a much needed facility for observations in the

rapidly expanding field of infrared astronomy.

Modern large telescopes, weighing up to several hundred tonnes yet capable of pointing to any part of the sky with arc second accuracy under computer control, are necessarily sophisticated and expensive items of equipment which put astronomy in the league of ‘big’ science. Unlike other sciences, astronomy has but one experimental techniqtie for studies outside our own solar system and that is by collecting and analysing the often tiny amounts of radiation received from objects far out in space. To study faint and distant objects out to the edge of the observable universe, the requirement for the telescope to have the largest possible collection area is therefore fundamental. Not only can fainter objects be studied but the observing time can be utilised more efficiently through shorter integration periods.

At present the observing time available on all existing large telescopes is heavily oversubscribed, a situation which is likely to be alleviated only if the cost of building large telescopes can be reduced. The United Kingdom Infrared Telescope (UKIRT) has made a significant step in this direction without sacrificing quality of performance.

The key to the cost cutting process is a reduction in overall weight. This has been achieved first by using a thin primary mirror, which would not have been considered feasible a few years ago; and second, by using a relatively lightweight supporting structure combined with a short tube length. Thus, within a fixed budget, UKIRT has been built with an aperture much larger than would otherwise have been possible.

The thin primary mirror The light collecting capability of a telescope depends upon the area of the primary mirror, the first optical element encountered by the incoming radiation. Traditionally, primary mirrors have been produced right up to the present

C. M. Humphries, B.Sc., Ph.D.

Was dppo~nted Project manager of the U.K. Infrared Telescope in 1977 and has been a memberof the research staff of the Royal Observatory. Edinburgh since 1966. His postgraduate work was in the field of ultraviolet molecular spectroscopy and his research interests include the study of the physical properties of stellar and interstellar environments.

T E. Purkins.H.N.C..F.R.A.S.

Was the Deputy Project Manager for the UKIRT project, having joined the Royal Observatory, Edinburgh in 1967. He is a mechanical engineer with experience in the fields of cryogenics, vacuum physics. optics, and space research.

Endemm,r, New Series Vohms 4, No. 4,1999 ( ~OPergamon Prom, Printed in Great Britain)

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day with an edge thickness equal to approximately one sixth of the diameter. An act of continuing faith in what went before rather than an analytical approach to determining the engineering limits of glass technology, this was always considered necessary to guarantee the structural stiffness needed to obtain the surface profile accuracies required (within a few millionths of a centimetre over a diameter of several metres) and to provide stability during use. As a result of experience gained with UKIRT we now know that large telescope mirrors can be less than half the thickness of earlier mirrors for a given diameter, and hence less than half the weight. This saving in weight amounts to several tonnes in mirror material alone.

Improvements in recent years have also been made possible by the development of low expansion materials which have replaced the older soda-lime and Pyrex glasses. The new glass ceramic materials such as CER-VIT or Zerodur have an expansion coefficient typically less than 1 part in 10 million per degree Celsius, some 30 times better than that of Pyrex. Thermal effects at this level may almost be ignored. To an astronomer, better thermal stability of the mirror profiles means better imaging properties, resulting in an improved signal to noise ratio which in turn allows objects to be studied at greater cosmic distances. There are two limits beyond which the image size cannot be reduced - the theoretical limit imposed by aperture diffraction of the telescope and the practical limit for a ground-based telescope set by turbulence of the Earth’s atmosphere. For large telescopes the latter effect is by far the greater of the two but can be minimised by careful choice of the site at which the telescope is located. Thus modern telescopes are generally situated at remote and elevated locations.

The 5.1 metre Palomar and the 3 metre Lick telescopes in California departed from earlier designs by using ribbed primary mirrors to save weight but technical complexities in their production and mounting prevented wide acceptance of this technique. In the new U.K. telescope, the paraboloid-shaped f/2.5 primary mirror (figure 1) is a solid CER-VIT disc with a centre thickness of only 19 cm and a diameter-to-edge thickness ratio of 13 :l, giving it a mass of 6.5 tonnes (instead of 15 tonnes). Similar diameter to thickness ratios have been used for the interchangeable convex secondary mirrors located at the top of the telescope tube.

The decision to use a thin primary mirror for UKIRT was not taken without some risk since at that time it was not known precisely how well such a mirror would perform nor

whether it could be supported adequately during the optical optical polishing should be continued to reach, if possible, a polishing stages. The original image specification was for 1 arc second image capability.

Figure 1 The thin 3.8 metre diameter primary mirror of UKIRT after aluminising.

an encircled energy diameter of 3 arc seconds containing 98 per cent of the incident light. Experience showed later that

Figure 2 Image profile of thedouble starADS 12 145 obtained with UKlRTona typical nighton Mauna Kea.Thepeaksare separated by 4.3 arc seconds showing that the individual images havea diameterof -1 arc second which includescontributions from atmospheric seeing and instrumental effects of the vidicon detection system.

this estimate was conservative and that the difficulties in grinding and polishing the thin mirror were not as severe as had been anticipated. At that stage it was decided that the

The primary mirror support system As a telescope is pointed around the sky, the changing gravitational force on a large diameter mirror, even one of

Normolised spatial frequency

Figure 3 Modulation Transfer Function (MTF) curvesfor the UKIRT optics at wavelength 0.5 u m (visible region) and wavelengths 1,2,4and 8um in the infrared. Each MTF curve shows how the imagecontrast varies for objectsof different spatial frequency. The MTF curves are compared with a typical atmospheric

MTFat Mauna Kea, Hawaii,and with the theoretical aperture diffraction MTF. It can be seen that the UKI RT optics are diffraction limited for wavelengths above 8 urn. (To obtain the absolute spatial frequencies (cycles/arc second) for each curve, divide the normalised frequenciesshown by 0.066A with wavelength A in u m.)

conventional thickness, is sufficient to distort the precision surface profile and hence the telescope image unless the mirror is supported adequately. In this context, ‘adequate’ means that the mirror has to be uniformly floated, axially

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and radially, for all attitudes of the telescope even when pointing low down on the horizon. With a thin primary

pads all of which are operated at the same pressure. Load sensing cells in contact with the rear surface of the mirror

mirror this becomes even more of a problem. provide the servo control which adjusts the air pressure to

4a

Figure 4 The development of the English equatorial telescope mounting, using a cradle or yoke to hold the telescope tube. One of the earliest examples of this type of mounting was Jesse Ramsden’s 9 cm refractor (4a (By courtesy of the Directorof the Science Museum, London)) which was completed for Sir George Shuckburgh in London in 1793.The Hooker 2.5 metre telescope (4b (By courtesy of the Hale Observatories)) at Mt. Wilson, California was completed in 19 17, and was at that time the largest in the world. The 3.8 metre U.K. Infrared Telescope (4~) was completed in Sheffield in 1977 and installed at Mauna Kea, Hawaii in the following

year.

Early work at Imperial College, London, indicated that rn airbag support system would not be satisfactory for UKIRT. The method finally adopted was relatively straightforward in engineering terms, yet elegant. The primary mirror rests on 80 servo-controlled pneumatic 134

compensate for varying tilts of the telescope. When floating, the mirror is raised by 5 mm relative to its rest position. Radial support is provided by 24 counterweighted lever arms which act on the mirror edge. When supported in this manner, the mirror profile stays accurate to & l/30 of a

wavelength (equivalent to 200 Angstroms or two millionths of a centimetre) and has proved to be exceptionally reliable and simple to operate.

Not only has the thin mirror concept now been proved sound for its application in an infrared telescope but results of tests performed during the final stages of the optical work, and commissioning of the telescope in Hawaii, show that the performance of a large, thin mirror telescope can

which required a short tube length, it was possible to keep the total weight of the telescope down to only 80 tonnes. Similar sized telescopes of conventional design typically weigh more than 300 tonnes. The telescope has an equatorially mounted English yoke and a Serrurier truss open tube structure.

The so-called English type of equatorial mounting dates back more than 200 years. In its original form it consisted

4b Tlw Houkcr 2.5 tnetre telescope at Mt. Wilson, California

approach closely, or even match, that of an optical telescope built on traditional design principles. At its f/9 Cassegrain focus UKIRT gives an image in clear seeing conditions of diameter less than 1 arc second (90 per cent of the incident radiation), and this includes the error contributions from the primary and secondary mirrors and the mirror support systems (figure 2).

At infrared wavelengths above 8 micrometres the f/9 image is diffraction limited (figure 3). Had it been required, the optical performance could have been improved even further during the final polishing stages but for its application in the infrared there was no advantage to be gained by this. At the f/20 coude focal plane, which utilises three additional mirrors, the image diameter in clear seeing conditions is 1.25 arc seconds.

The telescope structure By using lightweight optics and a fast f/2.5 primary mirror Em jr:4 *

of a spindle aligned along the polar axis, suspended by bearings held in two vertical support columns; the telescope tube was attached to the polar spindle but was offset and counterbalanced on the opposite side so that it could rotate freely about the declination axis. Later it was realised that extra stiffness of the structure could be obtained by supporting the telescope tube centrally within a cradle or yoke which rotates about the polar axis (figure 4). Although the sky coverage around the pole is reduced with this type of mounting, it has been used extensively in applications which require low flexure characteristics. The portion of sky lost around the pole becomes less important at low latitude sites, as for example in Hawaii (19”5O’N).

Several novel features have been incorporated into the design of UKIRT which together with the use of modern materials and cost-effective manufacturing methods add up to good engineering. The lightweight structure has allowed the use of roller bearings on the main drive axes instead of

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hydrostatic bearings which are considerably more expensive to manufacture and maintain. Thrust forces along the polar axis are taken up at the lower south column where a pressurised, toroidal steel capsule distributes the load equally over the bearing area. A similar arrangement has been adopted for the thrust bearings which support the telescope tube along the declination axis. These bearings

each axis. The original telescope specification called for a pointing accuracy of 30 arc seconds so that this requirement has been exceeded by a wide margin.

Earthquake protection is also included in the design to prevent the build-up of dangerous stresses which could otherwise damage the structure. The north and south support columns rest on 6 cups containing 60 steel balls

4c The 3.8 tnetre U.K. Infrared TelescoDe- Hawaii

are kept preloaded by tensioned tie beams attached to the declination beams that extend across the base of the telescope tube.

The telescope provides the standard range of sophisticated features to which astronomers are now accustomed. These include computer control, Camac electronic hardware interfaces, facilities for nodding and raster scanning motions in the sky, video monitoring of the sky field, and offset guiding facilities. The telescope is driven by a pair of d.c. torque motors on each axis which act either together, as in fast slewing, or in opposition, to remove backlash when tracking at the sidereal rate. The complete range of speeds for slewing, tracking, scanning and nodding is achieved using a single set of gears consisting of a 100/l reduction gear box and a main spur gear.

The sky pointing accuracy has a capability of about 5 arc seconds r.m.s. and is limited at present by the control servos which use a single 20-bit digital position encoder on 136

which therefore carry the weight of the whole telescope. The complete assembly is located laterally by two 8 mm diameter brass pins which are designed to shear should the horizontal acceleration exceed 0.3g. This has occurred on three occasions so far since the installation in Hawaii commenced in January 1978. The earthquakes which have caused the shear pins to fracture have been of magnitude b4.8 on the Richter scale with epicentres located 70

kilometres away on the south east rift zone of the active Kilauea volcano. Mauna Kea itself is designated a dormant volcano, the last activity there having occurred about 4500 years ago. During a severe earthquake the telescope can move laterally by as much as 5 mm but it takes only one hour to restore it to its original position with the polar axis aligned accurately along the north-south meridian.

The dome, building and site The Mauna Kea site (figure 5) on theBig Island of Hawaii

lies at an altitude of 4200 metres (13800 ft) and is Infrared observational techniques character&d generally by good atmospheric seeing The measurement of infrared radiation reaching us from conditions. The low amount of water vapour at that altitude faint and distant objects poses difficulties which require makes Mauna Kea a particularly favourable site for special observational techniques. All bodies of temperature infrared observations, since water vapour absorbs strongly greater than a few degrees above absolute zero are at certain infrared wavelengths. However, the mean characterised by thermal infrared emission and as such the barometric pressure at 4200 metres is only 60 per cent of Earth’s atmosphere through which the observations are

Figure 5 Perched on the arid 4200 metre (13800 ft) cinder cone summit of Mauna Kea are some of the world’s most powerful and advanced telescopes. The largest of these is UKlRTwhich is housed in the right hand (and smallest) dome of the three buildings on

the middle ridge.To the left of UKlRT is the 2.2 metre University of Hawaii telescope, and on the extreme left, in its onion shaped dome, is the 3.6 metre Canada-France-Hawaii telescope. The dome in the foreground is that of the NASA 3 metre infrared telescope.

that at sea level, thus reducing correspondingly the amount of oxygen available for use by the human body. Working conditions at the summit are arduous, therefore, and symptoms of high altitude sickness are sometimes experienced. Acclimatisation at a lower altitude is recommended before commencing a period of work at the summit, and catering and sleeping facilities are available for astronomers in a camp at the 2800-metre level.

The building and dome which house the telescope ‘Ifigures 6 and 7) are of minimum size and provide only the basic essentials for astronomers to work, a feature which in itself contributed greatly to the cost-cutting of the project as a whole.

made and the telescope itself both emit at infrared wavelengths. Thus, very faint astronomical sources have to be detected against a background which can be much brighter than the object being studied (figure 8).

These difficulties can be minimised by proper design of a telescope such as UKIRT for use in the infrared. Although UKIRT is an optical telescope which can be used perfectly well at visible wavelengths it has additional design features which enhance its performance in the infrared.

In the standard Cassegrain system, the primary mirror has a concave paraboloid surface and the secondary mirror at the top of the telescope tube has a convex hyperboloid surface (giving a beam which passes through the central ,

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hole in the primary mirror). The convex secondary mirror for UKIRT has a diameter which has been deliberately cut undersize so that it becomes an aperture stop for the system. In this way no thermal or scattered radiation from the structure of mirror cell can be seen by the detector at the focal plane. This applies to each of the interchangeable secondary mirrors which give f/9 and f/35 Cassegrain beams and also to the f/20 coud& beam (which produces a

Relatively cool, giant stars in our own galaxy, some enclosed in circumstellar clouds of dust which re-emit the trapped thermal energy, are known to be sources of infrared radiation. So, too, are the nuclei of galaxies which may lie millions to thousands of millions of light years away from our own Galaxy, and yet emit surprisingly large amounts of infrared radiation by a mechanism which is not yet understood. Then there are the massive clouds of

Figure 6 The dome which houses the U.K. Infrared Telescope.The cloud level in this photograph

is some 2000 metres below the summit of Mauna Kea.

final beam having a fixed direction in the building independent of the direction in which the telescope is pointing). A small flat mirror held obliquely in front of the convex secondary also deflects radiation away which would otherwise allow the detector to receive radiation from itself.

Infrared measurements are generally made using chopping techniques to improve the signal-to-noise ratio. Thus the telescope successively views the object to be studied and then a portion of sky adjacent to the object, and so on. This allows the sky background radiation to be subtracted and also eliminates any stray radiation outside the chopping frequency. With UKIRT it is possible to perform the chopping either by means of a small mirror near the focal plane or by oscillating the lightweight f/35 secondary mirror itself.

Other techniques are also necessary in the infrared. In particular, it is necessary to cool the solid state detectors to cryogenic temperatures (liquid nitrogen or liquid helium) to improve the detection sensitivity.

I nf rared astronomy In recent years developments in observational infrared astronomy have begun to parallel advances made in other regions of the spectrum. New sources of radiation have been discovered and new mechanisms have been postulated to explain the physical processes which take place in these distant regions. 138

interstellar gas and dust which are the sites of galaxy and star formation and which may hold clues to the origin ofthe universe and even of life itself. The solid matter provides the surfaces upon which hydrogen and other molecules form; of the 50 or so interstellar molecules identified so far, all have specific absorptions or emissions at infrared or radio frequencies. Only ten years ago such a complex array of interstellar molecular processes was totally unsuspected. It may also be possible to obtain an improved galactic distance scale by correlating infrared, dust-free magnitudes with measured velocity dispersions. These are but a few of the fundamental areas to which astronomers will be devoting themselves with UKIRT. The most exciting discoveries are no doubt yet to come.

Future developments The past 50 years have seen little change in the light collecting power of the world’s largest optical telescopes. Design of the 5.1 metre Palomar telescope begar, in the late 1920’s, although it was not until 1948 that it was completed. Currently the largest in the world is the Russian telescope completed in the northern Caucasus in 1976, which has an aperture of diameter 6 metres. This situation will change dramatically within the next ten years or so. Proposals in the United States are already being made for ground-based optical telescopes having apertures equivalent to diameters up to 25 metres, an increase of nearly 20 in collection area or a gain of more than 3 stellar

magnitudes on the logarithmic scale used by astronomers. this is by dividing the total aperture into separate segments, Although the engineering problems in achieving such large each using the thin-mirror technology pioneered so structures are formidable, the most likely way of achieving successfully by UKIRT. This could take the form of a

Figure 7 The U.K. Infrared Telescope inside its dome on Mauna Kea with mirror covers ooen

Figure 8 Oneof the first spectra obtained with UKIRT. The spectrum shown here between 1.5 and 2.5~ m is that of a Wolf Rayet carbon star h-ID 16523) and the main peaks are due to neutral helium (2.06f1m),a blendof neutral and ionised helium (1.86pm) and components of unknown origin at 1.73 and 2.4 1 p m.

single mosaic or an array of smaller diameter mirrors on a common mounting. In either case, large telescopes being used by the year 2000 will surely dwarf those of today.

Acknowledgements The excellent design and performance of the U.K. Infrared Telescope is a tribute to the skill of the British engineering firms who were involved in the project. Hadfields Ltd. of Sheffield were responsible for the design, manufacture, and installation of the telescope structure; the drive and control system; and the primary mirror support system, to meet performance requirements specified by the Royal Observatory, Edinburgh. Grubb Parsons (NE1 Parsons Ltd.) were responsible for the optical engineering and for thedevelopment work leading to the production of the thin mirrors. Credit goes to the teams led by Mr D. Hickinson (Special Projects Division, Hadfields Ltd.) and Mr D.

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Brown (Grubb Parsons) and to Mr D. J. Walshaw (design consultant to Hadfields Ltd.) for their expertise and for the many innovative ideas which they incorporated. Several other organisations were involved in the project including Imperial College, London; the Institute of Astronomy, Cambridge; and Heriot-Watt University, Edinburgh.

The entire project was financed by the Science Research Council and the Royal Observatory, Edinburgh was responsible for management of the construction and commissioning phases and is responsible also for the operational phase.

Bibliography Corm, G. K. T. and Avery, D. G. ‘Infrared Methods’. Academic Press,

London. 1960. Reddish V. C. ‘Evolution of the Galaxies’. Oliver and Boyd, Edinburgh

and London. 1967. Brancazio, P. J. and Cameron, A. G. W. ‘Infrared Astronomy’. Gordon

and Breach, New York. 1968. Allen, D. A. ‘Infrared The New Astronomy’, Keith Reid, Shaldon,

Devon. 1975. Pacini, F., Richter, W., and Wilson, R. N. Proceedings of ES0

Conference: Optical Telescope of the Future, Geneva, 12-15 December 1977.

Setti, G. and Fazio, G. G. ‘Infrared Astronomy’. D. Reidel Publishing Co, Dordrecht, Holland. 1978.

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