skip to content

Institute of Astronomy

36-Inch Telescope

The telescope was built in 1951-55 by the now-defunct firm of Sir Howard Grubb, Parsons & Co. at Newcastle-upon Tyne. It replaced a much older telescope of the same aperture, which was brought to Cambridge from South Kensington when the Solar Physics Observatory moved here in 1913. That telescope was returned to its owners (The Science Museum) before the new one was installed; the Director of the Observatories at the time (Professor R.O. Redman), who in his youth had made substantial use of the old telescope, always averred that it should never have left the Museum!

The 36-inch, which is thought to be the largest telescope in the country, has three possible focal stations. There is a prime focus with a focal ratio of f/4.5; the primary mirror is a paraboloid, so no corrector is needed to obtain good images on the optical axis.

In practice the prime focus has been little used: the telescope is large enough to make access to the focus difficult from the side of the tube. The other possible foci are coude, with a choice of two focal ratios, f/18 and f/30. The coude arrangement is unusual inasmuch as the light beam is directed UP the polar axis rather than downwards: that permits the shorter focal ratio to be exceptionally short for a coude, and results in a focus at a level near to that of the telescope, which is somewhat convenient for a lone observer who needs to operate both the telescope and whatever auxiliary equipment is placed at the focus. On the other hand, the arrangement lacks part of the advantage of a conventional coude focus, which is often in a basement that enjoys good passive thermal stability (and, from the point of view of the observer personally, protection from wind and extremes of cold!). Until recently the f/18 focus has been the favoured option, but new equipment that for the first time utilizes the f/30 arrangement has now been brought into use. The f/30 focus is just within the dome, high up to the north of the telescope, and its use involves a further reflection. In the present application, that reflection takes place close to the focus, and the beam is turned vertically downwards by successive internal reflections within two right-angle quartz prisms cemented together. The initial image is re-imaged at a focal ratio of f/14.5 at the position required for the auxiliary equipment. A simple plano-convex quartz field lens is cemented to the exit face of the quartz-prism assembly to image the telescope aperture upon the re-imaging lens.

In the early years of its operation, the telescope was used to send starlight into a spectrometer where the light intensities in several wavelength regions. which were accurately defined by masks in the focal plane of the spectrum, could be inter-compared. The intention (only partly realized, owing to the previously unrecognized individuality of the various stars) was to obtain astrophysically significant information about the chemical abundances and atmospheric characters of the stars surveyed. Three successive spectrometers, of progressively increasing size, resolution, and sophistication, were used in that effort.

The third spectrometer was further developed, some 30 years ago, to measure the doppler shift in the stellar spectra observed with it. It did that by means of a much more elaborate mask in the focal plane: instead of having just a few windows to isolate discrete bands of wavelength that were separately measured, it had a mask containing hundreds of narrow windows placed so as to match absorption lines in stellar spectra, the light from all of them being measured together by a single photomultiplier. The position of the spectrum could be sensed, and its doppler shift thereby accurately measured, by scanning the mask in the wavelength coordinate and looking for the more or less dramatic decrease in light transmission that occurs when every window is occupied by its corresponding absorption line. The plot of transmitted light against displacement of the mask is the cross-correlation function of the mask with the star spectrum, and has a pronounced minimum at the position of register.

That instrument, the orginal 'radial-velocity spectrometer', was the first application of cross-correlation to radial-velocity (or, indeed, any other astronomical) measurement. The method has now been adopted almost to the exclusion of the previous procedure involving the measurement of the positions of individual absorption lines, and has revolutionized the radial-velocity field, allowing observations to be made with enormously greater precision and sensitivity than was possible before. A few years before the instrument was brought into operation, a compilation of all known stellar radial velocities included only about 70 stars of 7.0 magnitude or fainter whose radial velocities were supposed to be known to an accuracy of 1 km/s; more and fainter stars than that were sometimes observed to at least that accuracy on individual nights in Cambridge - a site that has not generally enjoyed much of a reputation for its excellence for observation. The original instrument remained in operation for 25 years, during which it provided most of the data for about 200 published scientific papers, and when it was de-commissioned it went straight to the Science Museum as an historic instrument. There were delays in commissioning its successor, which is however operating now and provides sensitivity, precision and convenience well beyond those of the pioneering instrument.

Northumberland Telescope


The Northumberland is the only remaining large instrument from the early days of the University Observatory, and is preserved because of its great historical interest. It was for some years one of the world's largest refracting telescopes with an accurate clock-driven equatorial mounting to follow a star in its diurnal motion across the sky.

The Duke of Northumberland, later Chancellor of the University, indicated his wish to present a large telescope to the recently founded Observatory in 1833, and was enthusiastically encouraged by the Director, G.B. Airy.

The lens was an achromatic doublet of 11.6 inches clear aperture and focal length 19ft 6in, made by Cauchoix of Paris. Airy recognised that the mounting needed to be of great rigidity and adopted the 'English' form (of which the telescope is indeed one of the prototypes). The polar axis is composed of two massive triangular prisms of ingenious design, in which the components are kept in permanent tension and compression to attain the desired resistance to torsion and flexure.

The main structure was built by the engineers Ransomes of Ipswich, and the fine mechanical work by the London instrument makers Troughton and Simms. The polar axis frame and the telescope tube are of Norwegian fir. The observing chair which gives access to the eyepiece in all positions is the original. The polar axis points upwards to the North celestial pole, at an altitude equal to the latitude of the Observatory (+52degrees 13minutes). A small electric motor, now replacing the original mechanical clock, turns the polar axis once in a sidereal day. Once directed to a star the telescope tube remains in a fixed orientation in space, while the Earth turns beneath it.

A program of automation was started at the end of 2001 to provide high-precision coordinate capability.

The original Cauchoix lens is not (by present day optical standards) very good and it is now in store. The optics on the telescope are modern: a 12 inch aperture visual achromatic doublet designed by Dr R.V. Willstrop of the Institute and constructed by the local firm A.E. Optics Ltd. was installed to mark the 150th anniversary of the telescope.

The steel dome covering the telescope was made by Cooke, Troughton and Simms Ltd. of London & York in 1932 to replace the original wood structure which had become increasingly dilapidated after 96 years.

The telescope was last used in a regular Observatory research programme, for the micrometrical measurement of double stars, in the 1930s. It continues, however, to be actively used for visual observations by members of the University Astronomical Society (founded 1942), who have an Observing Guide on their website, and for Public Observing on clear Wednesday evenings in the winter months, and so continues a useful life of now over 150 years.

Thorrowgood Telescope


The 'Thorrowgood Telescope' was built by T. Cooke & Sons of York & London in 1864, as recorded by the small embossed plate at the top of the pillar. The achromatic doublet object glass has an aperture of 8 inches and a focal length of 114 inches (f/14), and is of excellent quality. The mounting is an example of the 'German' form of equatorial mounting, named after Wilhelm Struve's telescope, the 'Great Dorpat Refractor' of 9.5 inches aperture (1824). Many large refracting telescopes have used this form of mounting, including the Lick 36-inch and Yerkes 40-inch telescopes. The entire sky is accessible to a telescope on this type of mounting. When an object is in the eastern sky it is most convenient to have the tube of the telescope on the west side of the pillar, and for an object in the western sky the tube should be to the east. When an object crosses the meridian between the zenith and the pole, it is usually necessary to interrupt the observations to reverse the position of the telescope tube. This problem does not arise with an 'English' mounting, such as the Northumberland telescope. As on the Northumberland, the sidereal drive is now provided by a small electric motor, but here most of the original mechanical driving clock can still be seen on the North side of the pillar.

The history of this telescope is known in considerable detail. The first owner was Rev. William Rutter Dawes, described as 'eagle-eyed' when he was presented with the Gold Medal of the Royal Astronomical Society for his work on the measurement of double stars (1855), most of which had been done with telescopes made by the American optician Alvan Clark. Dawes started to use this telescope on 1865 April 13, but died in 1868 on February 15. He was qualified in medicine before taking Holy Orders, and in his obituary it is recorded that he ''was ever ready to impart gratuitous advice to the sick'' - a form of generosity more liable to misunderstanding now than then. In 1867 an attempt to buy the telescope for the Observatories was made by J. C. Adams. He argued that it was of superlative quality, superior to the 9.5-inch at Dorpat and to Herschel's 18-inch reflector at the Cape. Dawes had asked only 580 pounds but after four months the Observatory Syndicate withdrew its provisional approval.

George Hunt (1823 - 1896) bought the telescope in 1869. His father and uncles wished him to become a member of their business, but after eighteen months' trial he found it so distasteful and the loss of time for his literary studies so trying that he determined to give up business. After his father's death, he spent most of his time with an uncle, who eventually made him his heir. It is not known how much use he made of the telescope, but he published only one paper, "On the Identity of the Triple Star H I."

The first long fruitful period in the history of the telescope was in the ownership of William Henry Maw (1838 - 1924). Left an orphan at 16, Maw studied engineering and became an excellent draughtsman. Employed by the Eastern Counties Railway, he became head of the drawing office, aged 21. In 1865 he left the then Great Eastern Railway to join in the founding of the technical journal Engineering, of which he was an editor until his death. He was a founder member of the British Astronomical Association in 1890, at a time when the Royal Astronomical Society refused Fellowships to ladies, and became Treasurer for many years and President for a standard term of two years. Maw erected this telescope at his house at Outwood, Surrey, in 1896, and used it for measurements of double stars.

The telescope was offered for sale in 1927 by C. Baker of 244 High Holborn, London, at a price of 500 pounds. An advertisement published in the Journal of the British Astronomical Association listed the extensive range of accessories, including 10 astronomical eyepieces, which we still have.

William John Thorrowgood (1862 - 1928) was the last private owner of this telescope. He had spent his professional life in the service of the Southern Railway. After his retirement in 1927 he installed the telescope at Wimbledon, but had little time to enjoy it. He bequeathed it to the Royal Astronomical Society, which, having no suitable place to erect it or to store it, offered it to Professor Eddington, Director of the then University Observatory, initially for a period of 10 years. It was erected on its present site early in 1929. An engraved brass plate, made recently in our Workshops, serves to remind us that the telescope is here on extended loan.

The telescope is now used by members of the University Astronomical Society and is also available on Public Observing Nights. Double star measurements are currently being made by Mr. R.W. Argyle of the Institute of Astronomy.

Three Mirror Telescope

The Three Mirror Telescope (3MT) has been developed at the Institute of Astronomy by Dr. Roderick Willstrop. Its optical design is unique because it is the only form of telescope which combines the three advantages of a wide field of view, very small sharp images, and all-reflection optics.

Reflecting telescopes have been built with apertures up to 10 metres, but they have fields of view limited to 40 arc minutes (Ritchey- Chretien two-mirror design) or 1 or 2 degrees with a 3- or 4-lens Wynne corrector near to the focus of the main mirror. Schmidt cameras can give fields of view of 7 or 8 degrees, but their apertures are limited to about 1.3 metres because they use a thin glass lens large enough to cover the whole aperture.

The 3MT has a field of view 5 degrees in diameter, and the ray- theoretical image size is less than 0.33 arc seconds everywhere, and less than 0.1 arc seconds over the central 1 degree of the field. Because no lenses are needed the images are perfectly achromatic, and furthermore it is possible, in principle, to build this telescope with a larger aperture than any Schmidt camera.

The design is based on a simpler one discovered in 1935 by the French optician Maurice Paul and rediscovered independently in 1945 by the American James Baker. The original Paul-Baker design had a paraboloidal primary mirror, a convex spherical secondary mirror and a concave spherical third mirror. If the aperture of the primary mirror was f/4, it would have given acceptable images, and a field of view of about a degree.

The good performance can be explained as follows: if the second mirror were a convex paraboloid (instead of spherical) the light from a distant star would be made parallel again after the second reflection. (This arrangement of two coaxial and confocal paraboloidal mirrors was described by Mersenne in 1636.) The third mirror would then also have to be paraboloidal to focus the light, and the field of view of the whole system would be no larger than that of a single paraboloidal mirror. The essential feature of the Paul-Baker design is that the second mirror is spherical, so the light is not exactly parallel after the first two reflections, but is deviated in just the same way as by the corrector lens of a Schmidt camera. Then the third mirror must also be spherical to focus the light, and a large field of view with sharp images is obtained. This telescope has also been called the Mersenne-Schmidt.

Baker suggested that the Hale 200-inch (5.08 metre) telescope might be given a wider field by using two auxiliary mirrors. It would not have been acceptable to enlarge the central hole in the irreplaceable 200-inch primary mirror. The third mirror, 2.1 metres (84 inches) in diameter and weighing one or two tons, would therefore have been mounted directly above it, so the system was never built. The field of view could have been just over 1 degree. In the f/3.3 Hale telescope, it would have been necessary to modify the shape of the secondary mirror to retain small, sharp images.

The field of view has been increased here to 5 degrees by placing the third mirror behind the primary, and by increasing the relative aperture from f/3.3 (in the 200-inch proposal) to f/1.6. To retain excellent image quality with this large aperture and field it was also necessary to make small changes in the shapes of all three mirrors from the paraboloidal and spherical shapes that were satisfactory in the original Paul-Baker design.

Two versions of the 3MT have been built; a working model of 100 mm (4 inches) aperture (completed in 1985) has been used to take some photographs of the sky, and a prototype of 0.5 metres (20 inches) aperture (1989) has been used both for photography and with a CCD, and to test methods of aligning the mirrors so as to obtain the clearest images. The sky in Cambridge is nowadays too bright for front-rank research on faint extended objects with such a fast camera as these. The justification for building the prototype was to demonstrate that the design works as well as the Schmidt camera, and so to lead to the building of larger cameras of this type on much darker sites.

The prototype 3MT is on an equatorial mounting which accounts for nearly half of its height of 2.5 metres. The focal length is 800 mm, and the tube length 1.2 metres, which compares very favourably with the Schmidt camera in a neighbouring dome: this has a tube more than 4 metres in length, although its light-collecting area is very little larger than that of the 3MT. The 0.5 metre 3MT stands in a simple wooden building about 3.6 metres (12 feet) square, with a roof that runs off on rails to the North. It has been provided with a control system.

A proposal, by a committee of the Royal Astronomical Society in 1986, to build a telescope of this type with an aperture of 5 metres was not pursued after a committee of the Science and Engineering Research Council in 1987 recommended that UK funds should be used to purchase a one-quarter share of the 8-metre Gemini telescopes. However, a proposal by J.R.P. Angel of the University of Arizona to build a telescope of this type of 8 metres aperture, slightly modified to allow the use of many CCDs, was approved by the U.S. National Science Foundation in 2000.

Star field, 30' x 38', in Cygnus, centred at R.A. 21h 50.7m, Dec. +52 11'. Recorded using the 3MT on 1997 Sept. 26, 120s exposure, Hour Angle 20m W at mid-exposure, with a Kodak KAF 1400 CCD behind a Schott RG630 filter. The field of the telescope is ten times the width of this frame. North is up.