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973 viewsJun 25, 2005
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The Schmidt Camera ?1650 viewsThis instrument was built in 1952 by Grubb-Parsons of Newcastle-upon-Tyne and replaced an older telescope in the existing dome, which had been made by T. Cooke & Sons Ltd. of London & York at the time of the move of the Solar Physics Observatory from South Kensington to Cambridge.
It is a `Classical Schmidt' - the simplest and most efficient form of the ingenious wide-field camera invented in 1930 by Bernhard Schmidt of Hamburg Observatory. Light from the sky falls upon a 61 cm (24-inch) mirror with a spherical reflecting surface, at the bottom of the tube. It is reflected to a focus in the centre of the tube and half-way up it, 163 cm (64 inches) from the primary mirror. At the focus a photographic plate P 15 cm (6 inches) in diameter, which must be bent to fit a curved surface, records the star images in an area of sky 5 degrees in diameter. (The full Moon is half a degree in diameter.)
Without any further optical element the star images would be of poor quality owing to ``spherical aberration'': light falling near the edge of the mirror would come to a focus too close to it, and light falling near the centre of the mirror would be focused a little too far away. Schmidt's invention was to place at the centre of curvature of the primary mirror, near the top of the tube, a weak meniscus lens (in this case 43 cm (17 inches) in diameter) with one aspheric optical surface: this makes the light which passes through it near the edge diverge slightly, lengthening the focus of the outer parts of the mirror, and makes the light passing through near the centre converge, shortening the focus of the centre of the mirror. This optical combination of lens and mirror forms a fast, efficient camera giving sharp star images of uniform quality over the full 5 degrees field. It is an ideal sky-surveying instrument; by contrast the 36-inch (91.4 cm) telescope, with its paraboloidal mirror of 4.1 metres (162 inches) focal length (f/4.5), has a field of view only 7.2 arc minutes in diameter with images smaller than 2 arc seconds.
The auxiliary 15 cm (6 inch) telescope is for guiding. The exposure time is usually of the order of 10 minutes, and during this time the image can wander about on the photographic plate mainly because of irregularities in the refraction in the earth's atmosphere. These are corrected by maintaining a star image at the intersection of the cross-wires in the guiding telescope.
The Palomar and U.K. 48-inch (1.22 metres) Schmidt cameras which were used to make the all-sky surveys (now kept in the Cambridge Astronomical Survey Unit) have apertures nearly three times as large as our telescope, but focal lengths (and tubes) only twice as long. Only one Schmidt camera (the 53-inch (1.35 metres) at Tautenburg, Germany) has ever been built larger than these two. The reason is that, if a Schmidt camera is simply scaled up, its image size is also scaled up, and as Bernhard Schmidt himself predicted, the 48-inch Schmidts are close to the practical limit. The main image defect arises because the thin lens can correct the ``spherical aberration'' of the mirror exactly in only one colour of light, (usually blueish green), red light is under-corrected, and blue or ultra-violet light is over-corrected. To minimise the length of the tube, and so the size and cost of the dome, the 48-inch Schmidts have been made with an aperture of f/2.5 (ours is f/3.7) and the ``spherical aberration'' of the mirror is then 3.2 times as large as in our camera. Three of the largest Schmidt cameras have been fitted with ``achromatic'' lenses which reduce the residual colour errors, but astronomers now use very fine-grain emulsions, and wish to observe a wide range of colours of light, so these large Schmidts are still at the practical limit of size.
Fortunately, a new design of wide-field telescope, using three mirrors and no lenses, has been developed; a prototype can be seen nearby.
Description source: Institute of AstronomyJun 25, 2005
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The Thorrowgood Telescope ?859 viewsThe `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 13''.
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.
Description source: Institute of AstronomyJun 25, 2005
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The 36-inch Telescope971 viewsThe 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.
Additional information about the telescope and radial-velocity spectrometer is obtainable at any time from Dr. R.F. Griffin (rfg@ast.cam.ac.uk), who will also be glad to arrange informal demonstrations of the equipment both by day and at night.
Description source: Institute of AstronomyJun 25, 2005
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The Northumberland Equatorial Telescope, University Observatory, 18388450 views`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 the CUAS website, and for Public Observing on clear Wednesday evenings in the winter months, and so continues a useful life of now over 150 years.
Description source: Institute of AstronomyJun 25, 2005
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