The St Andrews Schmidt-Cassegrain Telescope
On Tuesday, 5th July 1938, a graduation ceremonial took place in the Upper Hall of the University Library in St Andrews to celebrate the tercentenary of the birth of James Gregory, a brilliant mathematician and astronomer, who from 1668 to 1674 was Professor of Mathematics in the University of St Andrews. By the age of twenty-four Gregory had written an important treatise on mirrors and lenses, his Optica Promota; this treatise, containing as it did the earliest description of a reflecting telescope, brought him fame. The first successful Gregorian telescope to be constructed was made by Hooke in 1674 and presented by him to the Royal Society. In the meantime Isaac Newton, working independently, had constructed a reflecting telescope of a somewhat different pattern; the second Newtonian telescope to be made was presented to the Royal Society in 1672.
The hall in which the graduation ceremony took place was the very room in which Gregory carried out the greater part of his work during the few years he spent in St Andrews. In this room is preserved an astronomical timepiece made to the order of Gregory about 1673 by the celebrated clockmaker Joseph Knibb, and an inlaid line on the floor of the room is said to mark the position of Gregory's original meridian line; at all events this room was certainly used by him as an observatory. Although an observatory, planned by Gregory four years before the founding of Greenwich Observatory, was built on a site near the southern end of West Burn Lane it is doubtful whether it was ever occupied by him. No trace of this building now remains, but to-day a new observatory stands on a site adjacent to the University Playing Fields.
Sir Peter Redford Scott Lang who, from 1879 to 1921, was Regius Professor of Mathematics in St Andrews expressed the desire that a lectureship in astronomy should be founded at St Andrews, and that Baron Napier of Merchiston should be commemorated by the association of his name with the foundation. In pursuance of her father's wishes Miss Edith Mary Valentine Scott Lang made a bequest for this purpose which was gratefully accepted by the University. In 1939 Dr E Finlay Freundlich was appointed to the lectureship, and work on the erection of a new observatory began. The building was completed in 1940 but by this time the outbreak of war had made the prosecution of serious astronomical research impossible. A Chair of Astronomy has now been founded and, on Ist January 1951, Dr Freundlich became the first Napier Professor of Astronomy in the University of St Andrews. Nearly three hundred years have passed since the invention of the Gregorian telescope and now, in St Andrews, Professor Freundlich has produced a new type of telescope. This telescope, completed in 1949, is a pilot model, the first stage in the development of a larger instrument thirty-eight inches in diameter at present in the course of construction.
The early history of the telescope is somewhat obscure but the credit for the discovery that two lenses may be combined in such a way as to make distant objects seem near is generally attributed to Hans Lippershey, a spectacle maker of Middleburg in Holland, about the year 1608. Realising the importance of this discovery, Galileo devoted much time and skill to the improvement of the telescope. The Galilean telescope, still commonly used in the form of field or opera glasses, consists of a combination of a convex or converging lens as object glass and a concave or diverging eye lens. The theory of the telescope had, by this time, become the subject of scientific study and, in 1611, Kepler gave the theory of a telescope consisting of two converging lenses; the practical development of this form referred to later as the refractor is largely due to Huygens.
The two great aims in the further development of the telescope were to obtain increased magnification and increased light-gathering power. In order to achieve the first of these aims it was necessary to increase the length of the instrument and this resulted eventually in the construction of telescopes several hundred feet long; the mechanical difficulties associated with the mounting of such instruments need hardly be stressed. The second aim calls for an object glass of large diameter. The amount of light which ultimately enters the eye from a star increases in direct proportion to the area, that is to say to the square of the diameter, of the object glass. Increasing the diameter brings fainter and more distant stars into view and thereby enables the astronomer to probe more deeply into space.
The early refractors suffered from many defects, and it was with a view to overcoming some of these defects that attention was turned to the use of mirrors. Despite great improvements in modern lenses the refractor has been superseded almost entirely for astronomical observation by the reflector, which makes use of mirrors. Perhaps one of the most important reasons, to-day, for the choice of mirrors concerns stability of mounting. A lens supported only by its rim sags under its own weight and, when the diameter exceeds some forty inches, the resulting deformation is sufficient to impair seriously the optical performance of the telescope. Since, however, a mirror can be supported over the whole of its back surface the same limitation of size does not apply to the reflector.
When light from a star falls on a concave mirror it is reflected as a convergent beam. For various reasons, not the least being ease of manufacture, the most convenient form for the surface of the mirror is the portion of a sphere, but this form gives rise to spherical aberration which means that the reflected light does not come to a point focus and the resulting image is blurred. Gregory showed that spherical aberration can be avoided by the use of a parabolic instead of a spherical mirror and that light from a distant object falling on a parabolic mirror, parallel to its axis, produces a clearly defined image of the object. This principle, however, applies only to objects on the axis, and for points even a small distance off the axis the quality of the image deteriorates to such an extent that the field of good definition is limited to a very small region of the sky. With all its faults, and the inherent difficulty of figuring, the parabolic mirror has for centuries formed the basis of the reflecting telescope, on the principle that half a loaf is better than no bread, and for astronomical work the Newtonian telescope has had no serious rival.
In the reflecting telescope light from a heavenly body, say a star, passes down to the lower end of the telescope tube where it falls on a large concave parabolic mirror and is reflected back to a focus near the top of the tube. In the Gregorian telescope this reflected beam of light is intercepted by a small concave secondary mirror which causes the beam to travel down the tube once more to a hole cut in the centre of the main mirror where it emerges through the eyepiece. A modification of this construction, in which the secondary mirror is convex instead of concave, was introduced by Cassegrain. Newton, on the other hand, used a plane secondary mirror placed obliquely so that it reflected the light sideways through an eyepiece set in the side of the tube near its upper end, a most inconvenient position for the observer. The two-hundred inch Newtonian reflector at Mount Palomar actually carries the observer inside the tube, high above the floor of the observatory.
Restriction of the field of view consequent on the use of non-spherical mirrors is not very serious for certain types of visual observation because the telescope can be turned so as to bring each star to be studied into the centre of the field in turn. With the advent of the photographic plate, however, came the strong desire for a wider field. The photographic method provides, in a relatively short time, a permanent record from which measurements can subsequently be made. An even more useful property of the photographic plate arises from the cumulative action of light on the sensitive material which renders the plate capable of recording objects far too faint ever to be seen by the human eye. It is therefore evident that any improvement in telescope design which increases the useful field of view means much to the astronomer whose worst enemy is the weather.
Many attempts have been made to improve the performance of parabolic mirrors but none of these has been really successful. In 1932 B Schmidt approached the problem from a new angle; he introduced the idea of counteracting the inherent defects of the spherical reflecting surface by adding a correcting plate through which the incident light passes before reaching the mirror. This idea has revolutionised telescope design; Schmidt has done for the telescope what the ophthalmic optician has done for the eye; defects of vision have for centuries been 'corrected' by spectacles and now the defects of the reflecting telescope can be corrected by means of a Schmidt correcting plate which operates on the same principle.
The Schmidt telescope consists of a single concave spherical mirror at the lower end of the tube, together with a correcting plate situated at the top of the tube where the light enters. This plate is so figured that the incident light is retarded and deviated by just the amount necessary to counteract the aberrations of the mirror. The correction, instead of being confined to an image on the axis of the system, now results in the production of well defined images over a wide field. The advantage over the parabolic mirror is obvious, but two quite serious difficulties remain; the image formed lies on a spherical surface which demands the use of specially curved photographic plates, and it occupies a position inside the telescope which makes it inaccessible for visual observation. It was with the idea of overcoming these drawbacks of the classical Schmidt system that the present project was undertaken in St Andrews.
Basically, the new instrument is a Cassegrain telescope in which both the main concave mirror and the secondary convex mirror are spherical. This arrangement is preferable to the Gregorian system since the combination of a convex and a concave mirror reduces the overall spherical aberration and at the same time reduces the length of the tube. A Schmidt plate, situated at the upper end of the tube near the centre of curvature of the main mirror, provides the necessary correction. The Schmidt-Cassegrain telescope has in fact achieved its main object of giving a plane image conveniently located just outside the back of the main mirror, that is to say, at the lower end of the telescope.
Before starting work on the full-scale instrument, it was decided that a pilot model of approximately half scale should first be constructed to allow the principle involved in the new design to be tested and to provide experience in the behaviour of this type of telescope. The pilot has a main mirror of diameter twenty inches and radius of curvature forty-nine and a half inches ; the diameter of the secondary mirror is nine inches and its radius of curvature forty-seven and a quarter inches. Since both mirrors are spherical the formidable problem hitherto encountered in the figuring of an accurate paraboloid is avoided and there only remains, in this connection, the accurate figuring of the Schmidt plate, in itself a very delicate operation. This plate, twenty-one inches in diameter and three-quarters of an inch thick, has one plane face while the other face is figured to a profile differing from a plane by only a few thousandths of an inch.
In order to expedite the crucial tests on actual star photographs the optical parts, after being subjected to the preliminary optical tests in St Andrews, were inserted in the mounting of the Newtonian telescope at the Mills Observatory in Dundee, a procedure to which the Custodians of the Mills Observatory readily agreed. The erection of the pilot in February of last year was followed, within a few weeks, by evidence in the form of stellar photographs which demonstrated clearly that the high hopes with which the project had been launched were entirely justified. Sharp star images were obtained over the entire area of the plate, the size of which is naturally limited by the size of the hole in the main mirror. Even with this small telescope the area of plate covered represents a field of five by five degrees. The area available for accurate measurement is, in fact, more than five hundred times as great as the useful area on a photograph taken with a Newtonian telescope of comparable size. Apart from the enormous saving of time and the better utilisation of favourable weather conditions, the mere fact of having a larger region of the sky recorded on a single photograph is in itself a great boon. For the study of extended systems, globular star clusters and spiral nebulae, the Schmidt-Cassegrain telescope has placed a powerful tool in the hands of the astronomer.
The success of the pilot model is most encouraging. It must not be assumed, however, that the making of the thirty-eight inch telescope is now a mere matter of routine; the computation of the form of the Schmidt plate is an extremely difficult and tedious task and this computation has to be carried out afresh for the larger instrument. The increase in size adds to the difficulty of construction, and so the experience gained on the pilot model will be invaluable; besides, the pilot is not merely a model but is in itself a telescope capable of being usefully employed in astronomical research. Although the pilot was, to all intents, completed in the autumn of 1949 it does have its teething troubles and refinements are still being added. These refinements do not concern the fundamental optical system but are rather problems of a mechanical nature relating to the mounting of the optical parts and of the telescope as a whole.
The mounting of a telescope for accurate astronomical observation introduces engineering problems of very great difficulty. The whole system must be delicately balanced and must be capable of moving continuously during an exposure, often extending to several hours, so that the star images on the photographic plate show no trace of movement arising from the rotation of the earth. The relative position of all its component parts must remain fixed to a very high degree of accuracy, no matter in what position the telescope may be set. This is no new problem, but every advance in the optical performance of the telescope brings with it the need for higher precision and stability of mechanical construction. Space does not permit of elaboration but one point may be mentioned here. In order to prevent distortion of the main mirror under its own weight without adding unduly to the weight of the telescope the mirror, besides being supported by its edge, 'floats' on a number of pads delicately balanced on levers and counterpoised so that each pad bears the weight of that section of the mirror against which it rests. These pads can be seen through the grid, in the photograph of the lower end of the pilot model which is reproduced here; the ring of cylinders on the outer edge is also associated with this balancing system. The plate holder seen in the centre of this photograph may be replaced by an eyepiece to allow of visual observation. Visitors to the Festival of Britain Exhibition will be able to see an accurate full-sized model of the pilot telescope as at present mounted in the Mills Observatory. This fine model is in view in the Dome of Discovery.
The entire work of construction of the pilot model has been carried out in the workshops of the University Observatory in St Andrews by R Waland, assisted by W Threadgill, and J G Bruce, Curator of the Mills Observatory. This work involved the design and construction of special tools, the grinding, polishing and testing of the mirrors and the Schmidt plate, and the design and construction of the mechanical devices for supporting the various optical parts of the instrument. Part of the machine which was specially designed for grinding and polishing the optical parts is illustrated in a photograph showing the Schmidt plate in position for the process of producing an optically flat surface. The edge of the plate can be distinguished in the photograph by its tone which is slightly darker than the support on which it rests. This plate is made to rotate in one direction, while the grinding tool above the plate rotates in the opposite direction. The tool can also be given a lateral motion and, when a spherical surface is being ground, it rocks about the universal joint at the lower end of the driving shaft. Work is now in progress, with this machine, on the grinding of the main mirror for the larger telescope. The entire course of construction demands a high degree of skill, and even the layman cannot fail to be impressed by the accuracy and the fine workmanship which is everywhere evident in the workshops of the Observatory.
The extensive calculations which led to the design of the new telescope have been carried out under the direction of Dr E H Linfoot, John Couch Adams Astronomer in the University of Cambridge and Assistant Director of the Cambridge Observatory, who has made a special study of new and improved optical systems for telescopes. The whole project is under the direction of Professor E Finlay Freundlich who first proposed this development of the Schmidt telescope.
The University has received financial assistance in undertaking this very important piece of research from the Education (Scotland) Fund, from the Carnegie Trust, and from the Royal Society of Edinburgh who through the Robert Cormack Bequest have, in addition to other financial help, provided the optical glass for the full-scale instrument.
It is interesting to recall that the first design ever to be produced for a reflecting telescope was due to a Scotsman, for Gregory was a native of Drumoak in Aberdeenshire, who a few years after publication of the design was elected to a Chair in the University of St Andrews. To St Andrews now falls the credit of producing the first Schmidt-Cassegrain telescope which marks yet another vital advance in the history of the astronomical telescope.
Article by: D JACK
JOC/EFR April 2007
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