A GREGORIAN TELESCOPE
From a portrait in the Museum of the History of Science, Oxford
58/1284 = 24
Institute, Southhold, New York
Click on picture for more pictures.
Short and His Work
Pictured above is a Gregorian reflector made by the renowned English (Scottish) instrument maker James Short, pictured above also. Short was born in Edinburgh, Scotland June 10, 1710 and educated at Edinburgh University around 1726. He migrated to London where he set up shop as a maker of telescopes and was eventually elected a fellow of the Royal Society. Although he produced only a few Newtonians, he primarily engaged in the making of Gregorian reflectors. Short apparently had an excellent understanding of optical theory for his time and was perhaps the first individual to produce relatively fast parabolic mirrors of good accuracy. John Hadley had produced the first parabolic mirrors slightly before Short's production of telescopes, but Short advanced the art of making aspheric reflecting surfaces to a level previously unattained. The great proportion of Short’s mirrors were of the speculum metal type, an alloy of copper and tin which results in an extremely hard and brittle metal suitable for grinding and polishing into a rather good optical surface. Considerable experimentation was being done at this time with reflecting telescopes inasmuch as the single lens, non-achromatic refractors of the period were extremely unwieldy and produced dissatisfying images. One of the more interesting aspect of Short’s optical work is that he made approximately six Gregorian telescopes with what we know as Mangin mirrors; glass mirrors silvered on their rear services like ordinary looking glasses and the front surfaces curved to balance aberrations. While it is entirely possible to create substantially perfect achromatic optical systems using rear surface mirrors of ordinary glass, optical theory of the day was not sufficiently advanced to fully understand the properties involved. Fortunately, the process is fairly intuitive and producing mirrors having concentric radii will result in fairly well-corrected systems. Nonetheless, Short must have conducted an extensive series of practical tests grinding and polishing a large number of such rear-silvered mirrors until the right combination was achieved. Notwithstanding, these early experiments with glass mirrors quickly gave way to the production of telescopes with speculum or metal mirrors. His production methods were such that he usually did not manufacture the mechanical components of the telescope himself but only the optical components, hiring the mechanical work out to others and maintaining only a small shop where he worked alone in relative seclusion. Although it has been said that he told no one of his methods, indications are that his brother may have been privy to at least some of his methods. Much of his tooling and appliances were taken into his bother's possession after his death, though he did not succeed to any degree at imitating James's skill as an optician and telescope maker. Short’s telescopes were renowned throughout Europe as being the finest available. Being an astute businessman, Short commanded high prices for his instruments and died in relative wealth, leaving an estate of approximately £25,000.
Some 1,360 instruments were manufactured over a period of approximately 30 years. These instruments were mostly sold in Britain but many found their way all over Europe and even into Russia and Turkey. The largest instrument he ever made was for the king of Spain, a 24 inch. Many of his telescopes are still preserved throughout the world. One is still preserved in Leningrad, another at Uppsala and several in the United States. In April of 2000 I viewed one in the Science Museum in Florence, Italy; a magnificent 8 inch equatorially mounted instrument that appeared to be in excellent condition.
Short did not confine his activities strictly to the production of optical instruments but undertook astronomical observations on occasions. He was principal British collator and computer of the Transit of Venus observations made throughout the world on 6th June 1761. His instruments traveled on Endeavour with Captain Cook to observe the next Transit of Venus on 3rd June 1769, but Short died in July 14, 1768, a year before this event took place.
The Present Instrument
The instrument under examination is located at the Custer Institute, Southhold, New York. This instrument is to be examined further but for now the following information is available. The overall condition of the instrument appears to be excellent. There are no signs of wear or misuse and the mechanical parts appear to be include working order. The primary and secondary mirrors appear to be removed from the instrument and are in the case open to examination. The primary mirror has a diameter of 4.44", a thickness of approximately 3/8" and a focal length of 24". The secondary has a diameter of .95" and a thickness of approximately 1/8". Examination of the exterior of the tube would suggest that the final focus from the secondary mirror would form approximately 1" behind the rear end plate, but this is conjectural. The instrument is focused by moving the secondary mirror by means of twisting a long brass rod threaded at the secondary end and moving a nut attached to the mirror holder. The secondary mirror appears to be approximately 5" beyond the focus of the primary mirror. The distance from the face of the secondary mirror to the final focus is estimated at 32", yielding an amplifying ratio of 6.4. This would indicate a total focal length of approximately 153", or an effective focal ratio of 34.46. This is approximately what I would suspect of one of Short's Gregorians.
The instrument bears the high manufactures number 58/1284 = 24, indicating that it was probably made toward the end of Short's career. According to Short’s method of identifying his instruments this would be the 1,284th instrument he ever produced and the 58th of 24" focus.
The Heliometer Objective
The instrument pictured above has been fitted at its front-end with a split objective heliometer. These were quite commonly adapted to existing Gregorians, often by such notables as Jesse Ramsden and John Dollond. The ability to adapt a heliometer to an existing telescope has caused some concern as to exactly how this was done, inasmuch as the heliometer objective requires at least some small amount of optical power in order to function, thereby altering the overall focal length of the telescope. Considering the overall effective focal length of a Gregorian and the large amplifying power of the secondary mirror, it becomes apparent that the introduction of even a slight amount of optical power at the front end of the instrument would cause the focal plane at the eyepiece to change by several inches. However, Short's Gregorians were focused by moving the secondary mirror and not by the focuser at the eyepiece. Examination of these instruments indicates that as much as one inch of focus travel was typically available. This is significant in as much as the slight amount of optical power needed to make an effective heliometer objective would alter the focal length of the primary mirror, typically working at approximately f/5, to be altered only very slightly; perhaps no more than a quarter or half an inch. (Assuming an amplifying power of 6 for the secondary mirror this would result in overall change at the focus of 1.5 to 3 inches.) Thus, movement of the secondary mirror is more than sufficient to compensate for the focal change due to the heliometer objective. Also, the introduction of so slight an amount of optical power would have no significant effect on the overall wavefront produced by the telescope.
It is my intent to examine this instrument further and hopefully to conduct optical tests. The people at the Custer Institute have been extremely helpful and seem to indicate that future examination would be possible.