Coin Shilling
Make Telescopes: From History To Home
An investigation into early telescopes makes for interesting reading. In 1722, John Hadley, an English mathematician, completed a form of reflector for the telescope in the style of Newton in which the mirror evidently was suitably figured. This instrument attracted considerable attention, and presently other makers were turning out Newtonian reflectors, following Hadley’s technique, which consisted of removing the spherical aberration as it was revealed by the extra-focal diffraction rings of a star image.
Hadley then turned his attention to a design by James Gregory and in 1726 he began to make a telescope slightly over 2″ in diameter and 12″ in focal length. Observatories purchased his larger instruments, a tribute to his skill, and the smaller ones were marketed chiefly among the aristocracy and amateur astronomers.
The principal attraction to make telescopes according to the Gregorian design was the erect image it gave, which made it suitable for terrestrial use. This circumstance influenced its preference over the Newtonian, notwithstanding the fact that its images must have been pretty dull. Well into the 19th century, however, the Gregorian rode a wave of popularity that no type of telescope has known, until overwhelmed in comparatively recent years by the flood of amateurs who have flocked to Newton’s design.
From the time of the invention of the telescope, and the startling discoveries of Jupiter’s moons and the rings of Saturn, interest in astronomy had become something infectious. Many began to make telescopes or become interested in astronomy as a hobby. Each new discovery was accorded the widest publicity, stimulating a desire among those of learning to gain at first hand a glimpse of these celestial wonders. Those whose means permitted bought telescopes, and envied was the gentleman who possessed one of three or four inches aperture, by an “exclusive” artist. But, judged by present-day standards, many of those reflectors were tiny. There is one (maker unknown) in the Fugger Collection at Augsburg, barely 1″ in diameter and 6″ in focal length, that was concealed in a walking stick! Eyepiece lenses of 1/6″ or less in focal length were quite common.
To facilitate the work, the comparatively thin disks were cast to the approximate curve, the backs also being curved to give uniform thickness and equalization of temperature effects. Grinding was done on convex iron tools of similar radius, using emery, and sometimes sand. Polishing was done on a pitch lap, with rouge.
Manufacturers usually devised their own machines to do the work of grinding and polishing. Except where the utmost perfection was imperative, figuring seems to have consisted for the most part of a final brief variation of the stroke, in an unguided attempt to concentrate the polishing at the center. In reflective ability, speculum was only about 60 per cent efficient, and the surface tarnished rapidly, effecting a further serious light loss. This meant frequent repolishing, and repolishing meant refiguring.
It is interesting to inquire into the prices that were asked for telescopes in that period, the latter half of the 18th century. The early telescopes were certainly gaining popularity by this time and amateur telescope making was beginning to take off.
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