Newtonian Telescopes

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 produced an instrument slightly over 2" in diameter and 12" in focal length. This proved so successful that construction was undertaken by others.

Notable among these was James Short, who made both Newtonians and Gregorians in great numbers, from about 1732 to the time of his death in 1768. 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 of 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. 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. It was not practicable as yet for the average individual to make his own speculum, but many contrived to fit spectacle lenses into tubes, much as Galileo had done some 150 years earlier. This in fact was one of the first of the early telescopes.

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.

The metal used in those early mirrors was an alloy of copper and tin, the usual proportion about 75 to 25, which could be given a beautiful polish. But the metal was extremely hard to work, and a prodigious amount of labor was involved in grinding and polishing the curve. 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. Critical testing, undoubtedly seldom indulged in on account of its laboriousness, could as yet only be performed on a star. 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. Listed below are prices and sizes of a few of the Gregorians made by Short, selected from his catalogue. Newtonians in similar sizes were priced only slightly lower.

Diameter (inches)	Focal Length (inches)	Magnification	Price (guineas)*

1.1 3 18 3
1.9 7 40 6
4.5 24 90-300 35
6.3 36 100-400 75
18 144 300-1,200 800

*An English gold coin, issued until 1813, equivalent to 21 shillings, or about five dollars.

The early telescopes were certainly gaining popularity by this time.

You'll Soon Be Gazing At The Stars Through Your Very Own Telescopes

i just bought a newtonian celestron telescope 127mm, what eyepieces shall use to see saturn?

it comes with a 20mm and a 4mm, and a barstow 3x, what oher pieces, filters shall i buy?

Start small and work your way up. You should be able to just drop in each eyepiece and then fine tune the focusing.

As magnification increases, you'll have more trouble locating your target. Vibration in your scope will also be highly magnified. The image will get less bright, too.

It will become painfully obvious if you use a 3x barlow. Magnification is not always your friend.

How to Use Telescopes : Using Newtonian Telescopes

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