Hadley then turned his attention to a design by James Gregory and in 1726 he began to make telescopes 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 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. 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 telescopes, 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 and amateur telescope making was beginning to take off.

The development of building a telescope was greatly aided by the construction of the achromatic lens.In 1733, the achromatic lens was invented by Chester Moore Hall, an English barrister. This helped to give better instructions to those intending to make a telescope.

Dollond’s efforts led to a demand for clearer glasses of more varied densities and of less equal dispersions, needed to improve achromatism, and chemists pursued experiments in learning how to control the refractive indices of melts, and in the pouring of large disks of limpid, homogeneous glass.

Dollond began make telescopes of the refractory variety (spyglasses) with single-lens objectives as early as 1742, his price for a 2-foot telescope then being 7s 6d. In comparison, in 1762 he sold a 2-foot telescope with a two-lens objective (achromat) for 2 guineas. The lens diameters in each case were just under 2″.

In 1783, with a view to combining the benefits of the wide field of Huygens’ eyepiece with a means of making micrometric measurements of an image in the focal plane, Jesse Ramsden, an English optician, designed the compound eyepiece. Building a telescope was becoming more like the process undertaken today. It can be seen that a measuring device, such as adjustable parallel wires, set in the focal plane would be magnified along with the image. With the advent of the achromatic lens, the erecting or terrestrial eyepiece assumed considerable importance. In the early part of the 19th century, small achromatic refractors were being manufactured by several concerns. For those not having the means to buy achromats, telescopes with single-lens objectives continued to be made. Enterprising opticians were also offering lens sets that could be assembled into simple refractors.

The method of chemically depositing silver on glass discovered about 1840 by Justus von Liebig, of Nuremberg, was successfully applied to a small glass telescope mirror in 1856 by Karl Steinheil, a German physicist, and independently in the following year by Jean Foucault, the famous French physicist.

Various processes of plating glass with metal for the making of mirrors had been known and practiced for centuries, but for one reason or another, the coatings were unsuited for front-surface reflection.

Then, in 1858, Foucault announced the development of his amazingly delicate and simple test for a concave reflecting surface, using an illuminated pinhole and a straightedge placed in the vicinity of the center of curvature of the mirror. The pinhole and straightedge were the outgrowth of earlier experiments in which simultaneous microscopic comparison was made of a pin point, likewise placed at the center of curvature of a mirror, and its reflected image, which was caused to fall alongside.

Silver-on-glass mirrors replaced the more expensive and difficult-to-work speculum.

After this point in history a telescope was becoming closer to being constructed in the style we know it today. You can also make telescopes at home that is similar is style and functionality to the telescoped made by histories great astronomers.

Amateur telescope making is a fascinating hobby. It is also one you can begin with a little time and a little patience. Many chose to begin to make telescopes themselves. Telescopes are usually designed to perform particular kinds of work. In general, for visual work, low-ratio telescopes with their wide fields are useful for comet seeking, variable star work, and the like.

The higher ratios are used in planetary study, double star observations, and in other fields where high powers and fine definition are required.

Some of these instruments are portable, and others must be mounted on a solid pier.

From the experience gained by amateur telescope makers, it has been found that the most practical and popular instrument for amateur use is the 6-inch f/8 Newtonian reflector. Its concave mirror is 6″ in diameter and its focal length 48″. The delicate task of parabolizing the mirror, while not easy, is not beyond the ability of a careful worker. The 4-foot focal length makes for comfortable observing, and with a low-power eyepiece, the field of view is a trifle over one degree in diameter – more than twice that of the full moon.

The magnifications that may be employed permit of a modest size of mounting, which can be made portable. Such a telescope should reveal stars of magnitude 12.8, as compared with the 6th-magnitude limit of the unaided eye, and the 9th-magnitude limit of the average small binocular.

This telescope will show the divisions in Saturn’s rings; surface markings on the moon little more than a mile across should also be visible.

The purchase price of such an instrument of professional make is necessarily high, and many an amateur feels compelled to do without it. Of course, many engaged in amateur telescope making feel that their mirrors are inferior to the professionals’, but this is not necessarily true. It has been frequently demonstrated that mirrors of professional make will seldom stand up to a test, because it is impossible for the professional optician to spend sufficient time on the mirror without losing money, whereas the amateur can, if he will, devote all the time and care necessary to produce a mirror of admirable figure. There are great benefits when you choose to make telescopes yourself.

Upkeep is slight when you choose to make telescopes yourself. For those involved in amateur telescope making, the task is time consuming but extremely rewarding. Good luck!

Very early in the 19th century, when advocates of the speculum mirror began to feel the challenge of the refractor, Dr. Nevil Maskelyne, English Astronomer Royal, ventured the opinion “that the aperture of a common reflecting telescope, in order to show objects as bright as the achromat, must be to that of an achromatic telescope as 8 to 5.”

The relative inefficiency of the reflector of that day was due to the fact that, even under most favorable circumstances, barely 40 per cent of the original light escaped absorption by the metal mirrors, the greatest losses occurring in the short and medium wave lengths. Even silver-on-glass mirrors are subject to considerable deterioration, especially under certain conditions of the atmosphere.

If you wish to make telescopes a good thing to remember is that the reflectivity of aluminum, however, is more-or-less constant, and from a standpoint of image brightness, it placed the reflector on a more equal footing with the refractor. In fact, until the quite recent development of anti-reflection lens coatings, an aluminized mirror has had the same efficiency, in light-transmitting qualities, as an air-spaced achromatic objective lens of equal aperture.

Coming down to figures – due to reflection there occurs in an untreated lens a light loss of slightly more than four per cent at each of its surfaces.

With reflection losses to be accounted for, plus an absorption loss in the substance of the glass (amounting to about two per cent for lenses of moderate size), it is evident that about 82 per cent of the original light is transmitted. In the reflector, after first deducting that area of the mirror’s surface obscured by the diagonal, an equal percentage of the original light is found to be transmitted.

Of course, this transmitted light is subject to another reflection by the diagonal, but the refractor will probably employ a star diagonal, the function of which is similar to that of the diagonal or prism of the Newtonian, so an equivalent loss may occur there. Therefore, for those engaged in amateur telescope making, with either instrument, the same amount of light reaches the eyepiece.

It was discovered, however, in the latter part of the last century, that some lenses which had been tarnished by the elements transmitted more light than ones that were newly polished; it was found that this resulted from lessened reflections at the tarnished surfaces. Various processes of producing an artificial tarnish were attempted. At present, in the most satisfactory method, metallic salts (such as magnesium fluoride) are evaporated in a high vacuum onto the glass. Ideally, the refractive index of an anti-reflection fluoride coating should vary from that of glass at the glass-fluoride surface to that of air at the fluoride-air surface, in which case no reflection would occur.

Practically, the index of the coating should be equal to the square root of the index of the glass, and its thickness equal to a quarter of a wave length of yellow-green light. Only the light at opposite ends of the visible spectrum is then reflected, amounting in general to less than one per cent of that of the whole, and is detected by the purplish color given to the reflection.

From the standpoint of an introduction to the optician’s trade, the experience of thousands of amateurs has shown that when deciding to make a telescope one’s teeth should first be cut on at least one good mirror. Then, if a refractor is contemplated, additional experience can be gained by making the optical flat that is so essential in the testing and figuring of the objective lens.

The history of the telescope makes for interesting reading from a theoretical perspective, but also from the perspective of someone interested in beginning to make a telescope. During the development of the telescope, practical experiments with reflectors had already begun in 1639, but it was not until 1663 that they gained any prominence.

The Gregorian Telescope

High magnification could be had with this instrument, the second reflection amplifying the focal length of the primary in the ratio of fs to Fs.

He began to make telescopes, but whatever chance it may have had of performing creditably was lost by polishing the speculum on a cloth lap – putty (tin oxide) being used as the polishing agent. The Cassegrainian Telescope
Sieur Cassegrain, a Frenchman, in 1672 designed a second compound reflector, differing from Gregory’s in that it employed a convex secondary, to be of hyperboloidal figure, placed inside of the focus of the paraboloidal primary .

The Newtonian Telescope

The history of the telescope takes an interesting turn at this point. In the same year, Newton designed and began to make telescopes that had  two small reflectors, of the type so popular with amateur astronomers today and which still bears his name. They were not large, as we know telescopes today, the effective apertures of the concave specula being about 1 1/3″. Their focal length was 6″, making the focal ratio f/4.5.6

Newton, according to his Opticks (1704), polished his specula on pitch, using putty as the polishing agent. It might be concluded that if the center of the mirror were properly deepened, that is, given a shorter radius, or if the radii of the outer zones were progressively lengthened, or if a little of each were done, all the reflected rays could be brought to a common focus. The standard practice is to deepen the spherical mirror so that, for a 6-inch f/8 mirror, the glass removed in the operation is but half a wave length of light in thickness at the center. The field lens, like Galileo’s concave lens, is placed before the focal plane of the objective.

Ever since Galileo took a Dutch invention and adapted it to astronomical use, astronomical telescope making has been an evolving discipline. Many astronomers after the time of Galileo built their own telescopes out of necessity, but the advent of amateurs in the field building telescopes for their own enjoyment and education seems to have come into prominence in the 20th century.

Today telescopes of the size and technicality used by NASA experts are out of an amateurs grasp (and price range), but an amateur can easily begin to make telescopes of the kinds mentioned above both inexpensively and easily.