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.

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