Many people have contributed to the development of telescope making, none more significantly than William Herschel.

Herschel's Contributions:

Born in Hanover, Germany, Herschel settled in England in 1757, where he became interested in astronomy and later (1776) turned his attention to telescopes. Working entirely by hand, at first as an amateur, he practiced and developed his technique on a great number of telescopes in the style of Newton and learned how to figure the mirrors far better than had any of his predecessors. He performed the polishing in the conventional manner, with the mirror on top, and used a sweeping, circular stroke for parabolizing.

Later, Herschel applied himself to the design now referred to as the Herschelian type, which had been proposed by LeMaire, a French scientist, in 1728. In this design, the mirror is tilted so that the image is thrown to one side of the open end of the tube, where it can be examined in comfort, with the observer's back to the object, and without the introduction of a second reflection.

This latter feature was of tremendous importance in the days of speculum, when 40 per cent of the light was absorbed in undergoing a single reflection. Of less importance, but nonetheless gainful, was the elimination of the harmful diffraction effects from the secondary mirror. But unless a suitably high focal ratio was chosen, astigmatic images resulted from the inclination of the mirror. And this introduced another problem; the lengthening of the tube meant placing the observer at an awkward height.

In 1789, Herschel completed his largest reflector, of the tilted-mirror type, which was installed at Slough, near Windsor. The speculum was four feet in diameter, with a focal length of 40 feet. It was about 3½" thick, and weighed about 2,100 pounds. An elaborate and ingenious trestlework was built to carry the observer.

This great mirror was exceeded with the completion in 1845 of the largest of all specula, one six feet in diameter and 54 feet in focal length, by the Irish astronomer, Lord Rosse. This was a major step in telescope making.The metal disk was nearly 6" thick, and weighed about 8,380 pounds when cast. Rosse's gigantic instrument was mounted at Parsonstown, Ireland.

As representative of the prices Herschel charged for his reflectors, a Newtonian model of 6½-inch diameter and seven feet focal length sold for 100 guineas (30 guineas for the optical parts). Another 8.8-inch Newtonian, 10-foot focus, cost 200 to 300 guineas. Herschel advised buying two mirrors for this latter instrument (which probably accounts for the variable price) so that one could be used while the other was being repolished!

His talents were not confined to the making of fine specula; he also made his own eyepieces, some of which were truly remarkable. His frequent references to the use of magnifications of some 7,000 on his 6½-inch reflector occasioned some speculation and controversy among the English astronomers, but his claim appears to have been justified by the discovery, comparatively recently, of some very tiny eyepieces made by Herschel.

Among his effects at Slough, W. H. Steavenson found several of these eyepieces, varying in focal length from about 1/16" downward. The smallest of these was examined in a microfocometer, and found to have a focal length of 0.011". It was bi-convex, about 1/45" in diameter, and 1/90" in thickness.

It was tried out on a 6-inch refractor, and performed as creditably as its power would permit, but its field in that instrument was only about 20 seconds of arc in diameter. If Herschel actually used this eye-piece on his 85.2" focal length reflector, it would have given a magnification of 7,668.