William Herschel's Contributions

Fig. 12. The Herschelian reflector. Large mirrors might be tilted at but half the relative angle shown, as only a negligible fraction of the incoming light from a star would then be obstructed by the observer's head.

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.
(See Fig. 13.)
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. The metal disk was nearly 6" thick, and weighed about 8,380 pounds when cast. Rosse's gigantic instrument was mounted at Parsonstown, Ireland.

William Herschel continued