The First Truly Large Telescopes

Like Newton’s telescope, Herschel’s telescopes used concave mirrors rather than lenses. Concave mirrors are not easy to make, since they must be ground and polished to within a few millionths of an inch. Nonetheless, they are much easier to fabricate than lenses which require at least two glass surfaces. Also the glass of a telescope lens must be perfectly clear with no bubbles since light from a star must pass all the way through the glass without distortion.

The example at left: Herschel’s largest telescope was nearly 4 feet in diameter and 40 feet long. Notice the man on the observing platform for scale.