Two identical telescopes called Keck I and Keck II are the largest reflecting telescopes in the world. Each has a segmented mirror 10 m (33 feet) in diameter. The telescopes are on Mauna Kea, a mountain in Hawaii.
The telescopes in an optical observatory which stands under a large dome with shutters. The dome and the shutters protect the telescope from the weather. Motors and precision gears keep the telescope pointed in the desired direction as the Earth rotates. Observatories use two principal kinds of optical telescopes—reflecting telescopes and refracting telescopes. Reflecting telescopes use a curved mirror or a set of such mirrors to focus light, and refracting telescopes use a system of lenses.
Refracting telescopes need thick lenses of high magnification. The refracting telescope at the Yerkes Observatory in Williams Bay, Wisconsin, is the world's largest lens. It measures 102 cm (40 in) in diameter.