Telescope

For almost 40 years the Hale telescope at Mt. Palomar was the world's largest with a primary mirror diameter of 200 in (5.1 m). During that time improvements were made primarily in detection techniques, which reached fundamental limits of sensitivity in the late 1980s. In order to observe fainter objects, it became imperative to build larger telescopes, and so a new generation of telescopes is being developed for the 1990s and beyond. These telescopes use revolutionary designs in order to increase the collecting area; 2,260 ft² (210 m²) is planned for the European Southern Observatory. This new generation of telescopes will not use the solid, heavy primary mirror of previous designs, whose thickness was between 1/6 and 1/8 of the mirror diameter, but will use a variety of approaches to reduce the mirror weight and improve its thermal and mechanical stability, including using 1) many hexagonal mirror elements forming a coherent array, 2) a single large meniscus mirror (with a thickness 1/40 of the diameter), with many active support points which bend the mirror into the correct shape and 3) a single large mirror formed from a honeycomb sandwich.

These new telescopes, combined with quantum-limited detectors, distortion reduction techniques, and coherent array operation will allow astronomers to see objects more distant than have been observed before.

One of this new generation, the Keck telescope located on Mauna Loa in Hawaii, is currently the largest operating

telescope, using a 32 ft (10 m) effective diameter hyperbolic primary mirror constructed from 36 6 ft (1.8 m) hexagonal mirrors. The mirrors are held to relative positions of less than 50 nm using active sensors and actuators in order to maintain a clear image at the detector.

Because of its location at over 14,000 ft (4,270 m), the Keck is useful for collecting light over the range 300 nm-30 æm. In the late 1990s, this telescope was joined by an identical twin, Keck II, which resulted in an effective mirror diameter of 279 ft (85 m) through the use of interferometry.