telescope: Reflecting Telescopes
Because a lens can be supported only at its edge, the weight of the lens itself produces unavoidable distortion in the shape. Because a mirror can be supported from behind, it can be much more massive without incurring distortion, and mirrors many feet in diameter have been constructed. The first reflecting telescope, built by Isaac Newton in 1672, had a mirror made of a metal alloy. When techniques for depositing metal films on glass surfaces were developed, reflecting telescopes became comparable in precision to refractors. An important advantage of the reflecting telescope is the absence of chromatic aberration. Because only one surface must be ground to an exact shape, the reflector is also easier to manufacture. Although increasingly larger mirrors provide increasingly greater light-gathering ability, the cost increases even more rapidly. Several innovations were introduced toward the end of the 20th cent. to achieve the goal of increasing light gathering more economically.
One of these innovations is the use of segmented or multimirror reflectors and of computer programs to control their optics. The first, and among the largest of these telescopes, are the twin W. M. Keck telescopes at the Mauna Kea Observatories, Hawaii. Each has a segmented primary mirror, composed of 36 separate hexagonal pieces. Each segment is about 72 in. (1.8 m) across but only 3 in. (76 mm) thick, creating a 394-in. (10-m) diameter primary mirror. The position of each 880-lb (400-kg) segment is computer controlled to a tolerance of less than one millionth of an inch. The 430-in. (11-m) primary mirror array of the Hobby-Eberly telescope at the McDonald Observatory, Tex., is made of 91 250-lb (113-kg) hexagonal segments. The revolutionary design, which resulted in an effective aperture, after upgrades, of 394 in. (10 m), enabled it to be constructed at 20% the cost of similar telescopes. The largest segmented mirror telescope is the Great Canary Telescope at the Roque de los Muchachos Observatory on the island of Palma in the Canary Islands. Active optics and its 36 hexagonal mirrors create a 410-in (10.4-m) primary mirror.
Another technique for compensating for smaller mirrors is called optical interferometry. The signals from two or more smaller telescopes at separate locations are combined so that the resulting image is equal to that which would be received from a very large telescope, or virtual telescope. The largest of these installations is at the European Southern Observatory in Chile. Completed in 2003, it comprises four 315-in. (8-m) fixed telescopes and several movable 72-in. (1.8-m) auxiliary telescopes, the images from which can be combined to provide the total resolving capability of a 630-in. (16-m) conventional reflecting telescope. Similar approaches to solving the problem of building a large reflector were the multiple-mirror telescope (MMT) at the Fred Lawrence Whipple Observatory, Ariz., the COAST (
The largest single-mirror reflecting optical telescopes are the 327-in. (8.3-m) Subaru telescope, formerly called the Japanese National Large Telescope, at the Mauna Kea Observatories, the four Unit Telescopes (each 323 in./8.2 m) of the European Southern Observatory's Very Large Telescope at Cerro Paranal, Chile, and the Gemini Observatory's Gemini North telescope (319 in./8.1 m), also at Mauna Kea, and its twin, the Gemini South telescope, at Cerro Pachon, Chile. Other large conventional optical telescopes include those at the Special Astrophysical Observatory near Zelenchukskaya, in the Caucasus (236 in./6 m), the world's largest solid-mirror optical telescope; Palomar Observatory (see under Palomar Mountain), Calif. (200 in./5 m); the Cerro Tololo Inter-American Observatory, Chile, and the Kitt Peak National Observatory, Ariz. (158 in./4 m each); the European Southern Observatory, Chile (142 in./3.6 m); Lick Observatory, Calif. (120 in./3 m); and McDonald Observatory, Tex. (107 in./2.7 m). Large Schmidt telescopes are at Palomar, Siding Spring Observatory, Australia, and the European Southern Observatory. The
Sections in this article:
- Reflecting Telescopes
- Refracting Telescopes
- Evolution of Telescopes
- Mounting the Telescope
- The Schmidt Telescope and Other Innovations
- Arrangement of Mirrors in a Reflector
- Resolving and Magnifying Power
- Images Produced by Optical Telescopes
- Types of Optical Telescopes
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