Evolution of TelescopesRefracting Telescopes
The first practical telescopes were refracting telescopes produced at the beginning of the 17th cent. By 1610, Galileo had made extensive astronomical use of the simple refractor. The best telescopes of this period had very long focal lengths to minimize the chromatic aberration inherent in the single-element objective. The multielement objective, invented in 1733, allowed the construction of telescopes of large aperture. The art of building refracting telescopes reached a high point in the 19th cent. The largest refractor in existence, with an objective lens 40 in. (102 cm) in diameter, is located at the Yerkes Observatory in Williams Bay, Wis. A 36-in. (91-cm) refractor is located at the Lick Observatory in California and a 33-in. (84-cm) refractor is located at Meudon, France. These telescopes represent the practical limit on the size of a refractor.
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. The largest of these 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 of 362 in (9.2 m), enabled it to be constructed at 20% the cost of other 350-in (9-m class) telescopes.
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 ( C ambridge O ptical A perture S ynthesis T elescope) system at the Univ. of Cambridge observatory, England, and the CHARA ( C enter for H igh A ngular R esolution A stronomy) Array at the Mount Wilson Observatory, Calif. The MMT, which became operational in 1979, consisted of six 72-in. (1.8-m) telescopes on a common mounting and having a resolving capability equal to that of a 176-in. (4.5-m) reflector of conventional design. It was replaced by a conventional 256-in. (6.5-m) single-mirror telescope in 1999. The COAST system, which became operational in 1996, combines light from a trio of small telescopes spaced about 20 ft (6 m) apart. The twin Keck telescopes, in domes a few hundred feet apart, have adaptive optics that make them equivalent in resolving power to a telescope with a mirror 280 ft (85 m) across. The CHARA Array, fully operational in 2002, consists of six 39-in. (1-m) aperture telescopes arranged in a Y-shape and contained in a 1,300-ft (400-m) diameter circle; the combined signals from the six telescopes provide the equivalent of the resolving capability of a telescope 1,080 ft (330 m) wide.
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 and the Gemini North telescope (320 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 Hubble Space Telescope is a 94.5-in. (2.4-m) reflector.
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