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reflecting telescope(ri-flekt -ing) (reflector) A telescope that employs a mirror (the primary mirror) to bring light rays to a focus. The various configurations include the Cassegrain, Ritchey–Chrétien, Newtonian, Gregorian, and coudé telescopes. There are also the Maksutov and Schmidt telescopes, which are catadioptric telescopes: these instruments differ in the shape of the primary, and in the additional optical system used to bring the image to a convenient point where it can be viewed through an eyepiece, photographed, or detected and recorded electronically, and subsequently analyzed. The aberrations in reflectors can be reduced to a very low level by suitable shaping of the primary mirror and with the possible use of a correcting plate. In addition, a mirror, unlike a lens, suffers no chromatic aberration.
The first working reflector was a Newtonian. In the period following its introduction in about 1670, reflecting telescopes with mirrors made from speculum metal displaced the simple Galilean and Keplerian refracting telescopes whose performance was limited by the chromatic aberration of their single-lens objectives. In the late 18th century, William Herschel developed a special ability in making speculum-metal mirrors of really accurate figure. In 1783 he built a reflector with an aperture of 46 cm and in 1789 one of 122 cm. He used these telescopes for a wide range of observations, making many important advances in stellar astronomy.
The end of the era was marked by the construction in 1842–45 of a 183-cm diameter speculum-metal mirror by William Parsons, 3rd Earl of Rosse, at his family seat of Birr Castle, at Parsonstown, Co. Offaly, Ireland. Known as Leviathan, this reflector was the largest such instrument built until the Mount Wilson 254-cm reflector – the Hooker Telescope – was brought into operation in 1917.
In the latter half of the 19th century, refracting telescopes with achromatic objectives overtook reflectors in performance and held their lead until the start of the 20th (see achromatic lens). By then astronomers had come to realize that the reflectivity of silvered glass in mirror design made it far superior to speculum metal, which rapidly became obsolete. It was also realized that refractors had reached a real size limit at about 100 cm diameter: a lens can only be supported around its edge and larger lenses sag under their own weight to an unacceptable degree. A primary mirror can be supported over its whole rear face and thus escapes this limitation.
The modern era opened with the construction of the giant reflecting telescopes at Mount Wilson Observatory and then Palomar Observatory, both in California. The 5-meter Hale Telescope at Palomar, which saw first light in late 1947, remains one of the world's most powerful reflectors in use today. It was surpassed by the 6-meter Bolshoi Teleskop Azimutalnyi (Big Altazimuth Telescope) at the Zelenchukskaya Observatory in the Russian Caucasus mountains, which was the largest optical telescope in the world when it came into operation in the 1970s. The Big Altazimuth Telescope illustrates a trend among the most modern reflectors, whereby advantage has been taken of the developments in drive technology pioneered by radio astronomers to return from the equatorial to the simple altazimuth mounting. The 9.2-meter-effective-aperture Hobby-Eberly Telescope at the McDonald Observatory on Mt Fowlkes, Texas, a lightweight segmented primary mirror in an instrument set up at a fixed elevation angle of 55° to the horizon and able to rotate in azimuth to access 70% of the sky visible from its location, was built at about one-fifth of the cost of comparable instruments of similar diameter. Like the Hobby-Eberly, most of the world's largest telescopes have apertures greater than 8 meters. Next-generation telescopes, such as the 10-meter Keck Telescopes in Hawaii, with their new designs of primary mirrors, are also making use of other advanced technologies, including adaptive optics to reduce the effects of atmospheric turbulence (see seeing), active optics to reduce longer-term effects, and computer control. Telescope performance has also greatly improved as a result of new techniques for detection and analysis of the image, using complex electronic equipment and computer systems (see imaging). Further improvements in instrumentation is, however, unlikely to increase sensitivity very much. The observation of very faint objects is today being accomplished by the new generation of ground-based telescopes in combination with orbiting telescopes, such as the Hubble Space Telescope and the planned James Webb Space Telescope, operating in an environment undisturbed by the turbulence and absorption of the Earth's atmosphere.
See also infrared telescope.