The Special Astrophysical Observatory of the Russian Academy of Science (SAO RAS; Russian: Специальная Астрофизическая Обсерватория) is an astronomical observatory, set up in 1966 in the USSR, and now operated by the Russian Academy of Sciences. Based in the Bolshoi Zelenchuk Valley of the Greater Caucasus near the village of Nizhny Arkhyz, the observatory houses the BTA-6 and RATAN-600, an optical and radio telescope, respectively. The two instruments are about 20 km (12 mi) apart. The BTA-6 (Bolshoi Teleskop Altazimutalny; Большой Телескоп Альтазимутальный, or Large Altazimuth Telescope), with first light in 1975, was for several years the world's largest single primary mirror optical reflecting telescope. The BTA-6's primary mirror has a diameter of 6 metres (236 inches) and is housed in a 48 m (157.5 ft) diameter dome at an altitude of 2,070 m (6,791 ft). It held the record from its completion until 1993, when it was surpassed by the Keck 1 telescope, Hawaii. Telescopes of comparable or larger size have subsequently employed flexible or segmented mirrors, and the BTA-6 remained the world's largest rigid-mirror telescope until the advent of spin-casting technology (which produced, for example, the single 8.4-meter primary mirror of the Large Binocular Telescope in the late 1990s). Its altazimuth mount dictates the need for a field derotation mechanism to maintain the orientation of the field of view. More information...
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