High Quality Content by WIKIPEDIA articles! The BTA-6 (Большой Телескоп Альт-азимутальный, Bolshoi Teleskop Alt-azimutalnyi, Eng:Large Altazimuth Telescope) is a 6 m aperture optical telescope at the Special Astrophysical Observatory located in the Zelenchuksky District on the north side of the Caucasus Mountains in southern Russia. The BTA-6 achieved first light in late 1975, using a computer-controlled alt-az configuration, supporting a single piece glass mirror.