The BTA-6 (Russian: Большой Телескоп Альт-азимутальный, Bolshoi Teleskop Alt-azimutalnyi, Large Altazimuth Telescope) is a 6-metre (20 ft) aperture optical telescope at the Special Astrophysical Observatory located in the Zelenchuksky District on the north side of the Caucasus Mountains in southern Russia.