Large Altazimuth Telescope

The Large Altazimuth Telescope (BTA-6 or Bol'shoi Teleskop Azimultal'nyi) is a 6-meter reflector telescope in southern Russia, that began operation in 1975 and was the world's largest optical telescope until Keck's 1990 first light. BTA-6 has a monolithic mirror and is of the Ritchey-Chrétien telescope design. It has an altazimuth mount and was a pioneer in using such a mount under computer control for a large research telescope. Over much of BTA-6's life, it has had mirror flaws, with a third primary mirror installed in 1978 and used until 2018, when the second was reinstalled after substantial regrinding to eliminate its problems. Instruments:

• MSS - Main Stellar Spectrograph.
• NES - High Resolution Echelle Spectrometer.
• Speckle - speckle interferometer.
• SCORPIO - Spectral Camera with Optical Reducer for Photometrical and Interferometrical Observations.
• SCORPIO-2 - multi-mode spectrograph.
• MPPP - Multicolor Panoramic Photometer-Polarimeter.