6m telescope

The 6-m alt/az mounted telescope (BTA - Big Telescope Alt-azimuth) is a national instrument operated by the SAO RAS. The observatory continually provides upgrades to the operation of the telescope and its data acquisition systems, thus ensuring that the most modern of techniques are used for the collection of the astrophysical data and its processing. The telescope is located near Mt.Pastukhova at an altitude of 2070m above sea-level. The co-ordinates of the site are longitude 41^o26'30", latitude +43^o 39'12".

Main mirror diameter 6 m Focal length 24 m Light collecting area 26 sq.m Wavelength range 0.3 - 10 mkm Angular resolution 0.6 arcsec - with the application of speckle   interferometry techniques 0.02 arcsec Mass of the main mirror 42 tons Total telescope mass 850 tons Telescope height 42 m Dome height 53 m Limiting stellar magnitude (1994) 26

Optical scheme of the telescope

The main mirror (MM) is parabolic in shape and has a focal length of 24 metres. Its diameter is 605 cm. The diameter of the cylindrical prime focus cage where optical devices are located, as are the drive mechanisms for the movements of the lens corrector and the hyperbolic secondary mirror, is 2 metres. Laboratory tests have shown that 90% of the energy is concentrated into a circle 0.8" in diameter. The image diameter is determined by the micro-climate in the dome, and also by the thermal state of the mirror. Under favourable conditions (little temperature difference between the MM, the air inside the dome and also that outside it), the seeing is limited by atmospheric turbulence. Such conditions result in the width of the seeing disc (FWHM) being ~1" for 20% of observational nights.

The optical scheme of the telescope provides for working at the prime focus (f/4) and also at two Nasmyth foci (f/30). To correct for field aberrations when working at the prime focus, a Rosse corrector is used. This is made of K-type glass (the short-wave bandwidth is 3800 ). The field of view with coma and astigmatism corrected to a level of < 0.5" is approximately 14'. The time taken to switch the beam from one focus to another is 3-4 minutes, which makes it possible to conduct different observing programmes during a single night.