The RATAN-600 (, an acronym for the "Academy of Sciences Radio Telescope – 600") is a radio telescope in Zelenchukskaya, Karachay–Cherkess Republic, Russia. It comprises a 576 m diameter circle of rectangular radio reflectors and a set of secondary reflectors and receivers, based at an altitude of 970 m. Each of the 895 2×7.4 m reflectors can be angled to reflect incoming radio waves towards a central conical secondary mirror, or to one of five parabolic cylinders. Each secondary reflector is combined with an instrumentation cabin containing various receivers and instruments. The overall effect is that of a partially steerable antenna with a maximum resolving power of a nearly 600 m diameter dish, when using the central conical receiver, making it the world's largest-diameter individual radio telescope. Operating modes The telescope can operate in three modes: Two-mirror system: A sector of the ring focuses waves to the cylindrical secondary mirror which directs them onto the receivers. Three-mirror system: The linear plane mirror reflects the waves to the south sector of the ring, which focuses them on a cylindrical secondary, that reflects them onto the receivers. This is a Kraus-type periscope mirror system. Entire ring: For observations near the zenith, the entire ring can be used, together with the conical secondary mirror and its receivers. Simultaneous independent observations at various discrete azimuths are possible. For this, a sector of the ring is coupled with one of the secondary mirror-and-receiver units, which can be positioned using railway tracks, while another sector, in conjunction with another secondary mirror, is similarly used for an independent observation. Optical specifications It has a resolving power in the horizontal plane of 1 arcminute at a wavelength of 8 cm (3.75 GHz). The effective collecting area of the entire ring is which is 0.33% of that expected of a completely filled reflector of this size. Transit telescope The RATAN-600 is primarily operated as a transit telescope, in which the rotation of the Earth is used to sweep the telescope focus across the subject of observation. Radio frequency observations can be made in the frequency band 610 MHz to 30 GHz, though primarily in the centimetric waveband, with an angular resolution of up to 2 arcseconds. Observation of the Sun at radio wavelengths, in particular of the solar corona, has been a long-standing focus of the RATAN-600's scientific programme. It has also contributed to radio observation for the SETI