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siderostat(sid -ĕ-rŏ-stat) A flat mirror that is driven in such a way as to reflect light or infrared radiation from a celestial body to a fixed point, such as a spectrograph slit at the coudé focus of a telescope, over the duration of an observation. A siderostat is often installed outside the main dome of an observatory and can be used with a coudé spectrograph, say, at the same time as the main telescope is being used to study a different field of view. Siderostats often have computer-driven altazimuth mountings.
an auxiliary astronomical instrument whose principal component is a plane mirror. The mirror moves in such a way that it reflects the light rays from a heavenly body in a fixed direction, in spite of the apparent diurnal rotation of the celestial sphere. A siderostat is used to direct the rays of a heavenly body into a fixed telescope. The mirror of the siderostat rotates around two mutually perpendicular axes. A siderostat used to observe the sun is called a heliostat. A primitive form of siderostat was used as early as the 17th century. In the 18th century the use of a clockwork mechanism was introduced for the rotation of the mirror.