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an astronomical instrument for photographing celestial objects. Astrographs are constructed according to the designs of refractors, reflectors, or mirror-lens telescopes (Schmidt telescope, Maksutov telescope, and others). A filmholder with a photographic plate is fitted on the eyepiece end of the astrograph. The rotation of the astrograph in synchronism with the daily motion of the celestial sphere is achieved by an accurate timing mechanism and is controlled by the observer with the aid of a guide—a second optical tube which is fixed parallel on the same mounting. Some astrographs just use a photoelectric guide to automatically hold the star at a fixed point on the photographic plate.
The astrograph’s main feature is the focal length of the lens or the main mirror and the aperture size of the instrument. Short-focus, wide-angle astrographs are used to photograph stars over large areas of the sky, meteors, artificial earth satellites, comets, and asteroids. These astrographs have focal lengths of less than 1 and cover several tens or more square degrees of the sky. For more accurate measurements of the positions of stars and planets as well as of the proper motion of stars, astrographs with focal lengths of several meters are used. These are the so-called normal astrographs (focal length 3.5 m) and zonal astrographs (2.0 m). Astrographs with the longest focal lengths (7–19 m) are used for the highly accurate work of determining star parallaxes and measuring binary stars. The main advantage of mirror astrographs is their high aperture, which allows relatively short exposure times for photographing very faint objects, specifically of space probes receding from the earth.
A. N. DEICH