Astrograph
Encyclopedia : A : AS : AST : Astrograph
Astrograph (astrographic camera) is a photographic telescope used in astrophotography. Its main parameters are diameter and focal length of an objective which determine optical efficiency and image scale in photographic plate.An astrograph is basically a newtonian reflector but uses a hyperbolic miiror instead of a parabolic mirror or spherical mirror.Astrographs have a very fast focal ratio which make its focal plane extremely curved.An astrograph also suffers from coma as it has a very fast f/ratio.
Wide-angle astrographs with focal distance less than 1 m are used for photographing a huge sky areas with images of meteors, comets and artificial satellites. Astrographs with focal distances between 10-15 m are used in precise measurements. Many observatories of the world are equipped with the so-called normal astrographs with focal length 3-4 m.
In 2004, Celestron announced they were producing an astrograph of 20 inch aperture (51 cm) based on a corrected Dall-Kirkham design. Priced at $40,000 USD, this brings this class of instrument within reach of the very affluent end of the amateur astronomy market. However, since the announcement, all mention of the 20" has disappeared from Celestron's web site, and no further information is known on whether or not the company still plans to release the telescope.
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