of Cassegrain reflectors, and Newtonian reflectors. Most optical designs do not produce large, flat, and well-corrected imaging fields and therefore require some type of optical correction by way of field flatteners or coma correctors. Amateur astrographs typically have purpose-built focusers, are constructed of thermally stable materials like carbon fiber, and are put on heavy duty mounts to facilitate accurate tracking of deep sky objects for long periods of time. See also BOOTES List of telescope types References External links The Double Astrograph of the Yale Southern Observatory The Carnegie Double Astrograph Pluto Imaging Challenge: Images Construction of the Tycho Reference Catalogue – 2 Source Catalogues Telescopes Cameras by type Optical devices Astronomical instruments ja:天体写真儀