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Catadioptric Afocal Telescopes For Scanning Infra Red Systems

Reflecting and catadioptric lenses have been used in astronomical telescopes for many years. They have recently been widely used in large aperture and man portable image intensified night vision equipment. The afocal telescope used with scanning infra red systems operating in the 8-12μ waveband generally use refracting telephoto objectives. These can be configured either as a single field of view lens, as part of a dual or multi field of view switchable system or as the basis for a mechanically or optically compensated zoom system. However, for physically large high magnification telescopes, catadioptric systems can offer advantages over refractors. Two types of catadioptric lens are described, the first (x7.5) utilising a full aperture and the second (x30) a sub-aperture germanium element to correct spherical aberration. The two telescopes were designed using an 11 mm. diameter exit pupil and a scanner space field of view of 60°. Only spherical lens surfaces were used.