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Abstract

A forward looking infrared optical system comprises a refractor telescope 20 in combination with a powered window and fold mirror arrangement 30. Arrangement 30 comprises a powered window element J with concentric spherical refractive surfaces having their common center of curvature on the common optical axis 19 where the latter is folded by the reflective surface 27 of the mirror element I. Because the window element J is powered it introduces significant spherical aberration and some color aberration to the system however telescope 20 provides compensation for such aberrations by being non-afocal and non-diffraction limited. This is achieved by the objective system 21 of the telescope 20 being telephoto and formed of two elements H,G of which H is positively powered and G is negatively powered and color corrective. The refractive surfaces of element G are each spherical whereas at least one refractive surface (15) of element H is aspheric. Element G is made of a material having a lower refractive index than that of element H.

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First Claim