Studies On Cooled Off-axis Reflective Optical Systems

Traditional infrared materials are expensive, and the material homogeneity can not be guaranteed when the lens aperture is large. So the infrared optical system with large aperture size is difficult to finish.Reflective systems have wide working spectral band and no chromatic aberration, and the reflective materials are cheaper. So reflective structures can be widely used in infrared systems. The traditional reflective systems have central obscuration, and the system energy and imaging quality are seriously affected. In order to remove the central obscuration, off-axis of mirrors are necessary.So faroff-axis reflection optical system are usually uncooled, but in order to improve sensitivity and signal-noise ratio, cooled optical systems are needed.This paper makes an exploratory study on cooled off-axis reflective optical systems. The basic characteristics and the vector aberrations of the systems are analyzed. A design method of cooled off-axis three-mirror optical systems is proposed. A cooled re-imaging off-axis three-mirror optical system composed of three mirrors is proposed. To ensure 100% cold shield efficiency, cold stop should be set as optical stop. In order to control the size of the primary mirror, the image of the optical stop through the second mirror and the third mirror should coincide with the primary mirror. Reflective elements in the system are tilted and decentered properly to make the system unobscured. Freeform surfaces are used to enlarge the field of view and improve the image quality. The designed system focal length is 450 mm, working waveband is 3-5?m, F-number is 2, and field-of-view is 3.662°×2.931°.The modulation transfer function of the system is above 0.5 all over the field of view in the temperature range with-40?~60?. The result shows that the system has compact structure, and the athermalization is achieved.