Design Of Infrared Optical System In Despun Device

With the development of the infrared imaging technology, the performance of airborne tracking and targeting equipment system has been well improved. Relative rotation of system in the working process results in image rotation and unstabilization, so it’s very necessary to real-time despine airborne CCD image. Focusing on the infrared cooled CCD imaging system with mechanical despinning, a long-wave infrared optical system to detect despinning amount was designed。The cooled CCD detector carries a cold shield, and is sensitive to heat radiation。In order to meet the requirement of a100%cold shield efficiency, the system must be ensured that the position and aperture of the exit pupil are equal to those of cold shield; Due to the wide working temperature, narcissus was controlled well in the optical system; with the help of design requirements and cooled thermal imaging characteristics, the method of a re-imaging configuration was to reduced the radial aperture and suppressed stray ray radiation.Due to the high refractive index temperature coefficient and the thermal expansion coefficient, the image quality of infrared optical system is affected by temperature severely, so athermalisation design is a need. Benefited from diffractive elements which have special temperature and dispersion characteristics and reasonable focal power distribution, the optical passive athermalization was realized with only two kinds of optical materials.Optical system was optimized with ZEMAX, and the results show as follow: the MTF in the center and the edge of the field of view achieve diffraction limit in the spatial cut-off frequency (211p/mm); The RMS of spot diagram is close to airy disk’s, and the spot diameter is not more than a CCD pixel size (24μm); After being Athermalized at the temperature range-40℃-60℃, the image of system is stable, and defocusing amount is less than the focal depth. The optical system consists of seven lenses, of which the third plane is diffraction surface. Because of aspherical base for the diffractive surface, the various aberrations including spherical aberration and chromatic aberration can be corrected. In addition, the aspheric plays a vital role in reducing the system size and weight and simplifying the system structure. In this paper, a method of compensating the aspherical base was used to fabricate diffractive element; the effect of profile accuracy of diffractive element on system imaging was analyzed; according to theoretical analysis and machining experimental, the technological factors which have influences on machining accuracy of diffractive elements were discussed.