Off-axis reflective optics, due to its no chromatic, no obscuration, small size, compact structure, easy to achieve wide field of view(FOV), high-quality imaging and other features, are widely concerned. In this paper, the conformal ellipsoidal dome of MgF2 material is used to allay the air resistance and reduce the aerodynamic heating effect of the missile head. The compact off-axis four-mirror is used to realize a conformal optical system of the seeker with medium wave infrared(MWIR),which the working waveband is 3~5μm, F number is 2, FOV is ±1 degree and the focal length is 55mm. The optics achieve 100% cold aperture efficiency and the MTF value of each field at 17lp/mm is greater than 0.7. It has a working distance of 17.4mm and a sapphire window 14.7mm apart from the focal plane, leaving plenty of room for the refrigerant detector. This system can also meet the high accuracy positioning with imaging quality requirements and ensure light design of the seeker.
Using progressive design method, we designed and built a wide field of view (FOV) catadioptric lens for LWIR earth sensors in the 14 to 16 μm range. The prototype lens is compatible with 640×480 uncooled FPA and 25 microns pixel pitch. Its full field of view is 170° and F number is 0.86. The f-theta distortion is less than 1%. Besides, the system works well during a temperature range of -40°C~+60°C.
The earth sensors on the satellites measure the attitude by observing the discontinuity between earth radiance and cold space background. As IR detector technology advances, the earth sensor has evolved from traditional scanning sensor to static sensors with uncooled infrared FPA. In order to obtain high pointing accuracy of the earth center and avoid the influence of the cloud layer, the optical system should have the following characteristics such as: excellent f-theta linearity, high MTF at corresponding frequency, and high transmittance in the 14 to 16 μm range. In addition, this earth sensor is intended to be used on a LEO satellite, so the optical system must cover a wide FOV larger than 130°, and be as light and compact as possible.
This paper contains the full design process of a wide-angle lens used for LWIR earth sensors from paraxial power allocation calculation to optimization of lens. The lens has a relative large image circle of 13mm, which is compatible with a 640×480 25μm uncooled FPA, and its full FOV is 180°. Its focal length is 4.177 and F number is 0.8. The f-theta distortion is less than 0.25%. By choosing chalcogenide glasses as lens material, the lens has a higher transmittance compare to Germanium lens. Also by analyzing the chromatic aberration of the lens, applying a DOE surface helps to improve the image quality, and pushes the modulated transfer function towards diffraction limitation.
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