For cooled 320 x 240 detector with staring focal plane array, a novel middle infrared athermal optical system is presented.The system is composed of 5 spherical lenses. The materials of lenses are silicon and germanium. The optical parametersand modulation transfer function (MTF) are investigated. The system has the diffraction limited image quality and stableimage plane from -30 °C to 70 °C. The characteristic parameters of the system are as follows:finumber of 4, cold shieldefficiency of 100, spectrum region of 3.7-4.8 lam and transmissivity of 80. The system has the merits of simple structure,low price, and it is easy to machining.
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