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Aiming at the problems existing in the optical system of laser communication terminal, in order to improve the isolation level of the communication terminal, the off-axis four-mirror afocal system with ultra-long exit pupil distance used in the coude optical path laser communication terminal is studied. In this paper, the method of four-mirror afocal system design based on primary aberration is explored. The structural parameters are calculated according to primary aberration coefficient. The procedure for calculating initial structural parameters is programmed. Then a four-mirror afocal system is designed with an entrance pupil diameter of 100mm, a field of view of 500μrad, the operating wave band of 1.55μm, and compression ratio of 15 times. The distance of exit pupil is greater than 500mm. After assembly and adjustment, the wavefront error root mean square (RMS) value of the four-mirror afocal system is 0.0172λ(λ=1.55μm). The four-mirror afocal system in this paper can have good imaging quality, reduce the backreflection and scattering of the sensitive mirrors, and solve the contradiction between the ultra-long exit pupil distance and the ultra-large compression ratio. It has application prospect in the field of space laser communication and space gravitational wave detection.
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