设计了一个无运动部件、结构紧凑并实现两档焦距的折反射混合式空间光学系统。长、短焦前组共用一个Ritchey-Chrétien(R-C)反射系统，使用分光棱镜对光路分光，由两路后组透射系统对前组进行缩放实现两档焦距。长焦焦距为4700 mm，F数为13.4，短焦焦距为2350 mm，F数为6.7。在光机设计上，还考虑了对温度变化敏感的间隔使用热膨胀系数小的材料；为了减小反射镜面形变化，采用了热性能好的材料作为反射镜基底，并对相机在空间环境下温度变化在(19±3) ℃范围内进行了像质分析，设计结果能满足使用要求。

A compact space optical system without moving parts which can get two focal lengths is designed. Ritchey-Chrétien (R-C) mirror is common for long-focus system and short-focus system. A splitter prism is used for splitting the light into two beams relayed by transmitting optical system. The focal length is 4700 mm and F number is 13.4 for the long-focus system, and the focal length is 2350 mm and F number is 6.7 for the short-focus system. Some measure is taken to make sure the less degradation of MTF for thermal distortion, such as a kind of material with small thermal expansion using comrection between the primer mirror and the second mirror and material with good thermal property as mirror substrate for reduce surface distortion. The modulation transfer function (MTF) of optical system in condition of (19±3) ℃ is analyzed, which shows a good result for user′s requirement.