结合反射系统中双反射和三反射各自的优缺点，提出一种特殊的两镜三反光学系统，给出其初始结构的确定方法。对其结构优化过程中，在系统次镜后加平面反射镜，将系统进行折轴处理，然后在折轴反射镜后加透镜来校正剩余像差。该系统初始结构中两反射镜组成的是一个无焦系统，在主镜发生2次反射后成像。分析该初始系统的缺陷，对补偿该缺陷并使其成像质量达到衍射极限的后续补偿系统进行了设计。应用该方法设计了焦距f=5000mm，F数均为5，全视场角分别为0.4°和1°的2个系统，从像质评价结果可以看出，系统成像质量接近衍射极限，具有长焦距、小尺寸等特点。

In combination with the advantages and disadvantages of ordinary double-reflection and three-reflection, one special two-mirror optical system with three reflections is designed and the method for calculating the parameters of its initial structure is given. This initial system composed by two mirrors is a focal system, and the rays are focused after reflecting twice on primary mirror and once on second mirror. The defects of this initial system are analyzed and some steps are taken, which are beneficial to compensate the defects and obtain the better image quality. Two systems were designed with the method. The focal length of them is 5000mm, the F/# of them is 5 and the field angles are 0.4°and 1°respectively. The result of evaluation demonstrates that the image quality of the systems is near diffraction limit and the systems are of high resolution, long focal length and small volume.