长焦距、多谱段、轻量化成像系统具有分辨率高、轻小型等优点，是目前成像系统设计的一个重要研究方向。针对制冷型红外同轴三反射结构中心遮拦大的不足，提出了一种改进型的同轴三反射镜成像系统，在系统三镜后加平面反射镜，将系统光线进行折转，利用三个反射镜的二次曲面系数校正轴外像差，给出了具体的设计思路及考虑问题，并给出了一个焦距f=400 mm, F/#为4，全视场角为1.72°的设计实例。设计结果表明，系统成像质量接近衍射极限。

The imaging system with long focal length, multi-spectrum and light weight has the advantages of high resolution and small volume, which is an important research direction in imaging system design. According to the large shelter in cooled infrared coaxial three-mirror, an imaging system with the improved coaxial three-mirror is introduced. A piece of mirror is added behind the third mirror, the light from the system is reflected by the mirror. Off-axis aberration is corrected by second surface coefficients of three pieces of reflective mirror. The detailed design ideas and relative problems are given. A design example with 400 mm focal length, 4 F/# and 1.72° full field of view is given. The result shows that the imaging quality of the system approaches to diffraction limit.