根据快速控制反射镜系统(快反系统)对平面反射镜的设计要求，对快反镜的轻量化结构进行了优化设计。选择比刚度大、热变形系数小的碳化硅为镜坯材料，采用背部开槽式结构减重，背部3点式支撑，对不同筋宽和镜面厚度等多种轻量化方案进行了有限元分析。结果表明: 筋宽越大，镜体刚度越好，但筋宽增大到一定程度时，反射镜刚度的改善程度减缓; 镜面越厚，镜体内应力、自重变形越大。结合反射镜设计要求和有限元分析结果，加工制备了筋宽为4 mm，镜面厚度为4 mm的碳化硅反射镜，镜体轻量化率达55%。实测反射镜的面形精度，其RMS值不超过λ/30，与分析结果相符，满足系统的使用要求。

The lightweight structure of a mirror in the Fast-steering Mirror(FSM) System was designed. Due to the high specific stiffness and low thermal deformation rate, SiC was used as a raw material to prepare the mirror. The lightweight mirror with a notched structure was supported by three points on the back. Several lightweight structures were analyzed by finite element method. The results show that the rigid of mirror increases with the width of rib. When the rib is wide enough, the rigid of mirror increases slightly. The stress and deformation of mirror caused by gravity increases with mirror thickness. Structure parameters of 4 mm rid and 4 mm mirror thickness are confirmed considering current mechanical manufacture process. The lightweight rate of mirror is 55%. The RMS value of mirror measured by Zygo interferometer is less than λ/30, which is in agreement with that of the analysis.