离轴三反光学系统的焦距是空间相机的重要指标之一,离轴三反空间相机在完成光学系统装调及光学镜头装配后需要进行焦距测量。通过对两种经典的焦距测量方法即放大率法和测角法进行对比分析,选用测角法作为焦距的测量方法,同时对用于同轴光学系统的焦距测量公式增加离轴角修正量使其适用于离轴三反光学系统,并以某离轴三反空间相机为例论述焦距测量方法及步骤。由修正后的焦距计算公式计算出相机的焦距为8 010 mm,证实了该测量方法的可行性。

The focal length of the off-axis three-mirror anastigmat (TMA) is one of the most important parameters of the space camera. The focal length measurement for the off-axis TMA space camera is done after the alignment of optical system and the assembly of optical lens. The magnification method and the angle measurement method are two classic methods for the focal length measurement. The angle measurement method is selected by comparative analysis. The calculation formula of the focal length which is applicable to the coaxial optical system is modified by adding the off-axis angle. Thus, it is applicable to the off-axis TMA optical system. Besides, the method and the procedure for the focal length measurement are discussed by the example of the focal length measurement for the off-axis TMA space camera. The focal length of the space camera is 8 010 mm, calculated by the modified formula of the focal length. The feasibility of the method of the focal length measurement for the off-axis TMA is confirmed.