卡塞格林系统观测不同距离目标时需要用螺纹微调旋转次镜的方法来改变焦距以获得清晰的像, 但次镜随螺纹旋转会导致系统成像的周期性晃动, 这在许多高精度光学仪器中是不允许的。针对该问题, 分析了系统成像周期性晃动的影响因素并给出具体的运算公式。通过具体事例结合工厂加工水平, 分析出该加工精度下的最大可能偏差, 若其不在仪器误差允许范围内, 就需采用无旋转的次镜微调机构。结果说明, 用旋转次镜的微调机构产生的晃动半径有0.65 mm, 而用无旋转的次镜微调机构产生的晃动半径只有 0.13 mm, 系统精度得到较大提高。

In order to obtain distinct images for the different distances of the Cassegrain system, the secondary reflector must be adjusted slightly by revolving with a screw thread so as to change the focus of the Cassegrain system. However, the revolving of the secondary reflector can lead to the cyclical shaking image, and this kind of shaking is forbidden in many high-precision optical instruments. Aiming at this problem, the factor causing the periodic shaking of the image in the system was analyzed, and the specific mathematical formula was given distinctly.Additional, through a particular instance based on the precision of manufactory, the maximum deviation under this precision was analyzed, and the fine adjustment mechanism of secondary mirror without revolving the screw thread should be utilized if the shaking radius is beyond the range of allowable error. Results show that the fine adjustment mechanism of secondary mirror with revolving lead can lead to 0.65 mm radius shaking, while the other method without revolving lead can lead to 0.13 mm, implying the precision of the system can be greatly improved.