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利用集光镜的辐射计或光度计,对于近物体,收集效率随距离而变化,造成显著的误差。研究了有效收集面以及立体角随距离的变化,把薄透镜和卡塞格林系统的误差作了比较。移动次镜调焦能大大减少这种变化。主镜和次镜焦距比也有重要作用。适当选择这个焦距比,能使视场变化减到最小,改善焦深。计算结果对于扩大仪表的应用范围和用近源作校正,都是有用的。
With a collecting mirror radiometer or photometer, for near objects, the collection efficiency varies with distance, causing significant errors. The variations of effective collecting surface and solid angle with distance are studied, and the errors of thin lens and Cassegrain system are compared. Moving secondary lens focusing can greatly reduce this change. The primary and secondary focal length ratio also has an important role. Proper choice of this focal length ratio, can minimize changes in the field of view, improve the depth of focus. Calculation results for the expansion of the scope of the instrument and the use of near-source correction, are useful.