Radiometric errors due to focusing collecting optics at varying object distances.

For radiometric or photometric instruments using collecting optics, the étendue or collecting efficiency varies with distance for near objects, causing significant errors. Variation in effective collecting area as well as in solid angle with distance has to be considered. Errors for a thin lens and a Cassegrainian system are compared. Moving the secondary for focusing greatly reduces this variation. The ratio of primary to secondary focal length also plays an important role. Proper choice of this ratio can minimize the variation in the field of view, improving the depth of focus. Calculations are useful for extending the range of instruments or for calibration by near sources.