Third Order Aberrations in Cassegrain-Type Telescopes and Coma Correction in Servo-Stabilized Images

Pointing errors in telescopes carried by space vehicles can be compensated by servo-control of the secondary mirror. This introduces some aberrations in the stabilized image, the magnitude of which depends on the asphericity of the telescope mirrors. A configuration is presented in which the centered image of an off-axis object point is free from coma. The required aspheric corrections and the residual aberrations are calculated. Also included is a summary of the third order aberration constants for some well-known centered cassegrain-type telescopes.