Zero-Coma Condition For Decentered And Tilted Secondary Mirror In Cassegrain/Nasmyth Configuration

Abstract

The tolerance on decentering and tilt of the secondary mirror of a Cassegrain/Nasmyth telescope is considerably broadened when the zero-coma condition is applied, the limit being set by the allowable residual coma and tilt of the focal surface. The dependency of field tilt on decollimation is defined. A simple set of steps for effective collimation of a telescope is outlined, along with practical engineering recommendations concerning the Ritchey-Chretien (R-C) astigmatism corrector and focal surface subsystem. It is shown that when the proper engineering degrees of freedom are provided, essentially ideal R-C imagery can be obtained over the original optical design field, even in the presence of relatively large angular errors in the optical axes of the primary and secondary mirrors. The specific example given is the case of the proposed University of Texas 7.6 m telescope.