A systematic performance analysis of aplanatic Cassegrainian telescopes has been accomplished by means of a ray trace program. The results indicate that the available field angle for a given geometric spot size is, to a very close approximation, a function of the relative back focal distance only. Image curvatures, as computed by third-order theory and by the ray trace program, were found to be in very close agreement. Image curvature is plotted as a function of the ray height ratio for a wide range of secondary magnification.
Charles L. Wyman and Dietrich Korsch, "Aplanatic Two-Mirror Telescopes: a Systematic Study. 1: Cassegrainian Configuration," Appl. Opt. 13, 2064-2066 (1974)