Abstract

At the last OSA annual meeting, I showed how the coma of a classical Cassegrain telescope can be fixed by adding a very weak meniscus lens next to the secondary mirror. The mirrors were not touched, so the design could be used with or without the lens in place. The distinctly less popular classical Gregorian telescope has a parabolic primary mirror and an elliptical concave secondary. This paper is about a simple way to fix both the coma and astigmatism of the Gregorian, again in such a way that the design can be used with or without the “fix” in place. Here, however, the “fix” is very small—a fraction of the secondary diameter. A small single lens is all that is added, in one version. A slightly more complex all-reflective component, equally small, is added in a different version. Performance numbers are given for a 2-m aperture example, with an f/1.5 primary mirror.

© 1985 Optical Society of America