Collimation of Fast Wide-Field Telescopes

Abstract

In this paper, I present a simple technique for collimating the secondary mirror of fast focal ratio wide-held Cassegrain telescopes. This technique minimizes both coma and astigmatism across the field. Because astigmatism is nearly zero on-axis even in a misaligned system, it is necessary to make measurements off-axis. This technique is useful on telescopes corrected for off-axis coma such as Ritchey-Chretien designs and classical Cassegrains with refractive correctors. Proper alignment for astigmatism is especially important in the latter type of telescope where there is no astigmatism across the field in a properly aligned system. The tools required for collimation are a camera that can examine images at several locations at the edge of the field and a secondary mirror that can be controlled in five axes. Also presented are analytic expressions for the amount of field-dependent astigmatism due to miscollimation. The technique is robust enough to collimate telescopes with fixed astigmatism in the telescope primary.