On the stability of Cassegrain spectrographs

Gravity-induced flexure has been a long-standing challenge in Cassegrain spectrographs at 4-m class telescopes; it is more so on the scale of 8-m telescopes. This is of particular concern for the Gemini High Resolution Optical Spectrograph, which will be Cassegrain-mounted for its routine mode of operation. In this paper we address the general flexure problem and make specific recommendations. In a companion paper we present results on experimental compensation for flexure in a specific Cassegrain spectrograph - ISIS on the 4.2-m William Herschel Telescope (WHT).