ON THE STABILITY OF CASSEGRAIN SPECTROGRAPHS

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