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Structural analysis and active flexure compensation of the high-resolution optical spectrograph for the Gemini South Telescope

HROS is the high resolution optical spectrograph which is being constructed by University College London for use on the Gemini South Telescope in Chile. We report here the structural analysis which was performed by CLRC on the proposed design as part of the conceptual design review. The UCL design uses an immersed echelle grating and has a number of very large and hence heavy optical components. Supporting these elements while providing a stable spectrum on the detector is particularly challenging as the instrument is designed to be mounted at the Cassegrain focus of the telescope and will therefore be subject to changing gravity fields during integrations. Our analysis demonstrates that it is possible to design a structure which will limit the bulk of the flexure in the spectrograph to rotation of the major elements with relatively little inherent decentering. This structure lends itself to active flexure compensation. Our analysis suggests that by careful design of the support structure, active control can provide compensation for quite large flexures with little or no degradation to the quality of the spectrum delivered at the detector.