ACTIVE COMPENSATION OF FLEXURE ON THE WHT ISIS SPECTROGRAPH

ISIS Spectrograph Automatic Active Collimator (ISAAC) was developed at the Optical Science Lab, University College London, as an experimenta l system to improve the image stability on the detectors of the Interm ediate-dispersion Spectroscopic and Imaging System (ISIS) at the Willi am Herschel Telescope (WHT). It is based on the concept of active comp ensation, where spectrum drifts, owing to the spectrograph flexing und er the effect of gravity, are compensated by the movement of an active optical element. ISAAC is a fine-steering tip-tilt collimator mirror; it was tested on the spectrograph in 1995 July. Here we report the de tails of the design and the laboratory tests together with the results of the experiment on ISIS at the WHT. The experiment shows that activ e compensation can reduce flexure down to less than 3 mu m over 4 h of telescope motions, dramatically improving the spectrograph performanc e. A way of implementing ISAAC as a common user facility is briefly di scussed, together with a study of the characteristics of flexure in IS IS.