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 experimental
system to improve the image stability on the detectors of the Intermediate-dispersion Spectroscopic and Imaging System (ISIS) at the
William Herschel Telescope (WHT). It is based on the concept of active compensation, where spectrum drifts, owing to the spectrograph flexing under 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 details of the design and the laboratory tests together with the results of the experiment on ISIS at the WHT. The experiment shows that
active compensation can reduce flexure down to less than 3mum over 4h of telescope motions, dramatically improving the spectrograph A way of implementing ISAAC as a common user facility is briefly discussed, together with a study of the characteristics of flexure in
ISIS.