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 3 μm over 4 h of telescope motions, dramatically improving the spectrograph performance. A way of implementing ISAAC as a common user facility is briefly discussed, together with a study of the characteristics of flexure in ISIS.