The William Herschel Telescope (WHT) has an altazimuth mount with a 4.2-m diameter f/2.5 parabolic primary mirror. At present, the Cassegrain and two Nasmyth focal stations (all f/11), and the corrected Prime focus (f/2.8) are in use. The three f/11 stations are all fed by the same secondary mirror, and they can be selected by the motion of the Nasmyth flat mirror. One Nasmyth focus is occupied permanently by the Utrecht Echelle Spectrograph; the other is used by the Ground-Based High-resolution Imaging Laboratory (GHRIL: see section ), which also contains the optical table for the WYFFOS spectrograph (section ). Schematic drawings of the telescope are shown in Figure .
The altazimuth design of the telescope means that the field of view rotates as the telescope tracks. In order to compensate for this, it is possible either to mount instruments on a turntable or to place image derotation optics in the beam. The former option is used at the Cassegrain and prime foci, but at Nasmyth, only light instruments may be mounted directly on the turntables and heavy apparatus, which must be kept stationary, is used with image derotators. There are currently two types of image derotators:-
The appropriate unit is mounted on one of the Nasmyth turntables and is rotated under computer control at an appropriate rate to keep the final image at a fixed orientation. This allows the instrument to remain stationary, at the cost of a reduced field, and a 30% reduction in throughput