The WHT has an alt-azimuth mount with a 4.2 metre diameter f/2.5 parabolic primary mirror. The focal ratio at the Cassegrain and the two Nasmyth focii is f/10.94, and at the corrected prime focus f/2.81. Any of the Nasmyth and Cassegrain focii can be selected by moving the Nasmyth flat mirror, however the change between prime focus and Cassegrain/Nasmyth can only be done during the day. The main instruments include a medium-resolution spectrograph, a high-resolution spectrograph, optical and infra-red imaging systems, a wide-field multi-object fibre spectrograph, an integral field fibre spectrograph and an adaptive optics system. This cutaway drawing pictures the telescope and building. For the most up-to-date technical information, see the WHT home page.
The alt-azimuth 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 derotation optics in the beam. The former option is always used at Cassegrain and prime focii, but at Nasmyth only light instruments can be placed directly on the turntables, and there image derotators for the Utrecht Echelle Spectrograph and for the GHRIL Nasmyth focus. Each derotator consists of a pair of fused silica prisms, each with lens surfaces, and these are mounted on the Nasmyth turntables and rotated under computer control at the appropriate rate to keed the final image at fixed orientation. This allows instruments on the Nasmyth platforms to remain stationary. The field of view at the Nasmyth focus with the derotator is 5 arcmin diameter; at Cassegrain it is 15 arcmin diameter.
Prime focus has a six element field corrector and atmospheric dispersion compensator, giving correction over an unvigetted field of 40 arcmin diameter, and atmospheric dispersion compensation at zenith distances between 0 and 73 degrees.