The 4.2-m William Herschel Telescope (WHT) "The king of the mountain", Sky and Telescope, April 1997.

The WHT is of classical Cassegrain configuration, with a paraboloidal primary mirror and a convex hyperboloidal secondary mirror. The diameter of the primary mirror is 4.2 metres. The mirror is made of a glass-ceramic, Cervit, with zero coefficient of thermal expansion. The mirror surface is figured to better than 1/50 wavelength of light on a scale of a few cm, and to better than half a wavelength over larger scales. The aluminium mirror surface reflects about 85% of the light falling on it. It is resurfaced in the aluminising tank every 18 months or so.

The WHT mounting is of alt-azimuth design, which requires a computer to calculate 20 times per second the motions needed in altitude and azimuth to track an object on the sky.

The telescope weighs about 200 tonnes and floats on a layer of oil 0.1 mm thick, pumped through 6 supporting pads. There is so little friction that the telescope can be pushed around by hand.