William Herschel Telescope
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William Herschel Telescope(her -shĕl) A 4.2-meter reflecting telescope with an altazimuth mounting sited at the Roque de los Muchachos Observatory in the Canaries and forming part of the Isaac Newton Group. It is largely a UK telescope with the Dutch and Spanish sharing observing time. It began regular operations in 1987. It has a glass-ceramic (Cer-Vit) primary mirror of great thermal stability and a focal ratio of f/2.4. The telescope can be used in a Cassegrain configuration (there is also an off-axis ‘broken Cassegrain’ focus). In addition, light from the secondary mirror can be diverted through the altitude bearings of the Y-shaped fork mounting to two Nasmyth foci, where heavy instruments can be mounted. Sophisticated electronic equipment detects and analyzes the light. In 2000, the William Herschel Infrared Camera (WHIRCAM) was replaced by the Isaac Newton Group Red Imaging Device (INGRID), a near-infrared camera for use mainly at the telescope's Cassegrain focus. INGRID is optimized for a wide field of view at relatively short wavelengths (0.8–2.5 μm). Its 1024 × 1024 near-infrared detector array provides imaging at a resolution of 0.25 arcseconds per pixel.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006