page_title: William Herschel Telescope
section_title: Dome
hierarchical_section_title: William Herschel Telescope / Design / Dome
caption_attribution_description: Sea of clouds and the WHT telescope dome.
context_page_description: The William Herschel Telescope is a 4.20-metre optical/near-infrared reflecting telescope located at the Observatorio del Roque de los Muchachos on the island of La Palma in the Canary Islands, Spain. The telescope, which is named after William Herschel, is part of the Isaac Newton Group of Telescopes. It is funded by research councils from the United Kingdom, the Netherlands and Spain.
  At the time of construction in 1987, the WHT was the third largest single optical telescope in the world. It is currently the second largest in Europe, and was the final telescope constructed by Grubb Parsons in their 150-year history.
  The WHT is equipped with a wide range of instruments operating over the optical and near-infrared regimes. These are used by professional astronomers to conduct a wide range of astronomical research. Astronomers using the telescope discovered the first evidence for a supermassive black hole at the centre of the Milky Way, and made the first optical observation of a gamma-ray burst. The telescope has 75% clear nights, with a median seeing of 0.7".
context_section_description: The telescope is housed in an onion-shaped steel dome with an internal diameter of 21 m (69 ft), manufactured by Brittain Steel. The telescope mount is located on a cylindrical concrete pier so that the centre of rotation is 13.4 m (44 ft) above ground level, which lifts the telescope above ground-layer air turbulence for better seeing. A conventional up-down 6m-wide shutter with wind-blind, several large vents with extractor fans for thermal control, and a 35-tonne (34-long-ton) capacity crane (used for moving the primary mirror e.g. for aluminising) are all incorporated. The size and shape of the shutter allow observations down to 12° above the horizon, which corresponds to an airmass of 4.8. The total moving mass of the dome is 320 tonnes (310 long tons), which is mounted on top of a three-storey cylindrical building. The dome was designed to minimise wind stresses and can support up to its own weight again in ice during inclement weather. The dome and telescope rest on separate sets of foundations (driven 20 metres (66 ft) down into the volcanic basalt), to prevent vibrations caused by dome rotation or wind stresses on the building affecting the telescope pointing.
  Attached to the dome is a three-storey rectangular building which houses the telescope control room, computer room, kitchen etc. Almost no human presence is required inside the dome, which means the environmental conditions can be kept very stable. As a result, the WHT obtains perfect dome seeing. This building also houses a detector laboratory and a realuminising plant. Because the WHT has the largest single mirror at the Observatorio del Roque de los Muchachos, its realuminising plant has a vacuum vessel large enough to accommodate the mirrors from any other telescope on the mountain. As a result, all of the other telescopes at the observatory contract to use the WHT plant for their realuminising (with the exception of the Gran Telescopio Canarias, which has its own plant).
caption: The WHT dome above a sea of clouds
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