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William Herschel Telescope

The William Herschel Telescope (WHT) is the second largest telescope at El Roque de Los Muchachos Observatory in La Palma. With 4,6 m in diameter it is also the second largest in Europe only following the 10,4m Gran Telescopio de Canarias. Despite being of modest size nowadays is one of the most scientifically productive telescopes in the world.

William Herschel Telescope La Palma


Its major results are:

  • The discovery of the first evidence for a supermassive black hole (Sgr A*) at the centre of the Milky Way galaxy
  • The first optical observation of a gamma-ray burst ever made


The Science and Engineering Research Council (SERC) began laying plans for the WHT in 1974. In 1979, with a price tag of 18M pounds, the project was on the verge of being scrapped. SERC set up a team to take another look at the design and they manage to reduce the budged to 10 M pounds. In 1981, SERC negotiated a 20% stake by the Dutch. That year was also the 200th anniversary of Herschel's discovery of Uranus, and the name of the new telescope was announced.
First light was 1987 June 1, using TAURUS-2.


  • The telescope is named after William Herschel (Germany 1738 - England 1822), a musician, with no formal scientific training who got interested into astronomy by the end of 35. He discover the planet Uranus with a 1.2-m reflector he constructed himself as he couldn't afford the refracting telescopes used then by professonals. He pioneered the kind of fork mounting used for the WHT.
  • Reducing the construction price in 8 M pounds was made by reducing the focal length of the main mirror and shrinking the dome, by making the dome onion shaped (which allows for simpler shutters) and by lowering the telescope closer to ground level (turbulent air currents only rise about 3 m off the ground on La Palma).
  • The WHT is like a swiss knife. In every focus it has an instrument that are relatively easy to switch from one to another. Nowadays professionals can apply for time with the following common-user instruments:
    • ISIS - single-slit spectroscopy, R < 10000, 4' slit, spectro-polarimetry
    • LIRIS - IR spectroscopy, R < 4000, and imaging (including imaging polarimetry), 4' field
    • ACAM - optical imaging, low-resolution spectroscopy, 8' field
    • Prime-focus imager - optical imaging, 16' field
    • AF2/WYFFOS - multi-object fibre-fed spectroscopy, R < 9000, 40' field
    • NAOMI/OASIS - integral-field spectroscopy with or without natural-guide-star adaptive optics (NGS AO), R < 4000, 17" field
    • NAOMI/INGRID - IR imaging with or without NGS AO, 40" field (coronagraphy is also possible, with OSCA, 25" field)