William Herschel Telescope, the pride of Europe

Versión en español

The William Herschel Telescope (WHT) is one of the the world's largest single-mirror telescope. Until 1993, when the American Keck I telescope was inaugurated, the WHT was considered the most powerful telescope in the world, because of its modern technology combined with the superb quality of the La Palma site. The WHT is the largest telescope in Western Europe.

Construction of the telescope building began in 1983. The telescope was shipped to La Palma in 1985 and first light was on 1st June 1987.

The WHT is financially supported by the British PPARC (Particle Physics and Astronomy Research Council) and the Dutch NWO (Nederlandse Organisatie voor Wetenschappelijk Onderzoek), in proportion of 80% and 20% respectively.

The WHT is operated on La Palma by the Isaac Newton Group of Telescopes (ING) which also operates the 2.5 m Isaac Newton Telescope (INT) and the 1 m Jacobus Kapteyn Telescope (JKT).

Astronomical time in WHT is distributed as follows: 60% for British astronomers, 15% for Dutch astronomers, 20% for Spanish astronomers and finally 5% is dedicated to large international projects.

WHT scientific highlights

The Edge of the Observable Universe

The WHT has played a very important role in modern observational cosmology. Deep exposures taken by WHT has led to the discovery of the most distant objects ever observed For example, the galaxy 8C1435+635, found in 1995, and more than 650,000,000,000,000,000,000,000 kilometers away, is the most distant such object known. Moreover, the analysis of faint blue galaxies at redshifts of about 2 which are probably in their first phase of star formation, show us that the space density of galaxies in the early universe must have been much higher than it is now.

Optical Counterparts of Gamma Ray Bursts

On the 28th February, 1997 the WHT took the first picture of the optical counterpart of a gamma-ray burst (GRB), an explosion which emits in a few seconds as much energy as the Sun in its whole life. This is probably the most powerful explosion mankind has ever witnessed. GRBs were discovered in the early seventies and their origin has remained unsolved since then. Thanks to this first detection in the optical wavelengths, and later investigations, we now understand that these explosions take place outside our Galaxy.

Brown Dwarfs

For decades researchers have speculated about the existence of brown dwarfs, celestial objects which probably constitute a link between stars with lower masses and giant planets, such as Jupiter. However, despite many searches having been carried out, their existence had never been unequivocably proved. In 1994 WHT took the spectra which confirmed the discovery of one of the coldest quasi-stellar objects known in the Universe, a brown dwarf.

Is the Universe open or closed?

For some time, astronomers have considered the idea of using type Ia supernovae to determine the extragalactic distance scale. Supernovae of this type occur in the late stages of evolution of a binary system consisting of a white dwarf star orbiting a companion star. When the white dwarf mass reaches a critical value, the nuclear fuel ignites explosively. The intrinsic luminosity of the explosion is thought to be independent of distance and therefore usable as a "standard candle". The distances and redshifts combined of these supernovae provide a measure of the deceleration parameter of the Universe and hence can be used to determine the ratio of the present density of the Universe to the critical density. Since 1992 a systematic search for high redshift supernovae is being made on the WHT and INT telescopes, which has led to the discovery of the most distant supernovae ever observed. Astronomers using this technique believe we will distinguish whether our expanding Universe is open or closed before the end of the millenium. An open Universe means there isn't enough matter in the Universe to stop the expansion, while a closed Universe would imply that there is sufficient material to stop the expansion, and as a consequence, the end of the Universe could be a 'Big Crunch').

Technical description

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.

History of William Herschel, the astronomer and the telescope

1738	William Herschel born in Hanover 15th November
1757	Herschel emigrates to Britain
1766	Herschel hired as organist at Octagon Chapel in Bath
1772	Herschel joined at Bath by sister Caroline; astronomical work begins in earnest
1774	Herschel builds a 1-m reflecting telescope
1781	Herschel discovers Uranus, attempts to name it after George III Rewarded with royal patronage (but never Astronomer Royal)
1780s	Herschel discovers nature of nebulae. Pioneering work on structure of Universe
1970	Anglo Australian Telescope (AAT) being built; astronomers thinking about Northern Hemisphere Observatory
1974	AAT finished; the Scientific and Engineering Research Council (SERC) plans a 4-m telescope
1979	Price tag now £18M, risk of cancellation. Redesigned for £10M
1981	Dutch buy 20%. 200th anniversary of discovery of Uranus, SERC names WHT, places contract
1983	Construction of building begins
1985	Telescope shipped to La Palma
1987	First light 1st June with TAURUS-2 instrument

William Herschel was a German-born musician, who emigrated to Britain at the age of 19 in 1757. His serious astronomical work began in the town of Bath at the age of 35, with the enthusiastic help of his sister Caroline. He never had any formal scientific training. He couldn't afford the refracting telescopes used then by professionals, so taught himself to grind mirrors. He was a skilled and much-admired builder of telescopes, the largest being a 1.2-m reflector completed in the 1770s. He pioneered the kind of fork mounting used for the WHT. He is probably most famous for discovering the planet Uranus, the first new planet to be discovered since ancient times. He attempted to name the planet after George III, and this flattery earned him the title King's Astronomer (although he never became Astronomer Royal). Herschel realised that some of the 'nebulae' were enormous clouds of stars like our own galaxy. He published a number of papers on the evolution of the Universe from a hypothetical uniform initial state to one in which stars were clumped into galaxies. This evolution remains a central problem in cosmology (and one on which much WHT dark time is expended).

The WHT's conception dates back to the late 1960s, when the Anglo-Australian Telescope (finished 1974) was being designed. Astronomers wondered what telescopes of comparable power they should build in the Northern Hemisphere. They settled on a suite of three: a 1.5-m, the 2.5-m from Herstmonceux (INT) and a 4.5-m. There was particularly strong pressure from the radio astronomy groups in Cambridge and Jodrell Bank for a powerful instrument which they could use to follow up their discoveries of optically very faint galaxies in the Northern Hemisphere. SERC started to lay plans for the WHT in 1974. In 1979, with a price tag of £18M, the project was on the verge of being scrapped. SERC set up a team to take another look at the design. They were able to cut the price of the telescope by £1M and the price of the building by £7M, by reducing the focal length of the main mirror and shrinking the dome, 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). 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 1st June 1987, using the TAURUS-2 instrument.

For further information please contact:
Javier Méndez:
jma@ing.iac.es
Phone: 0034-922-425464
Fax: 0034-922-425401