The street lamps in the village of Barlovento on the Canary island of La Palma wear hats to direct their light downwards. This is just one, but perhaps the quaintest, example of the symbiosis that exists between the people of La Palma and the Astrophysical observatory at the Roque de los Muchachos 7000 feet above sea level on the island’s bare and snowy summit.
Javier Méndez, public relations officer of the Isaac Newton Group of telescopes (ING) at the observatory explains the hats. Light pollution is one of the astronomers worst enemies, he says. Light from ground level contaminates the light from the stars. Consequently the Canary Islands’ government requires all street lights in the norther n part of La Palma to have hats to minimise the amount of light they direct upwards into the night sky. Méndez says that the 70,000 islanders are pleased to cooperate with the Observatory, which is one of the best in the world, because of the international renown that it brings the island.
At the Roque, as it is affectionately known, there are three telescopes run by the ING, a joint venture serving UK, Spanish, Dutch and Irish scientists. The main element of each telescope is a large concave mirror that concentrates light on a system of smaller mirrors and lenses. These can be configured either to form a high resolution image of a small part of the sky or to direct the light from a particular object onto measuring equipment.
The William Herschel telescope, the largest of the three, has a mirror 4.2 metres in diameter weighing over 16 tons. It can resolve objects 1 arcsecond apart – about sixty times better than the naked eye, – sufficient to read the Financial Times from a range of 100 metres.
The resolution would be better if the telescope were in outer space. Even in the calm clear conditions prevailing at the Roque, atmospheric turbulence distorts the image. “It’s like looking down into a swimming pool and trying to read a newspaper lying on the bottom” says Chris Benn, the astronomer who manages the telescope.
The Hubble space telescope, orbiting above the atmosphere, has resolution ten times better than the William Herschel with a mirror half the size. The ING cannot launch their telescope into space, but they have a couple of tricks for improving matters. They have already begun a “half arc-second programme” which will double resolution by removing all sources of heat from the telescope building, where convection currents cause air turbulence.
In the longer term they will use adaptive optics to bring resolution up to about 0.1 arcsec, Benn says. The system will work by monitoring the image of a bright star to measure the distortion induced by atmospheric turbulence and continuously changing the shape of the mirror so that it cancels the atmospheric distortions.
Research projects on the Willim Herschel telescope span the three big areas of astronomy – the origin and history of the universe, the behaviour of matter under extreme conditions not attainable on earth, and the search for extra terrestrial life.
About 80 per cent of this work involves measuring the intensity and the wavelength of the light from different objects, rather than making pictures of them. Big telecopes can make these measurements more precisely because they catch more light. Benn likens the William Herschel to a bucket for light. It catches about 1 million times more than the human eye, and four times more than the Hubble telescope. It could detect the light from a single candle on the surface of the moon, Benn says.
The colour of light, determined by the spectrum of wavelengths, carries a huge amount of information. The colour of a newspaper indicates whether it is the FT or a lesser broadsheet. The spectrum of light from a star can show which chemical elements it contains, its speed of motion and its distance.
Each chemical element emits a characteristic set of wavelengths. The gas Helium was first discovered on the sun, where it is produced continuously by nuclear fusion. If ta star is moving away from us, the distinctive signatures for each element are shifted towards red. Approaching motion causes ashift towards blue.
Benn uses the William Herschel telescope to study quasars, faint star-like objects that were first identified in the 1960s and are now known to be the exploding nuclei of distant galaxies. The red shift in their spectra tells him that they are moving away from us at tens of thousands of kilometres per second. The high velocity of quasars is caused by the expansion of the universe, and is the best clue to their distance. The quasars are thousands of millions of light years away: the light Benn is measuring was emitted when the universe was young.