THE BIG CHILEAN

On a mountain in Chile, the largest and most sophisticated telescope on Earth will track

stars billions of light years away - and 'de-twinkle' them. Hugh O'Shaughnessy explains

After an hour of pounding along a rough, stony track across the lifeless Atacama Desert, the Cerro Paranal space observatory in Chile comes as something of a surprise. You emerge over a ridge and see this angular, 30m-high structure sitting on the next mountain, whose crest has been cut off like the top of a boiled egg. Its angularity proclaims the same pure other-worldliness that is the mark of atronomy in every observatory in history, from Babylon to Jodrell Bank.

The difference - at 2,680m above the Pacific Ocean - is that, unlike in Babylon or at Jodrell Bank, the atmosphere is so dry, nature so demanding, that man has never made a home here. Apart from the occasional clump of weeds which seem to survive on what little dew there is, no life of any sort, animal or vegetable, is visible around the Cerro Paranal.

Despite its hostility to humans, this is the site nearest perfection on the planet for such an installation. Observation from a high altitude, which Isaac Newton first thought would be a good idea, is possible through the clear air for 360 days of the year. On the way to Cerro Paranal, my driver said: "I haven't seen a cloud in the sky all the time I've worked here. The only clouds are 1,000m below us, covering the ocean 12km from here."

At this inhospitable spot in southern Chile, the European Organisation for Astronomy is building a Very Large Telescope (VLT) - the largest and most powerful in the world - for the European Southern Observatory (ESO). Costing pounds 400m, it will be powerful enough to see what an astronaut is wearing on the surface of the Moon, or tell the difference between the main section of the Independent on Sunday and the pink business pages.

The telescope's main use, however, will be collecting light generated more than 10 billion years ago. "We'll almost be able to view the creation of the Universe," says Wolfgang Kowasch, deputy head of the construction site. "It is totally thrilling."

Building the Cerro Paranal observatory has not been without its challenges. Though there is no pollution to speak of from human settlements, dust is sometimes a problem. The other natural hazard, constant seismic activity in the Andes, appears to have been countered not least by the almost impossibly high exactitude in building that the ESO has demanded. For the telescope to work, the masonry of Cerro Paranal has to be exactly where it was planned to be, and not a hair's breadth thicker or thinner.

"Nobody has built such a thing to these tolerances before," says Kowasch. "We demand a horizontal tolerance of 4mm and a vertical tolerance of 1mm." Pointing to the ring on which the telescope will rotate on its pier, he says: "That has a tolerance of 0.4mm."

Such stability and precision will be essential when the VLT is trained on something billions of light years away. A fraction of a millimetre's difference in Chile could mean the machine is pointed at something in outer space totally different from what was intended. Such smoothness is also required when astronomers are trying to keep the telescope trained on a moving body emitting very little light and photograph it on an exposure that lasts hours.

Away from the site, similar precision is being achieved in the polishing and grinding of the VLT's giant 8.2m mirrors. A team in Paris is working on the first of four of them, to be installed in August or September next year, and the final optical surface is correct to within 0.00005mm. If such a mirror were expanded to an area the size of Paris, its surface would deviate by only 1mm.

When the whole installation is complete, probably at the end of 1997, its total light-collecting power will be that of a 16m telescope - twice as big as anything in existence, or known to be planned. Each of the big mirrors, 17.5cm thick, is made of Zerodur ceramic material which will not expand, covered with a film of aluminium. Each is activated by a support system of computer-controlled rams which mould the mirror into the most useful configuration.

All this will allow the VLT to literally take the twinkle out of the stars - and not just the little ones. The light the telescope collects will be passed through different combinations of mirror which will compensate for distortions as the light passes through atmospheric turbulence. It will in effect be "de-twinkled"; the resulting degree of definition will be unmatched anywhere.

A variety of astronomical instrument will be installed in the telescope. UVES, a high-resolution camera, will record and analyse ultra-violet light. Another instrument will be for infra-red spectrometry and imaging, enabling astronomers to tell how hot things are out there. By analysing the infra- red spectrum, ISAAC, as it is called, will be able to see some newly born stars through the clouds of dust and gas which surround them. Behind the primary mirror will sit FORS, an enormous wide-angle lens.

Together, these instruments will be able to tell the appearance, composition and temperature of bodies so far from our planet that they have never been examined in such detail before. Observation may be controlled and carried out via a satellite link, by scientists sitting at the ESO's headquarters at Garching outside Munich.

But why bother with a VLT when the US Hubble space observatory is sending back amazing pictures from space, capturing light before it has been filtered through the Earth's turbulent atmosphere? The answer is that the "de-twinkling" apparatus of the VLT will give a quality of observation on Earth similar to that achieved by Hubble in space. The Hubble's mirror is small, too, compared to this Chilean giant, which will be able to "see" much fainter celestial objects. Moreover, the whole operation in Chile is being carried out for a fraction of the cost involved in flinging Hubble's machinery into space and keeping it operating there.

When the whole scheme is completed, the site itself and the nearby camp will house an interferometric (light processing) complex, its own power plant and everything needed for the maintenance and annual service of recoating the massive mirrors. There will be permanent quarters for staff, lodging for visiting scientists and eventually an airstrip to save the trek of 1hr 40mins over the Atacama from Antofagasta.

The project has since its launch made many men and women sweat blood. The first thing to do was cut 28m off the top of the Cerro to provide a flat area about the size of five football fields. (Cerro Paranal was 2.664m high, now it is only 2,636m.) Natural disasters posed the second threat. Not long ago the Cerro was hit by an earthquake measuring eight on the Richter scale. The approach road was covered with loose boulders; in the offices files and computers were thrown on to the floor, and coffee and juice machines ruined the carpet - but the concrete bases of the observatory were not shaken out of alignment.

Life is demanding on top of the Cerro Paranal. "You suffer from the dryness," says Wolfgang Kowash. "It gets to your respiratory system. I never take oxygen; once you start, you don't want to give it up. But I can't get to sleep without a humidifier in my room.

Peter Aniol, a German businessman and amateur astronomer who has installed his own telescope with Chilean help on neighbouring Cerro Amazonas, spent six consecutive weeks here. "It was very difficult," he says. His friend Kowash adds: "He survived only by going down now and then to the fleshpots of Antofagasta."

There is the occasional sign of the other kind of wild life. "We see desert foxes," says Kowash. "When we arrived they wouldn't come within 30m. Now they come up to 1m. There's the sound of an owl from time to time. On rare occasions we see a condor."

Astronomers wishing to accelerate their careers should surely apply for Chilean nationality at the earliest opportunity. The Chileans have negotiated access to 10 per cent of the VLT's time, and so far have not trained a full complement of scientists able to put such an asset - whose running cost is put at pounds 10,000 an hour - to the best use. A Chilean identity card will perhaps secure priority in the queue to use the most sophisticated instrument on Earth to explore the heavens. !