Scientists working on the Very Large Telescope say it will be powerful enough to show an astronaut walking on the moon.
THE SILVER buildings on the summit of the Cerro Paranal mountain in the Atacama desert, in Chile, looks like the set for a science-fiction film. In fact, this is where the European Southern Observatory (ESO) is building a very large telescope. The name? The Very Large Telescope - more usually, VLT. When complete, it will be the largest, most powerful optical telescope available to astronomers, capable, in theory, of seeing an astronaut on the Moon. Except that it isn't a telescope - it is four telescopes.

Engineers and technicians are working feverishly to complete the first of four large telescopes. Unit Telescope 1 - or UT1 - is expected to see its "first light" at the end of May. Together with three other identical telescopes, to be completed in the next few years, its 8.2- metre mirrors will allow observation of celestial objects not seen before. "What is unique is that the VLT will combine very high sensitivity with very high resolution," says the ESO director-general Riccardo Giacconi.

The key to its success is the co-ordination of the telescopes. Together, they will gather as much light as a single 16m mirror. So why not just build a single 16m telescope?

Until recently, ground-based telescopes suffered two problems. First, the size of the mirror was limited to about 6m, because if it was any larger it would sag under its own weight when pointed in different directions, destroying optical quality. Second, the atmosphere itself. Turbulence in the air causes images to wiggle, as anyone can see in the twinkling of stars. In a telescope, images become smeared and resolution is lost.

One solution came from Star Wars (the defence concept, not the film). During the 1970s, American military engineers developed adaptive optics devices - deformable mirrors that would allow control of the beam of a laser weapon over long distances in air. They compensated for the effects of air turbulence by continuously changing shape. Much of the technology was declassified in the 1980s and astronomers were quick to use it to improve the resolution of ground-based telescopes.

In the VLT, each 8.2m mirror is supported by 256 actuators, all driven by a computer which continuously monitors the reflection of a reference star on different parts of the main mirror. The same actuators also compensate for gravitational deformations of the mirror as its alignment changes.

The greatest challenge is linking the optical signals of the four telescopes so they function like a single mirror with an aperture as large as the distance between the two most distant telescopes. It is a technique already widely used in "long- baseline" radio telescopes. But radio waves are comparatively easy to combine. To achieve the same results in the VLT, light from the four telescopes will travel through 60m optical delay systems that will continuously equalise the distance each beam of incoming light travels. At a central point, the four beams will meet and overlap, creating the interference fringes familiar from school laboratory experiments. Computer processing of these fringes will allow the reconstruction of images with a resolution 100 times greater than by direct imaging. "We expect the first fringes in 2001," says Giacconi. Then the second telescope will be completed and linked up with UT1.

Because the four light beams must cover exactly the same distance, the control margins of the moving mirrors will have to be much smaller than one wavelength. "This is really at the forefront of optical technology," says the astronomer Eduard Zuiderwijk, an astronomer of the University of Groningen in the Netherlands. "If this succeeds, we will be able to observe objects with such detail that, up to now, astronomers could only dream of," he says. We expect to see galaxies at larger distances and larger redshifts... we will study the formation of galaxies at an epoch that is within five per cent of the life of the universe."

The astronomers also expect to expand their information on planets that circle around stars. "There is a campaign being put together to measure extrasolar planets that have already been found," says Giacconi. Their investigation will shed light on "how likely it is that conditions for the emergence of life exists in the universe", he adds. "We will be able to measure directly the diameter of a large number of nearby stars and confront this with our theories. I'd bet my right arm that we are in for some surprises," says Zuiderwijk.

Many people will point out that there is already a very serviceable telescope at work - the orbiting Hubble Space Telescope (HST). But Alan Moorwood, who is responsible for the development of the instruments that will be attached to the VLT, explains that the observations by the HST and the VLT will complement each other. "The HST will see the very sharp points of light in distant galaxies, but not necessarily see the extended diffuse part, while an 8m telescope on the ground is much better at seeing the diffuse part of the galaxy, but less good at seeing the sharp spots."

Such sophistication doesn't come cheap. The VLT will cost about pounds 347m. But Professor Rolf Kudritzki, director of the University of Munich's institute for astronomy and astrophysics, believes the cost is justified. "Mankind wants to know where it comes from, where it is going. Astronomy stimulates people's curiosity by following up these questions more deeply and thoroughly than any other science. Astronomy gets to the roots of knowledge."