Far-sighted telescope opens eyes

Chilean desert: Some of the clearest skies in the world

By BBC News Online Science Editor Dr David Whitehouse

One of the most powerful arrays of telescopes on the planet is about to open to astronomers.

Its telescopes will peer further into space and see objects in finer detail than any other telescope, including the Hubble Space Telescope.

The European Southern Observatory's Paranal facility will be officially opened for use by astronomers from Monday. It is due to be completed in 2004.

Each mirror is 8 metres in diameter

The observatory is located on Cerro Paranal in the clear air of the Atacama Desert in northern Chile. There are few clearer skies for astronomers on Earth.

The finished observatory will consist of four reflecting telescopes, each with light-collecting mirrors that are 8 metres across. Only one of the telescopes has been completed but during its performance tests it has produced some stunning images of the sky.

The second of the four telescopes will come online within weeks, followed by the remaining two in 2000.

Each of the four telescopes at Paranal is world class, but it is when they work together that the great leap forward is made.

By combining the light from more than one telescope astronomers can simulate the performance of a much larger telescope.

This technique, called interferometry, has been successfully carried out with radio telescopes for decades but is only a recent development for optical telescopes.

This is because combining the light beams from more than one telescope involves some very fine measurements. Radio waves have much longer wavelengths than optical light waves, so less accuracy is required to combine the radio beams.

A planetary nebula imaged from Paranal

For optical light, measurements must be accurate to within of a few 10 millionths of a millimetre.

Astronomers could mimic the performance of a telescope whose mirror was 16 metres across, by far the largest in the world. They would do this by combining the light from two or more of the Paranal telescopes and augmenting them with three smaller telescopes in the vicinity.

In interferometric mode, the observatory will be able to peer deep into the cosmos. It will be able to search for planets circling nearby stars.

Many Jupiter-sized planets have been found circling nearby stars in recent years. The Paranal Observatory will be able to detect much smaller planets, possibly as small as 10 Earth masses around thousands of Sun-like stars.

It will also get a detailed view of the mysterious centre of our own galaxy, which is difficult to observe. But by measuring the velocities of objects in the central core, it is hoped to learn more about the super-massive black hole that many astronomers believe resides there.

The telescopes could even make detailed studies of the surfaces of nearby stars. They may also compile sharp maps of the central regions of quasars, exploding galaxies at the edge of the observable Universe.