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Saturday, 6 July, 2002, 09:28 GMT 10:28 UK
Big eye on the sky
McGourty, BBC

It cost more than 500m to build, but in the VLT, the European Southern Observatory organisation has a world-beating telescope facility.


We would be able to read the registration plate of a hypothetical car driving on the Moon

Dr Roberto Tamai, Paranal Observatory
The Very Large Telescope actually consists of not one, but four 8.2-metre telescopes located on a mountaintop in a desert thought to be the driest in the world.

This remote and desolate corner of the world is near-perfect for astronomers. Short of being in outer space, it is hard to imagine a better spot to put your telescope.

The mountain is 2,635 metres high, more than 110 kilometres (70 miles) from the nearest city and 13 kilometres (8 miles) inland from the Pacific Ocean. It all makes for minimal light pollution and atmospheric conditions that are unusually stable.

New technologies

There are around 330 clear nights a year and humidity is virtually zero. Water vapour may be good for plants, animals and humans, but it hampers observations, particularly at infrared wavelengths.

So, the dry atmosphere is crucial. "It's bad for astronomers here, but very good for astronomy," joked Dr Roberto Gilmozzi, director of the Paranal Observatory, which operates the VLT facility. But it is not just the location that is unique.

VLT, Eso
The clear, dry air gives an exceptional view of the sky
A host of revolutionary new technologies have been used in constructing the VLT. It is the first large optical telescope purpose-built as an interferometer.

This means that the light gathered from all four telescopes can be combined to provide images of the quality you would expect from a far larger single instrument.

So far, two of the VLT units have been made to work in tandem, producing important scientific results. But when all four work in unison next year, they will represent the most powerful optical-infrared telescope system on Earth.

'Magic' tunnel

Dr Roberto Tamai, head of engineering at Paranal, said the images would be as sharp as if they came from a single telescope 200 metres across.

"We would be able to read the registration plate of a hypothetical car driving on the Moon," he told BBC News Online.

VLT, Eso
The tunnel: The heart of the interferometer
The key to the system is the Interferometric Tunnel, an underground cavern where some magic is performed on the light gathered by the four telescopes.

The light from the object under observation will reach the VLT units at slightly different times, because they are in different positions.

So, each beam is directed - via a series of up to 16 mirrors - into the 130-metre-long tunnel, where they are passed through one of several "delay lines", which compensate for the different paths the beams have taken.

Engineering expertise

The delay lines help to synchronise the beams, before redirecting them to a central laboratory.

The interference fringes produced when the beams are finally recombined provide the information needed to reconstruct the original image in unprecedented detail.

Radio telescopes have been combined as interferometers for years but the engineering precision required to match up the much shorter wavelengths in optical light take the VLT to another level.

VLT, Eso
The light from the main units can be combined
 WATCH/LISTEN
 ON THIS STORY
The BBC's Christine McGourty
"Britain has joined the world's premier astronomy club"
The BBC's Christine Mcgourty
"Scientists believe this telescope is the best in the world"
The UK joins the European Southern Observatory.


VLT FORUM

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