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Last Updated: Tuesday, 28 February 2006, 11:09 GMT
'Laser star' enhances cosmic view
Laser in action (Stefan Seip/www.astromeeting.de)
The adaptive optics laser in action on Yepun (Image: Stefan Seip)
The world's largest optical telescope facility can now use an "artificial star" to improve its vision.

The star is created with a laser, which is fired 90km into the sky to make sodium atoms in the thermosphere glow.

This spot of light is then used as a reference to process and correct the blurring effects of our turbulent atmosphere on astronomical images.

The system will improve the already stunning pictures obtained by Europe's Very Large Telescope (VLT) in Chile.

The achievement is the culmination of five years of collaborative work by a team of scientists and engineers from the European Southern Observatory (Eso) organisation, and the Max Planck Institutes for Extraterrestrial Physics in Garching and for Astronomy in Heidelberg, Germany.

'Bathroom window'

The VLT is actually made up of four 8.2m telescopes, which have been built on the summit of Cerro Paranal, in the Atacama Desert. Each unit takes a name from the Mapuche language of the Chilean Indians.

The new laser "adaptive optics" system has been attached to the telescope known as Yepun.

The laser is hosted in a dedicated laboratory under the platform of Yepun (Eso)
The laser is prepared in a lab under the telescope (Image: Eso)
The artificial star it creates is extremely faint - about 20 times fainter than the faintest star that can be seen with the unaided eye - but bright enough for the adaptive optics to measure and correct the "twinkling" that impedes a clear view of the Universe.

The system is dependent on some powerful computing, sophisticated electronic camera and laser technology, and a 10cm mirror that can rapidly change its shape.

"As light comes down through the atmosphere, it gets 'corrugated'; as if it has gone through a bathroom window and has a particular shape on it," explained Ric Davies, project manager for the laser source development at the Max Planck Institute for Extraterrestrial Physics.

"By looking at the reference source, the computer calculates what the shape is and makes the deformable mirror take exactly the right shape to make the light flat again," he told the BBC News website.

In unison

Adaptive optics does not need a laser star; it can use a real star as a reference, provided it is bright enough. But there are not many of sufficient luminosity and those that do fit the task are not always in the part of the sky astronomers want to look.

The VLT is the first big telescope in the Southern Hemisphere to use the laser system. In the Northern Hemisphere, the Keck Observatory in Hawaii and the smaller Lick Observatory in California are already using the approach.

Adaptive optics gives ground-based telescopes resolutions that exceed those achieved by space-based observatories such as Hubble; and with their bigger mirrors, the ground telescopes can also see much fainter objects.

The VLT already uses a technique known as "active optics" to enhance image quality.

The VLT (Eso)
The plan is to make all four units work together (Image: Eso)
This system employs tiny actuators on the units' main mirrors, to keep their heavy, curved, reflecting surfaces in the optimum shape as they are tilted to look at different parts of the sky.

The next stage of the observatory's development will be to bring the light beams from all four telescopes together and combine them.

After much computer processing, this would produce an image similar to that which could be achieved by one giant telescope.

Known as interferometry, it is a technique that has been used in radio astronomy for decades. However, making it work on telescopes that see in the visible part of the spectrum is a tall engineering challenge.

"So far, we've tied three units together," said Alan Moorwood, Eso's head of instrumentation. "Instrumentation is being defined now and we should be able to tie four together in the next few years; but this interferometry is by no means easy."

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