Image caption The Paranal Observatory is located 2,635m above sea level

Astronomers have created the world's largest virtual optical telescope, linking four telescopes in Chile so that they operate as a single device.

The telescopes of the Very Large Telescope (VLT) at the Paranal Observatory form a virtual mirror of 130m (424ft) in diameter.

A previous attempt to link the telescopes last March failed.

Thursday's link-up was the system's scientific verification - the final step before scientific work starts.

Linking all four units of the VLT will give scientists a much more detailed look at the Universe than previous experiments using just two or three telescopes to create a virtual mirror.

The process that links separate telescopes together is known as interferometry.

In this mode, the VLT becomes the biggest ground-based optical telescope on Earth.

Besides creating a gigantic virtual mirror, interferometry also greatly improves the telescope's spatial resolution and zooming capabilities.

The VLT is one of several telescopes in the Atacama Desert set up by the European Southern Observatory (Eso).

Eso is an international research organisation headquartered in Munich, Germany, and sponsored by 15 member countries.

Vital milestone

Even prior to the start of the operation, as the domes of the four VLT units opened on the desert mountaintop, excitement filled the Paranal Observatory's tiny control room.

Image caption The combination of four units of the Very Large Telescope facility creates a virtual 130m-wide mirror

It was going to be a special night, said one of the astronomers.

The head of instrumentation at Paranal, Frederic Gonte, called the event a "milestone in our quest for uncovering secrets of the Universe".

"It's an extremely important step because now we know that we're ready to do real science," he tld BBC News.

"From now on, we'll be able to observe things we were not able to observe before."

To link the VLT units, the team of international astronomers and engineers used an instrument called Pionier, which replaces a multitude of mirrors with a single optical microchip.

Although the first attempt to combine the four telescopes occurred in March 2011, it did not really work, said Jean-Philippe Berger, a French astronomer involved in the project.

But this time, it was already pretty clear that all the instruments were working correctly, he added.

"Last time, the atmospheric conditions and vibrations in the system were so bad that the data was just worthless," he recalled. "We stopped after half an hour knowing that it wouldn't improve.

"So, this attempt is the real first one to carry out observations for several hours straight to test the system in different conditions."

From now on, the system will be offered to the astronomical community, he added - any astronomer working at Paranal or visiting the observatory will be able to use it.

Image caption The first attempt to link up the telescopes in March 2011 failed

Gigantic mirror

VLTI, or the VLT Interferometer, has been used since 2002 to link together up to three VLT telescopes, as well as four small auxiliary telescopes that reside beside the big ones on the same platform at Cerro Paranal mountain, at 2,635m altitude.

The main component of an optical telescope is a mirror, and the light coming from a particular object being observed with separate telescopes - such as a star, a nebula or a galaxy - first gets reflected off individual mirrors.

And this is where the interferometer comes into play.

It directs the light underground into tunnels, where specific instruments compensate for the delay that inevitably exists when more than one telescope is used.

Once there is no delay, the light is combined into a single beam - and the image astronomers get is what would have been produced by one telescope with a gigantic mirror and a much better zoom.

In the case of the VLT, the zooming capability becomes almost 20 times better, said Mr Berger.

He explained that although the biggest "virtual" mirror of 130m in diameter has already been achieved by linking the two telescopes farthest from each other at Paranal, using all four units gives astronomers several advantages.

"The more telescopes the better - you want to generate a plane to fill that virtual mirror, to increase the efficiency to reconstruct an image, in order to observe more complex objects in the sky," he said.

"With two telescopes, you typically observe round stars, for which you're only interested in the diameter, or binary stars, where you can measure the separation between the two stars.

"With four telescopes, you can start thinking about triple stars or young stars surrounded by a protoplanetary disc - a disc of dust and gas that forms planets."

"Now, the zoo of objects accessible to us will be much bigger."

Katia Moskvitch was on secondment to ESO for two months leading up to her trip to Chile.