World's Largest Mirror created by Four telescope link-up

Astronomers have created the world's largest virtual optical telescope linking four telescopes in Chile, which they can operate as a single device. The telescopes of the Very Large Telescope (VLT) at the Paranal observatory form a virtual mirror of 130 metres in diameter.

A previous attempt to link the telescopes last March failed. 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 a desert mountaintop in Chile, excitement filled the Paranal observatory's tiny control room. The head of instrumentation at Paranal, Frederic Gonte, called the event a "milestone in our quest for uncovering secrets of the universe".

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 happened in March 2011, it did not really work. But this time, it was already pretty clear that all the instruments were working correctly.

From now on, the system will be offered to the astronomical community - any astronomer working at Paranal or visiting it will be able to use it. 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 Cerra 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 that one telescope is used. Once there is no delay, the light is combined into one single beam - and the image astronomers get is what would have been produced by a single telescope with a gigantic mirror and a much better zoom.

In the case of the VLT, the zooming capability becomes almost 20 times better. Although the biggest "virtual" mirror of 130m in diameter has already been achieved by linking two farthest from each other telescopes, using all four units gives astronomers several advantages.

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 disk - a disk of dust and gas that forms planets.