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Friday, August 1, 2008

Earth-Size Radio Telescope Opens Its Eye

Photo: Roger Ressmeyer/Corbis

Arecibo, Puerto Rico: The 1000-meter radio telescope dish at dawn.

This fall, the world's largest telescope will begin its scientific mission. Made up of radio telescopes in Chile, Germany, Italy, the Netherlands, Puerto Rico, South Africa, and Sweden, the e-VLBI—for electronic very long baseline interferometer—creates in effect a telescope with a diameter of 11 000 kilometers; Earth's own diameter is about 12 750 km at the equator. Because a telescope's resolution is proportional to its size, the e-VLBI should see farther out in space and time and elucidate the finer structures of the most energetic phenomena in the universe, such as supernovas, pulsars, and black holes.

Although a smaller, Europe-wide e-VLBI has been in operation for more than a year, the full multicontinent version opened its eye only on 22 May 2008, when all seven sites were linked to a custom-built supercomputer, operated by the European VLBI Network (EVN), in a test observation.

VLBI increases the resolution of a pair of radio telescopes by using the time a particular radio wave arrives at each of them to estimate its frequency and pinpoint its origin. Although the technique has been in use since the mid-1980s, linking radio telescopes between other countries and continents in real time has not been possible until now.

Each of the radio telescopes used in the May test produced up to 1 gigabit per second of data. Until the EVN supercomputer was built, the only way to transport such large volumes of data across the globe for analysis was by recording them on magnetic media and then physically shipping the media to the Joint Institute for VLBI in Europe, located in Dwingeloo, Netherlands.

The new system transports data via a large number of network providers. Just getting the data to EVN “was an enormous technical hurdle to overcome,” says Arpad Szomoru, head of technical operations and R&D at Dwingeloo. The supercomputer can handle flows of up to 100 terabytes per observation coming in from 16 radio telescopes.

Radio astronomers were initially skeptical that moving to real-time VLBI was worth the effort, says Huib van Langevelde, director at Dwingeloo. But the recent test showed that e‑VLBI collapses processes that would have taken weeks without the supercomputer networking into a matter of hours.

For the new intercontinental e-VLBI system to work, all the telescopes must observe the same astronomical radio source at exactly the same wavelength. All the stations have atomic clocks, synchronized to within a few millionths of a second, so that a radio wave's arrival can be precisely marked at every telescope. 

As Earth rotates, an observational target will drop beneath the horizon in relation to some telescopes and rise in relation to others. “The source is tracked by a changing set of telescopes,” says Szomoru. 

According to Szomoru, astronomers will get the most from the e-VLBI when they're tracking stellar explosions and other transient cosmic activity, including gamma-ray bursts and flaring microquasars. “It is now possible to observe a number of candidate cosmic sources and, if one of them is seen to come into an active state, do one or several follow-up observations, thus catching a flare in a microquasar at a very early moment, something which was not possible previously.”

Original here

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