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Virtual telescope provides pinpoint views of the cosmos


An international collaboration of astronomers including those at MIT's Haystack Observatory have created an Earth-sized virtual radio telescope capable of detecting never-before-seen features of the universe.

The virtual device, which was created earlier this year by linking signals from radio telescopes on several continents, can detect features 3,000 times smaller than the finest detail observed by the Hubble Space Telescope.

"The resolution achieved by this telescope is the equivalent of sitting in New York and being able to see the dimples on a golf ball in Los Angeles," said Sheperd Doeleman, a research scientist at Haystack.

The telescope, which has successfully picked up radio signals from galaxies more than 3 billion light years away, will be used to address one of the fundamental mysteries of modern astronomy - how so-called "active" galaxies produce their incredible energetic output.

Normally, a galaxy gives off a predictable amount of energy, one equaling the sum of the energies given off by each of its stars. Active galaxies emit an amount that is far in excess of their stars' combined energies. This excess energy tends to be concentrated at the galaxy's core. Researchers believe these energetic cores are powered by super-massive black holes, billions of times bigger than the Sun. Erupting from some of these cores are powerful streams of high-speed particles that can extend millions of light years from the host galaxy. But it isn't clear how these high-speed particle jets are launched from the galactic cores.

The new telescope is specifically designed to make detailed images of regions very close to where the jets originate.

"Locating the point at which these jets are turned on has been the Holy Grail in this field," Doeleman said. "Using the new telescope array, we have detected two galactic cores which are allowing us to observe jet behavior close to their nozzles."


A key to the power of the new telescope lies in its angular resolution - its ability to clearly separate two closely spaced objects in the sky. This ability can be improved by either increasing the size of a telescope or by observing at higher frequencies.

To increase telescope size, the scientists programmed telescopes on several continents to record radio emissions from the same object at the same time, using a technique called Very Long Baseline Interferometry (VLBI). Signals from each telescope were time-stamped with extremely accurate atomic clocks, recorded on magnetic tapes, then combined in a correlator (a special-purpose supercomputer). This technique forms a virtual radio telescope whose size can be as large as the diameter of the Earth.

To further increase resolution, the virtual telescope was programmed to observe at extremely high radio frequencies (129GHz and 147GHz). The angular resolution achieved by the telescope in its record-breaking test was 50 micro arc seconds - approximately one-hundred-millionth of a degree.

Telescopes in the United States (Arizona), Spain, Finland and Chile participated in the observations. Two telescopes in Arizona and one in Spain provided key long-distance detections that were especially critical to the project.

Scientists will be using this technique to target the core of our own Milky Way galaxy, where a suspected black hole three million times more massive than the Sun may be lurking. If this central radio source can be imaged using high-frequency VLBI, structures very close to the black hole could become visible.

Reports of the experiment were presented by Doeleman and colleagues at the annual European VLBI Network conference in June in Bonn, Germany. Scientists and engineers from the following institutes made this experiment possible: The MIT-Haystack Observatory in Westford; the Max Planck Institute for Radio Astronomy in Bonn; the Steward Observatory in Tucson, Ariz., which operates the Arizona Radio Observatory in Granada, Spain and Grenoble, France; the Mets������hovi Radio Observatory in Finland; the Swedish-ESO Submillimetre Telescope in Chile; and the three-telescope National Radio Astronomy Observatory, headquartered in Charlottesville, Va. The U.S. effort in this project is supported by the National Science Foundation and the Tucson-based Research Corp.

A version of this article appeared in MIT Tech Talk on October 2, 2002.

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