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Eagle's Eye in the Sky

Astronomers have produced a small prototype of what could one day become a telescope with almost unimaginable resolution. The instrument, a so-called x-ray interferometer, would enable astronomers to see the fiery rings around nearby stars or the gas swirling close to black holes.

Interferometry is telescope team work; by combining the signals from telescopes based far apart, astronomers can construct images with very high resolution. Currently, radio interferometers--with two or more radio dishes spaced thousands of kilometers apart--yield the sharpest images of the cosmos. For even better quality, researchers are dreaming of interferometry with radiation of shorter wavelengths, such as visible light or even x-rays. But the technical difficulties become ever more daunting as the wavelengths shrink. "The big [optical] telescopes in the world are just now beginning to do interferometry," says Webster Cash of the University of Colorado, Boulder.

Now Cash's team, together with scientists at NASA's Marshall Space Flight Center in Huntsville, Alabama, has taken a bold leap. They have constructed the first, simple x-ray interferometer. The apparatus is installed in a 120-meter-long vacuum tank, which mimics the conditions of space. On one end of the tunnel a set of almost parallel mirrors helps cast x-rays on a second set of mirrors, placed just 1 millimeter apart, instead of thousands of kilometers like the radio dishes. These mirrors send the x-rays to a detector some 100 meters away, at the other end of the tank, which records the x-ray fringe pattern.

The system produced images whose resolution was slightly better than that of the Hubble optical telescope, and several times better than that of Chandra, currently the world's best x-ray telescope, the team reports in the 14 September issue of Nature. "That was step one--to show that interferometry is possible," Cash says. Now the team is proposing an x-ray interferometer with mirrors about 1 meter apart. This device would see two times as well as the best radio interferometer: It would be able to spot a dime 6000 kilometers away.

Cash and his team have broken a psychological barrier, says Nicholas White of NASA Goddard Space Flight Center in Greenbelt, Maryland. "Up until he did it, people were just thinking, this is science fiction ..." White says. "It will definitely revolutionize x-ray astronomy." NASA is currently studying how to incorporate the new x-ray interferometer design into a future mission called Pathfinder, White says, which won't be launched until the end of this decade.

Related sites

Webster Cash's pages about x-ray interferometry