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The Sharpest Picture Ever Made.

RADIO ASTRONOMERS HAVE SET A NEW world record in the quest for ever higher resolution of celestial objects. In the process they showed off a technique that may eventually reveal the surroundings of the Milky Way's central black hole in unprecedented detail.

A telescope's resolution is limited by diffraction--the fundamental blurring caused by the limited size of any telescope's aperture combined with the wave nature of light (or any other radiation). Diffraction can be reduced only by making the aperture bigger or by observing at shorter wavelengths.

A radio-astronomy team led by Sheperd Doeleman (MIT/Haystack Observatory) and Thomas Krichbaum (Max Planck Institute, Bonn) did both. They used the technique of very long baseline interferometry (VLBI), in which radio dishes on different continents are integrated so tightly that individual wave-fronts arriving at each one can be correctly matched up. This difficult process can create a "synthetic aperture" thousands of kilometers wide (S&T: December 1999, page 36). The group managed to do this at a wavelength of just 2 millimeters, shorter than has ever been used with such a long baseline.

At a June conference in Bonn, Krichbaum and Doeleman described combining radio dishes in Spain and Arizona to achieve a resolution of 0.000024 arcsecond, or 24 microarcseconds (when resolution is defined as half the wavelength divided by the baseline). This is about 50 percent better than any previous effort and about 1,500 times sharper than the Hubble Space Telescope can see.

Other groups have achieved wider synthetic apertures using radio dishes on spacecraft, but they worked at longer wavelengths. Nor can today's optical interferometers compete. Although these work at much shorter wavelengths, they are limited to much smaller spacings between telescopes --a few hundred meters at most.

VLBI has severe limits; the "world's sharpest image" isn't going to win any awards for awesome astrophotos. Only the brightest, most concentrated radio sources can be observed with this technique. But quasars, the hot surroundings of galaxy-core black holes, lend themselves well to it. "Our ultimate goal is to probe the cores of quasars in their very innermost and deepest regions," said Krichbaum.

The most enticing target will be the nearly dormant black hole at the heart of our own galaxy, marked by the radio source Sagittarius A*. At its distance of 25,000 light-years, 24 microarcseconds corresponds to 0.2 astronomical unit--only a little larger than the size of the black hole itself. Millimeter-wave VLBI would help here not only because of its fine resolution but also because the inner galactic center is shrouded by plasma that scatters longer radio waves. Says Doeleman, "With this technique we can actually pierce that scattering screen to explore the environment of the black hole."

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Title Annotation:achieving higher image resolution in radio astronomy
Author:Winn, Joshua
Publication:Sky & Telescope
Geographic Code:00WOR
Date:Jan 1, 2003
Words:452
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