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Mercury, May/June 2001 Table of Contents

By constructing virtual telescopes the size of continents (and larger), radio astronomers are obtaining spectacular high-resolution results.

by T. Joseph W. Lazio

Early radio astronomers faced a problem: Their telescopes had lousy resolution. An optical telescope with a mirror only 10 centimeters in diameter has a resolution of 1 arcsecond, equivalent to separating an automobile’s headlights at a distance of 400 kilometers. By contrast, because radio wavelengths are millions of times longer than optical wavelengths, radio telescopes have to be larger by the same factor in order to achieve the same resolution. A radio telescope must be roughly 25 km across in order to achieve a resolution of 1 arcsecond. Even astronomers would object to a proposal to cover an area the size of San Francisco Bay with a single radio telescope.

Radio astronomers found a way around this problem by developing an idea from the turn of the last century: interferometry. With interferometers, small telescopes can be linked to achieve the same resolution as a virtual telescope as large as the distance between the telescopes. Radio astronomers have spent the last 30 years perfecting very long baseline interferometry (VLBI), synthesizing virtual telescopes the size of a continent or even the size of Earth! The large size of these interferometers means that their resolution can be better than 1 milliarcsecond (0.00000028°), or hundreds of times sharper than even the Hubble Space Telescope’s resolution. This is like separating a car’s headlights on the Moon.

In Europe and the United States, radio telescope networks engage in VLBI on a regular basis. The European VLBI Network (EVN) consists of 18 telescopes that spend about one-third of their time doing VLBI. In the United States, the Very Long Baseline Array (VLBA), operated by the National Radio Astronomy Observatory (NRAO), consists of 10 telescopes devoted to VLBI 100% of the time. The 10 VLBA telescopes are located from St. Croix, Virgin Islands to Mauna Kea, Hawai’i, forming an interferometer over 8,000 km wide, larger than the continental United States. On occasion, the VLBA and the EVN are linked together, sometimes with telescopes in South Africa, Japan, Australia, Russia, and other nations, to form a global interferometer.

With the resolution that EVN and VLBA achieve, radio astronomers can resolve details smaller than the Earth-Sun separation at the distances of nearby stars, structures as small as the size of the solar system at the center of the Milky Way Galaxy, or features a few light-years across in the centers of the most distant quasars. This incredible resolution allows us to peer into the heart of these and many other objects, answering old questions and raising new ones.