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 automobiles
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 Telescopes resolution. This is
like separating a cars 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, Hawaii,
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.
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