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The Question
(Submitted November 17, 1997)
I'd like to know exactly what interferometry is, how it works, and
what its benefits are to astronomy.
The Answer
There are two important advantages of a telescope over, for example, a
human eye. One is the collecting area (bigger telescopes scoop up more
photons) and the resolution (how close things can be together and still be
distinguished as separate). A telescope's resolution is inversely related
to its physical size; basically, the further apart the two furthest points
in the telescope are, the better the resolution (the smaller the distance
between two objects still distinct can be). If you take a big telescope,
like the Arecibo radio telescope, and start hacking away parts of it, you
are taking away surface area and its collecting area goes down. BUT, as
long as you leave the two parts that are the furthest apart, the telescope
still has the same resolution.
This is what an interferometer does. It is a bunch of small telescopes
that have the resolution of a big telescope (the size of the widest
separation of two telescopes in the interferometer). Interferometers are
great for observing fine detail, but because their collecting area is
small, the sources observed have to be fairly bright.
In order to use the individual telescopes together, the light from each of
them has to be added. This has to be done in a special way, however. Light
is a wave and different parts of the wavefront will reach the different
telescopes at the same time. This means that at one instant the trough of
the wave could be arriving at one telescope while the crest is arriving at
another. If these two were added they would cancel. To fix this, something
must be added to make sure that the light wavefronts from the source
arrives at each of the telescopes at the same time. This can be done after
the data have been collected at the telescopes, or it can be done by adding
called a "delay line" (a little extra path length) to the path that the
light travels to each telescope. In this case, the light travels the same
distance to each telescope, the wavefronts all arrive at the same time at
the telescopes and they can be added together to make one image.
Traditionally interferometry has been used in radio astronomy. You can
read about the interferometers that are part of the National Radio
Astronomy Observatory at
http://www.nrao.edu/. (this includes the VLA and
the VLBA). With new technological advances (in computer hardware and in
things like the CCD chips used in observations), optical interferometers
are starting to come on line. One working optical interferometer is the
Navy Prototype Optical Interferometer. To give you an idea of its
resolution, its entire field of view is the size of one Hubble Space
Telescope pixel. You can look at the NPOI home page at
http://www.nofs.navy.mil/projects/npoi/.
A historical note: Interferometers were first used by Michaelson, who won
the Nobel prize in 1907 for his work using an optical interferometer to
measure very accurately the speed of light. The next use of interferometers
was not until the 1970s, when the VLA came on line. You may wonder why there
was such a long time lag between the two events. The answer is that
Michaelson used his eye as a detector to see the interference, (the work
that won him the Nobel prize used a VERY bright star!) and it was not
until very recently that the technology has advanced to the point where
suitable electronic detectors and the computers necessary to crunch the
large amounts of data that an interferometer generates (remember the
signals from all of the telescopes have to be stored and then combined
with a computer, they cannot just be captured on photographic film) have
been available.
If you are interested in some MORE information on optical interferometers,
there is a pretty good (and fairly accessible) article in Physics Today
(a magazine that may only be available in University libraries) by
J. T. Armstrong. It is Vol. 48. page 42 (1995).
Hope this helps!
J. Allie Cliffe and Arsen R. Hajian
for Ask an Astrophysicist
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