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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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