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10.28 Interferometry

Astronomers have invented a clever technique to improve telescope performance. This trick makes it appear as if we have a bigger telescope than we really do. The technique, called interferometry, uses widely separated telescopes in a special arrangement to increase the resolution of fine details in astronomical objects. In our earlier discussion of telescopes, we noted that the resolution of a telescope increases with the telescope's diameter. If we could build a good-quality optical telescope or radio telescope a mile wide, for example, we could achieve fantastic resolution (especially if the telescope were in space, where the full resolution could be realized without being ruined by atmospheric shimmering). It is not possible to build a mile-wide telescope, but astronomers have devised a partial solution to the problem.

Engineering obstacles and high construction costs make it impossible to build a mile-wide telescope. However, it is much easier to build two big telescopes, locate them a mile apart, and have them work together to observe the same patch of sky. The trick is that their light (or radio waves in the case of a radio telescope) must be fed into a single detector, just as all the light from a single telescope goes into one detector. The two telescopes must act like one, even though they are separated. This blending is achieved by converting radio waves into electrical signals and sending them from separated telescopes to one central facility, where they are processed together. You can see why optical interferometry is harder than radio interferometry by recalling the wave nature of light. To combine waves of light from separate telescope, we must line the waves up to within a fraction of a wavelength; otherwise the waves might cancel as they do with interference. This requires us to know the distance between two telescopes to better than a millionth of a meter! With longer radio waves, the requirement is a lot less severe.

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Very Large Array radio telescopes in New Mexico, USA. Click here for original source URL.

Using the technique of interferometry, astronomers have been able to measure incredibly small angular details — 2 or 3 ten-thousandths of a second of arc — in distant stars and galaxies. The technique has been especially fruitful for radio telescopes, which have made beautiful images of the details of distant galaxies. One of the most advanced such observatory is the VLA of the National Radio Astronomy Observatory, with 27 movable radio dishes spread over an area up to 36 kilometers across. Astronomers in 1986 achieved the first space-based interferometry using a radio telescope in space linked to one on the ground, thus increasing the effective size of the telescope to a distance wider than the diameter of the Earth!

Optical interferometry is more experimental, and the technique is made very difficult by the short wavelength of light compared to a radio wave (it’s slightly easier at infrared wavelengths). When light from different telescopes is combined to gain higher resolution, the phase of the light signals must be preserved, which requires a precision of a small fraction of a light wavelengh. The price paid in this technique is usually that it requires a large light signal, so only bright astronomical targets can be observed. Large telescopes combining their light to do interferometry include the twin 10-meter Keck telescopes in Hawaii, and the four 8-meter Very Large Telescopes in Chile. In Arizona, the Large Binocular Telescope will soon be combining the light from its two 8.4-meter mirrors on a common mount. This technique holds much promise for the future. The "killer app" for optical or infrared interferometry is to null or subtract the light from a nearby star and look directly for an Earth-like planet orbiting it. This might be possible within the next decade.