Monday, June 16, 2008

Bigger Is Better: The Green Bank Telescope

In the case of radio telescopes, size really does matter. The resolution of a telescope depends not only on its diameter, but the wavelength of the detected radiation (the ratio of wavelength to telescope diameter determines the resolution). Radio waves are big (on the order of centimeters or meters), and the telescopes that detect them are correspondingly huge. Also, the radio signals that these instruments detect are very faint, and just as bigger optical telescope mirrors collect more light than smaller ones, bigger radio telescopes collect more radio waves and image fainter radio signals than smaller ones.
Collecting radio signals is just part of the task, however. You may recall that, for practical purposes, very good optical telescopes located on the earth’s surface can resolve celestial objects to 1” (1 arcsecond—1 ⁄60 of 1 arcminute, which, in turn, is 1 ⁄60 of 1 degree). The best angular resolution that a very large single-dish radio telescope can achieve is about 10 times coarser than this, about 10”, and this, coarse as it is, is possible only with the very largest single dish radio telescopes in the world. The National Radio Astronomy Observatory has just commissioned the world’s largest fully steerable radio telescope. The 100 m dish will have a best resolution of 14”.
The world’s largest nonsteerable single-dish radio telescope was built in 1963 at Arecibo, Puerto Rico, and has a dish, 300 meters (984 feet) in diameter sunk into a natural valley. While its great size makes this the most sensitive radio telescope, the primary surface is nonsteerable—totally immobile—and, therefore, is limited to observing objects that happen to pass roughly overhead (within 20 degrees of zenith) as the earth rotates.

No comments: