The standout property of radio telescopes is that they are big. The largest single-mirror telescope in the world is the Arecibo Observatory in Puerto Rico. That telescope has a mirror diameter of 305 meters. Its detectors are radio receivers located at the prime focus of the telescope. The main mirror of the Arecibo telescope does not steer; it is fixed in the ground. The steering is done mostly by the Earth's rotation, but the telescope can track an object for a few hours by moving the prime focus.

The main reason that radio telescopes must be big is the diffraction limit (see topic 4 above). Since radio wavelengths are much bigger than optical wavelengths, the telescope aperture must be bigger by the same ratio to give comparable angular resolution. For example, a radio telescope would need a diameter of 4 meters to observe at wavelengths of 10 cm with 20/20 vision. [Check this calculation. Assume that 20/20 vision corresponds to the limit that the diffraction limit of the human eye and scale the pupil diameter (2 mm) up by the ratio of the radio wavelength (10 cm) to a typical optical wavelength (5 ´ 10^-5 cm).] By the same reckoning, it would require a radio telescope of diameter of about 500 km (300 miles!) to observe 10-cm radio waves with the angular resolution of the Hubble Space Telescope.

You might think that it would be out of the question to build such a radio telescope, and of course that would be true if we had to build a single mirror 300 miles in diameter. But, fortunately, astronomers have found a way to combine the signals from several separate radio telescopes so that they act as a single-dish telescope with the effective diameter equal to the maximum separation of the telescopes. This technique is called interferometry. Under angular resolution, we mentioned astronomers' efforts to develop this technique to obtain very precise images at optical and infrared radiation. But actually, interferometry has been used with radio telescopes for more than 40 years. Doing interferometry with radio telescopes is much easier than doing so with optical telescopes for two reasons: (1) we only have to control the positions of the radio telescopes within a fraction of a radio wavelength (within say, 1 cm if we are observing radio waves at 10 cm); (2) radio waves are not distorted significantly by the Earth's atmosphere.

The two most important facilities for radio interferometry are the Very Large Array (VLA) of 27 radio telescopes located in New Mexico, which has a maximum telescope separation of 36 km, and the Very Long Baseline Array of 10 radio telescopes scattered from Hawaii to Puerto Rico -- a maximum separation of 8600 km, or about 3/4 the diameter of the Earth!

As you will see, very high frequency radio waves, with wavelengths in the range of about 0.1 - 3 millimeters provide one of the best "windows" to observe the formation of galaxies and stars. But to observe millimeter waves, we need special radio telescopes with very precisely shaped antennas. Moreover, because water vapor in the Earth's atmosphere absorbs millimeter waves, we need to place these antennas at very high and dry places. In the table below, which lists some of the world's most important radio telescopes, you will see that the telescopes designed to observe millimeter waves are all at high altitudes. In 1998, Congress approved funding ($160 million) for the National Science Foundation to build the Atacama Large Millimeter Array (ALMA) an array of 64 or more radio telescopes designed to observe millimeter wave radio signals.

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