Most of what we know about the Universe comes from information that has been carried to us by light. But we have seen that visible light is only a small part of the electromagnetic spectrum. In recent years the remainder of the electromagnetic spectrum has revealed extensive information about our Universe.

NRAO 140 Foot
The first non-visual spectral region that was used extensively for astronomical observations was the radio frequency band. Telescopes observing at these wavelengths are commonly called radio telescopes. Although they do not look like optical telescopes, radio telescopes are built to accomplish the same gathering and focussing tasks on radio frequency radiation that reflecting telescopes perform on visual frequency light. The adjacent image shows the 140 foot antenna of the National Radio Astronomy Observatory in Green Bank, West Virginia.

Radio telescopes are used for many purposes, but let us describe two here explicitly: the mapping of neutral hydrogen concentrations using the 21 centimeter spin-flip transition in hydrogen, and the use of multiple radio telescopes operated as if they were a single telescope of very high resolution.

Mapping Neutral Hydrogen

A neutral hydrogen atom (H I) consists of 1 proton and 1 electron. The proton and electron spin like tops with their spin axes either parallel or anti-parallel. When hydrogen atoms switch from the parallel to the anti-parallel configuration they emit radio waves with a wavelength of 21 centimeters and a corresponding frequency of exactly 1420 MHz. This is called the 21 cm line. Thus, radio telescopes tuned to this frequency can be used to map the great clouds of neutral hydrogen found in interstellar space. Here is a map of neutral hydrogen in the galaxy obtained by observation of the 21 cm line with radio telescopes.

Long-Baseline Interferometry

Another important way in which radio telescopes have been used is in long-baseline interferometry. The resolution of a telescope (whether optical or radio) is set by the size of the telescope. But it is possible to use more than one radio telescope at separated locations and have them function to some degree as if they were a single telescope. Such a device is called an interferometer, and the resolution of the corresponding device is dictated by the distance between the two telescopes, which can be enormous compared with the size of the individual telescopes. This larger effective size is termed the synthesized aperture of the system. Here is an illustration of a set of telescopes operated as a simple interferometer.

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