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Radio Astronomy Help

By — McGraw-Hill Professional
Updated on Sep 18, 2011

Radio Astronomy

The science and art of radio astronomy began as the result of an accident. Karl Jansky was conducting investigations at a wavelength of 15 m (a frequency of about 21 MHz) in the shortwave radio band to determine the directional characteristics of sferics , or radio noise that originates from natural sources in Earth’s atmosphere, particularly thunderstorms. The antenna was not particularly large. However, Jansky found, in addition to the radio noise caused by thunderstorms, a weaker and steady noise of unknown origin.

The Mystery Noise

Human-made noise was ruled out when Jansky noticed that the source of the faint noise seemed to change with the time of day. It was found to have a rotational period of 23 hours and 56 minutes, exactly the same as the sidereal rotation period of the Earth. Jansky concluded that the radio noise was of extraterrestrial origin, and he found that it was coming from the direction of the constellation Sagittarius, which lies in the same direction as the center of our Milky Way galaxy. Other parts of the galaxy also produced radio noise, Jansky found, but none of it was up to the amplitude of the noise coming from Sagittarius.

Jansky was interested in the phenomenon and wanted to continue the research in the field with equipment designed specifically for receiving signals from space, but his superiors and the people who funded his work weren’t impressed by his “mystery noise.” As a result, he did not pursue radio astronomy any further. However, Jansky’s discovery of the noise coming from the Milky Way did not pass entirely unnoticed. A radio engineer named Grote Reber began to get interested in radio astronomy as a hobby, in conjunction with his activities as an amateur radio operator. Radio amateurs, also called ham operators , have been known to make radical communications discoveries. Reber built a large parabolic dish antenna in his back yard. His neighbors were amazed (and fortunately, tolerant) as the assembly of the 10-m (31-ft) bowl-shaped reflector progressed.

Reber’s antenna was not fully steerable but could be moved only up and down along the celestial meridian from the southern horizon through the zenith to the northern horizon. As Earth rotated on its axis during the course of a day, different parts of the observable sky passed across the focal axis of the antenna. Many radio telescopes use this kind of steering system. By tilting the antenna from horizontally south through the zenith to horizontally north, the entire radio sky can be mapped if the astronomer is willing to take the necessary time.

Reber’s first tests were conducted at the fairly short wavelength of 9 cm, corresponding to a radio frequency of 3.3 GHz, or 3,300 MHz. Reber checked the most familiar objects in the sky, such as the Sun, the Moon, and the planets. No signals were detected. At a wavelength of 1.87 m, or about 160 MHz, however, Reber did find noise coming from the Milky Way.

Astronomers took notice of the work of Jansky and Reber, and plans were made to construct large radio antennas to receive signals from the Cosmos.

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