Radio Astronomy Help (page 2)
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.
The most important part of any radio or wireless receiver is the antenna. This is especially true of a radio telescope. Radio signals from space are much fainter than standard broadcast or microwave signals. To determine the location in the sky from which a signal is arriving, it is necessary that a radio telescope antenna have exceptional resolving power, also known as directivity . It must be sensitive only to signals in the direction in which it is aimed, and it must be able to reject signals coming from other directions. The old-fashioned television (TV) receiving antenna, which you still see occasionally on home and business rooftops, is a directional antenna, but the radio telescope requires a much more precise antenna than this. Radio telescope antennas more closely resemble outsized satellite TV antennas.
The gain (a logarithmic measure of the sensitivity) of an antenna is also important in the design of a radio telescope. The gain and directivity both depend on the physical size of the antenna. For a given amount of gain and directivity, a dish antenna must have at least a certain diameter, measured in wavelengths. For a given fixed antenna size, the sensitivity and directivity increase as the wavelength decreases.
Of course, even the most sensitive and directional antenna is useless without a good receiver. Most of the radio noise that comes from space sounds like the noise generated inside the electronic circuits of a radio receiver, and this compounds the problem of radio reception from the Cosmos. (Tune an old AM radio receiver to a frequency where there is no station. The faint hiss is internal noise; this is what radio astronomers generally hear from space.) The most advanced receiver designs must be used in a radio telescope to obtain the greatest possible amplification and sensitivity.
The location of the radio telescope antenna is important, just as is the site for any optical observatory. Human-made interference can ruin the operation of a radio telescope. Such interference comes from all kinds of electrical appliances, such as hair dryers, light dimmers, electric blankets, and thermostats. Automobile ignition systems are a severe problem for those who attempt radio reception of faint signals. A rural location is therefore superior to an urban site for a radio telescope.
With all these factors in mind, scientists set out to build sophisticated radio telescopes. One of the most famous early instruments employed a 250-ft steerable dish and was located at Jodrell Bank in Cheshire, England. This project, completed in the 1950s, was proposed and overseen by the physicist A. C. B. Lovell. He went through great personal difficulties in arranging the construction of this radio telescope.
Not all radio telescopes use single-dish antennas. There are schemes for obtaining exceptional directivity that are more physically workable than the construction and operation of one huge parabolic reflector. The interferometer , pioneered by Martin Ryle of Cambridge University and J. L. Pawsey of Australia, provides superior resolving power using two separate antennas. When two antennas, spaced many wavelengths away from each other, are connected to the same receiver, an interference pattern occurs. There are many lobes , or directions in which the signals arriving at the two antennas add together. There are also many nodes , or directions from which the signals cancel each other out. The farther apart the antennas, the more numerous are the lobes and nodes, and the narrower they become. Each lobe covers a smaller part of the sky than the main lobe of any single antenna.
Figure 18-3 shows horizontal-plane directional patterns of the sort used by antenna engineers for a hypothetical single antenna ( A ) and a pair of antennas in an interferometer arrangement ( B ). Imagine that you are high above Earth, looking straight down on the antennas and at such an altitude that the pair of antennas (at B ) looks like a single point. Also imagine that both radio telescopes are aimed at the northern horizon. The curves show the relative sensitivity of the radio telescope as a function of the azimuth. These are two-dimensional slices of the true pattern, which is three-dimensional. In three-space, the lobes are shaped like tapered cigars.
Interferometry cannot provide the sensitivity of a huge dish measuring many kilometers in diameter, but it does provide the equivalent directivity at a far lower cost and inconvenience. In some cases, the radio image resolution can be on the order of a few seconds of arc.
Today, there are radio telescopes in many countries throughout the world. These radio telescopes have proven worth the trouble and the expense of their construction. The mysterious, fascinating quasars and pulsars were found using radio telescopes; only later did astronomers start analyzing these objects with optical telescopes.
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