DISCOVERY OF CELESTIAL RADIO WAVES
In 1931 a Bell Telephone engineer, Karl Jansky (1905-1950), was trying to find where the interference disrupting transatlantic radiophone circuits came from. He discovered that some of the radio noise was not from the earth; it was extraterrestrial. The primary source was the center of the Milky Way, in the constellation of Sagittarius. In 1936 an Illinois radio engineer, Grote Reber (1911- ), pursued the phenomenon farther. He built the first parabolic radio telescope, 9.5 meters in diameter, and made the first radio map of the sky. The strongest signals he found came from the star clouds in Sagittarius and from several discrete sources toward the center of our Galaxy. The next major discovery was in 1942, by British radar operators and scientists tracking down suspected radar jamming during World War II; they discovered that the interference was radio emission from the sun.
At fjrst astronomers did not grasp just how significant Jansky's work was; they were preoccupied with their observations of the universe through the optical window of the earth's atmosphere. But after World War II radio astronomy came into its own when physicists, radio engineers, and astronomers joined forces to build larger and more efficient radio telescopes. Radio astronomy since then has led to startling discoveries, such as interstellar molecules, pulsars, and the enigmatic quasars. Today our concept of any cosmic body is based upon its appearance all across the electromagnetic spectrum, with the radio region an extremely important component.
RADIO-TELESCOPE DESIGN
Because the physical nature of a radio wave is exactly the same as that of a light wave, the problem of designing a radio telescope is similar to that of designing an optical telescope. There are some practical differences. Radio waves pass through most materials without any interaction; thus it is not feasible to design a "lens" for radio waves that will focus them in a refracting telescope. But any metal will reflect radio waves; so a dish-shaped metal mirror will focus radio waves, just as a glass mirror focuses light waves. The reflecting surface of the dish can be an open, fine-wire mesh or a solid metal with a parabolic shape. Radio waves are reflected from the su rface and converge toward a focal point, where a small collector aerial absorbs the concentrated energy turning it into an electrical current. From there the current or signal is carried by an electrical cable to the receiving equipment, which processes the signal just as in your home radio receiver.
After amplification the signal variations are recorded in one of several ways The signal changes can then be fed into a computer for analysis. When the computer has done its job, a formerly invisible part of the universe is revealed as in the radio map.
Astronomers can increase the sensitivity of radio telescopes, with better accuracy in pointing and with higher resolution, by expanding the collecting area of the dish or by improving the capabilities of the receiver. With the largest radio telescopes we can obtain a resolution of about l' of arc, comparable to that of the eye. That is like seeing a penny 65 meters away. The most powerful radio telescopes can detect energy from sources whose power is comparable to that of a terrestrial FM broadcast station many light years away.
The radio telescope is remotely controlled by the astronomer from an electronic console. Moderatesized radio dishes, up to about 100 meters or so in diameter, are steerable and have equatorial mountings that follow the rotating sky just as optical telescopes do. Larger and heavier dishes employ an attazimuth mounting-one that rotates about a vertical axis and a horizontal axis. This minimizes the distortion in the shape of the dish due to changing the orientation of the dish in the earth's gravitational field while tracking an object. A computer directs the rotation about both axes.
Even larger and more unwieldy antennas are fixed pointing upward while the rotating earth sweeps much of the sky by the antenna's field of view. Arecibo, Puerto Rico, has the biggest fixed antenna, 2. metal dish 305 meters across contoured out of a natu· ral bowl in the ground. It can survey the sky to within 20° of the zenith, allowing coverage of about 40 percent of the entire sky.
RADIO INTERFEROMETRY
Astronomers have searched endlessly for better resolving power. It can be achieved by building bigger telescopes or by observing at shorter wavelengths for a chosen aperture or by using the phenomenon of interference. Interferometry is a technique involving two or more radio telescopes. Radio radiation from an astronomical source received at the individual telescopes is combined to obtain data that have a spatial resolving power equal to that of a single telescope as large as the distance between the individual receivers. With interferometry an astronomer can obtain details about the spatial structure of a given celestial object that a single radio telescope could never reveal.
For many years the separation between the individual telescopes of the interferometer was limited by the lengths of the cables connecting the antennas because the technique depends upon combining, at the same instant, the signals received by the separate telescopes. With the advent of the atomic clock (a clock governed by the vibrations of certain atoms) it became possible to record the signals received by the different telescopes, along with the precise time, and to compare them later. This allowed the individual telescopes to be greatly separated, even on opposite sides of the earth. The technique is called Very Long Baseline Interferometry (VLBI). It has been used with a geosynchronous or geostationary satellite as the link in the communications channel between the telescopes.
In the Very Large Array (VLA) radio interferometer recently put into operation in New Mexico signals from each of 27 individual radio telescopes are combi ned by a computer. Each dish is 25 meters in diameter, and the 27 individual telescopes are moved along railroad tracks arranged in the shape of an enormous 21-kilometer Y. Nine dishes will be located on each branch of the Y, and the system can provide a total of 351 interferometer pairs of antennas. The energy-collecting power of the VLA is roughly equivalent to a single 122-meter telescope. The VLA will be able to achieve spatial resolution of about 1 second of arc in 10 hours of observing, or about that of the 5.1-meter Hale optical telescope. This ability makes it comparable to large optical telescopes in ferreting out the structure of various cosmic bodies.
