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.
