In addition to the visible radiation that penetrates Earth's atmosphere on a clear day, radio radiation also reaches the ground. Indeed, as indicated in Figure 3.11, the radio window in the electromagnetic spectrum is much wider than the optical window. 
(Sec. 3.3) Because the atmosphere is no hindrance to long-wavelength radiation, radio astronomers have built many ground-based radio telescopes capable of detecting cosmic radio waves. These devices have all been constructed since the 1950s—radio astronomy is a much younger subject than optical astronomy.
The field originated with the work of Karl Jansky at Bell Labs in 1931, but only after the technological push of World War II did it grow into a distinct branch of astronomy. Jansky was engaged in a study of shortwave radio interference when he discovered a faint static "hiss" that had no apparent terrestrial source. He noticed that the strength of the hiss varied in time and that its peak occurred about 4 minutes earlier each day. He soon realized that the peaks were coming exactly one sidereal day apart and correctly inferred that the hiss was not of terrestrial origin but came from a definite direction in space. That direction is now known to correspond to the center of our Galaxy. It took over a decade, and the realization by astronomers that interstellar gas could actually be observed at radio wavelengths, for the full importance of his work to be appreciated, but today Jansky is widely regarded as the father of radio astronomy.
Figure 5.20 shows a fairly typical radio telescope, the large 43-m (140-foot)-diameter telescope located at the National Radio Astronomy Observatory in West Virginia. Although much larger than reflecting optical telescopes, most radio telescopes are built in basically the same way. They have a large, horseshoe-shaped mount supporting a huge, curved metal dish that serves as the collecting area. The dish captures cosmic radio waves and reflects them to the focus, where a receiver detects the signals and channels them to a computer. Conceptually, the operation of a radio telescope is similar to the operation of an optical reflector with the detecting instruments placed at the prime focus (Figure 5.7a). However, unlike optical instruments, which can detect all visible wavelengths simultaneously, radio detectors normally register only a narrow band of wavelengths at any one time. To observe radiation at another radio frequency, we must retune the equipment, much as we tune a television set to a different channel.
Figure 5.20 The 43-m-diameter radio telescope at the National Radio Astronomy Observatory in Green Bank, West Virginia.
Large radio telescopes are very sensitive instruments and can detect even very faint radio sources. However, their angular resolution is generally poor compared with that of optical counterparts, despite the enormous size of many radio dishes. It is not Earth's atmosphere that is to blame—the radio wavelengths normally studied pass through air without any significant distortion. The problem is that the typical wavelengths of radio waves are about a million times longer than those of visible light, and these longer wavelengths impose a corresponding crudeness in angular resolution because of the effects of diffraction. Recall from Section 5.2 that the longer the wavelength, the greater the diffraction.
The best angular resolution obtainable with a single radio telescope is about 10" (for the largest instruments operating at millimeter wavelengths)—at least 10 times coarser than the capabilities of the largest optical mirrors. The resolution varies widely, depending on the wavelength being observed. The 43-m radio telescope shown in Figure 5.20 can achieve resolution of about 1' when receiving radio waves having wavelengths of around 1 cm. However, it was designed to operate most efficiently (that is, it is most sensitive to radio signals) at wavelengths closer to 5 cm, where the resolution is only about 6', or 0.1°.
Radio telescopes are large in part because that is the only way they can achieve good resolution. But, as with optical telescopes, there is another reason: light-gathering power. The amount of energy arriving at Earth in the form of radio radiation is extremely small. In fact, the total amount of radio energy received by Earth's entire surface is less than a trillionth of a watt. Compare this with the roughly 10 million watts our planet's surface receives in the form of infrared and visible light from any of the bright stars visible in the night sky.
Radio telescopes can be built so much larger than their optical counterparts because their reflecting surface need not be as smooth as is needed for shorter-wavelength light waves. Provided that surface irregularities (dents, bumps, and the like) are much smaller than the wavelength of the waves to be detected, the surface will reflect them without distortion. Because the wavelength of visible radiation is short (approximately 10–6 m), very smooth mirrors are needed to reflect the waves properly, and it is difficult to construct very large mirrors to such exacting tolerances. However, even rough metal surfaces can accurately focus 1-cm waves, and radio waves of wavelength a meter or more can be reflected and focused perfectly well by surfaces having irregularities even as large as your fist.
Figure 5.21 shows the world's largest and most sensitive radio telescope, located in Arecibo, Puerto Rico. Approximately 300 m (1000 feet) in diameter, the surface of the Arecibo telescope spans nearly 20 acres. Constructed in 1963 in a natural depression in the hillside, the dish was originally surfaced with chicken wire, which was lightweight and cheap. Although fairly rough, the chicken wire was adequate for proper reflection because the openings between adjacent strands of wire were much smaller than the long-wavelength radio waves to be detected. The entire Arecibo dish was resurfaced in 1974 with thin metal panels, and upgraded in 1997, so that it can now be used to study shorter-wavelength radio radiation. Since the 1997 upgrade the panels can be adjusted to maintain a precise spherical shape to an accuracy of about 3 mm over the entire surface. At a frequency of 5 GHz (corresponding to a wavelength of 6 cm—the shortest wavelength that can be studied given the properties of the dish surface), the telescope's angular resolution is about 1'. The huge size of the dish creates one distinct disadvantage, however: the Arecibo telescope cannot be pointed very well to follow cosmic objects across the sky; its observations are restricted to those objects that happen to pass within about 20° of overhead as Earth rotates.
Figure 5.21 An aerial photograph of the 300-m-diameter dish at the National Astronomy and Ionospheric Center near Arecibo, Puerto Rico. The receivers that detect the focused radiation are suspended nearly 300 m (about 80 stories) above the center of the dish. One insert shows a close-up of the radio receivers hanging high above the dish. The other insert shows technicians adjusting the dish surface.
Arecibo is an example of a rough-surfaced telescope capable of detecting long-wavelength radio radiation. At the other extreme, Figure 5.22 shows the 36-m-diameter Haystack dish in northeastern Massachusetts. It is constructed of polished aluminum and maintains a parabolic curve to an accuracy of about a millimeter all the way across its solid surface. It can reflect and accurately focus radio radiation with wavelengths as short as a few millimeters. The telescope is contained within a protective shell, or radome, that protects the surface from the harsh wind and weather of New England. It acts much like the protective dome of an optical telescope, except that there is no slit through which the telescope "sees." Incoming cosmic radio signals pass virtually unimpeded through the radome's fiberglass construction.
Figure 5.22 Photograph of the Haystack dish, inside its protective radome. For scale, note the engineer standing at the bottom. Also note the dull shine on the telescope surface, indicating its smooth construction. Haystack is a poor optical mirror but a superb radio telescope. Accordingly, it can be used to reflect and accurately focus radiation having short radio wavelengths, even as small as a fraction of a centimeter.
Despite the inherent disadvantage of relatively poor angular resolution, radio astronomy enjoys many advantages. Radio telescopes can observe 24 hours a day. Darkness is not needed for receiving radio signals because the Sun is a relatively weak source of radio energy, so its emission does not swamp radio signals arriving at Earth from elsewhere in the sky. In addition, radio observations can often be made through cloudy skies, and radio telescopes can detect the longest-wavelength radio waves even during rain or snowstorms. Poor weather causes few problems because the wavelength of most radio waves is much larger than the typical size of atmospheric raindrops or snowflakes. Optical astronomy cannot be done under these conditions because the wavelength of visible light is smaller than a raindrop, a snowflake, or even a minute water droplet in a cloud.
However, perhaps the greatest value of radio astronomy (and, in fact, of all invisible astronomies) is that it opens up a whole new window on the universe. There are two main reasons for this. First, just as objects that are bright in the visible part of the spectrum (the Sun, for example) are not necessarily strong radio emitters, many of the strongest radio sources in the universe emit little or no visible light. Second, visible light may be strongly absorbed by interstellar dust along the line of sight to a source. Radio waves, on the other hand, are generally unaffected by intervening matter. Many parts of the universe cannot be seen at all by optical means but are easily detectable at radio wavelengths. The center of the Milky Way Galaxy is a prime example of such a totally invisible region—our knowledge of the Galactic Center is based almost entirely on radio and infrared observations. Thus, radio observations not only afford us the opportunity to study the same objects at different wavelengths but they allow us to see whole new classes of objects that would otherwise be completely unknown.
Figure 5.23 shows an optical photograph of the Orion Nebula (a huge cloud of interstellar gas) taken with the 4-m telescope on Kitt Peak. Superimposed on the optical image is a radio map of the same region, obtained by scanning a radio telescope back and forth across the nebula and taking many measurements of radio intensity. The map is drawn as a series of contour lines connecting locations of equal radio brightness, similar to pressure contours drawn by meteorologists on weather maps or height contours drawn by cartographers on topographic maps. The inner contours represent stronger radio signals, the outside contours weaker signals.
Figure 5.23 The Orion Nebula is a star-forming region about 1500 light years from Earth. (The nebula is located in the constellation Orion and can be seen in Figure 1.6.) The bright regions in this photograph are stars and clouds of glowing gas. The dark regions are not empty, but their visible emission is obscured by interstellar matter. Superimposed on the optical image is a radio contour map of the same region. Each curve of the contour map represents a different intensity of radio emission. The resolution of the optical image is about 1"; that of the radio map 1'.
The radio map in Figure 5.23 has many similarities to the visible-light image of the nebula. For instance, the radio emission is strongest near the center of the optical image and declines toward the nebular edge. But there are also subtle differences between the radio and optical images. The two differ mainly toward the upper left of the main cloud, where visible light seems to be absent, despite the existence of radio waves. How can radio waves be detected from locations not showing any light emission? The answer is that this particular nebular region is known to be especially dusty in its top left quadrant. The dust obscures the short-wavelength visible light but not the long-wavelength radio radiation. Thus, our radio map allows us to see the true extent of this cosmic source.
