Radio Astronomy

Radio telescopes and optical telescopes have some features in common. Both instruments, for example, are designed to collect, focus, and record the presence of a certain type of electromagnetic radiation—radio waves in one case and light waves in the other. However, the details of each kind of telescope are quite different from one other.

One reason for these differences is that the human eye cannot detect radio waves as it can light waves. So an astronomer cannot look into a radio telescope the way he or she can look into an optical telescope. Also, radio waves have insufficient energy to expose a photographic plate, so an astronomer cannot make a picture of a radio source in outer space as she or he can of an optical source.

The first difference between an optical telescope and a radio telescope is in the shape and construction of the collecting apparatus—the mirror in the case of the optical telescope and the "dish" in the case of the radio telescope. Because the wavelength of visible light is so small, the mirror in an optical telescope has to be shaped very precisely and smoothly. Even slight distortions in the mirror's surface can cause serious distortions of the images it produces.

In a radio telescope, however, the "mirror" does not have to be so finely honed. The wavelength of radio waves is so long that they do not "recognize" small irregularities in the "mirror." (The word mirror is placed in quotation marks here because the collecting surface of the radio telescope looks nothing like a mirror, though it does in effect act like one.) In fact, it can be made of wire mesh, wire rods, or any other kind of material off which radio waves can be reflected.

For many years, the largest radio telescope in the world was located in a natural bowl in a mountain outside Arecibo, Puerto Rico. The bowl, which is 1,000 ft (305 m) wide and occupies 20 acres (8 ha), was lined with wire mesh, off which radio waves were reflected to a wire antenna at the focus of the telescope. The radio waves collected along the antenna were then converted to an electrical signal which was used to operate an automatic recording device that traced the pattern of radio waves received on the wire mesh.