A radio observatory is equipped to study the heavens with radio waves, either emitted by the objects being studied or transmitted from the earth and reflected from the moon or planets. Radio telescopes are much larger than optical telescopes. The greater size is necessary to focus radio waves, which are much longer than light waves. Two or more radio telescopes are often used together in arrays to study radio signals in greater detail. Radio telescopes generally are not housed inside special structures. Small ones are sometimes built on rooftops; large ones are built in open fields.
Radio observatories are best located in valleys surrounded by mountains. Light, clouds, and turbulent air have virtually no effect on radio telescopes. However, radio signals from broadcasting stations and electrical machinery interfere with observations in the same way that stray light from earth interferes with optical observations. Mountains or hills surrounding a radio observatory tend to block out stray radio signals.
Like optical observatories, radio observatories have computer departments, libraries, offices, and machine shops. The main difference is in the auxiliary instrumentation related to making and recording observations. Radio waves are not visible. In most cases immediate knowledge of what is being observed comes from radio receivers that present the amplified signals from space as graphical records. These records are made by devices such as an automatic pen that traces a line on a sheet of paper. The pen makes a wavy line, the height of the wave corresponding to the strength of the radio signal.
Radio observatories can operate 24 hours a day in all types of weather. Unlike optical equipment that can receive a variety of wavelengths of light simultaneously, radio equipment must be tuned to one wavelength (or band of wavelengths) at a time. Tuning sometimes can be done automatically at high speed to give a composite result in several wavelengths.