For centuries, astronomers learned about the sky by studying the light coming from astronomical objects, first by simply looking at the objects, and later by making photographs. Many astronomical objects emit radio waves, but that fact wasn't discovered until Since then, astronomers have developed sophisticated systems that allow them to make pictures from the radio waves emitted by astronomical objects. A number of celestial objects emit more strongly at radio wavelengths than at those of light, so radio astronomy has produced many surprises in the last fifty years. By studying the sky with both radio and optical telescopes, astronomers can gain much more complete understanding of the processes at work in the universe. Radio astronomers have made some exciting discoveries. Pulsars (rotating neutron stars) and quasars (dense central cores of extremely distant galaxies) were both discovered by radio astronomers.

Radio astronomy and optical astronomy both examine electromagnetic radiation from outside the Earth's atmosphere. Where they differ is in the tools used to detect this radiation and in the wavelength or frequency of the waves they study. Light and radio waves are both variations of the same energetic phenomena. Because radio waves are much longer than optical waves, the telescopes used to detect them must be much larger than optical telescopes.

This term refers to sharpness of image. With normal "20/20" vision, you can read letters 1 cm high at a distance of 10 meters. But with the Hubble Space Telescope, you could read letters 1 cm high at a distance of 12 km times farther! We characterise the angular resolution by the smallest angle, , that can be discerned in an image. For an eye with 20/20 vision, the angular resolution is  = 1 arcminute (1/60 th of a degree). The HST has an angular resolution  = 0.05 arcseconds, 1200 times sharper.