The size of a radio telescope is determined by the diameter of its "dish," a parabolic wire mesh which receives radio waves in much the same manner as the mirror of an optical telescope receives light waves.

The sources of the radio signals received by such apparatus are believed to be of two kinds. The most certain is that they come from the lonized gas-clouds of the Milky Way. But the other source are known as "radio stars," some of which are near smaller interstellar explosions. Research on thele nature is progressing by the coordinated use of optical and radio instruments.