In optical astronomy, the objective of an optical telescope collects the visible light ray from the celestial object being observed and concentrates them at a point whose enlarged image is then projected by the eyepiece.
Optical astronomy retains a special role since it is directly accessible to the eye.
In radio astronomy, large radio telescopes are used to intercept the radio waves emitted naturally by objects in space. Among radio sources in our Galaxy are the remains of supernova explosions. Strong sources of radio waves beyond our Galaxy include radio galaxies and quasars. The Earth's atmosphere absorbs or reflects, either totally or partially, most of the electromagnetic radiation emanating from the cosmos. Optical astronomy and radio astronomy are the only two realms observable from the ground.