Ground-based observatories, located on the surface of Earth, are used to make observations in the radio and visible light portions of the electromagnetic spectrum. Most optical telescopes are housed within a dome
or similar structure, to protect the delicate instruments from the
elements. Telescope domes have a slit or other opening in the roof that
can be opened during observing, and closed when the telescope is not in
use. In most cases, the entire upper portion of the telescope dome can
be rotated to allow the instrument to observe different sections of the
night sky. Radio telescopes usually do not have domes.
For optical telescopes, most ground-based observatories are located
far from major centers of population, to avoid the effects of light pollution.
The ideal locations for modern observatories are sites that have dark
skies, a large percentage of clear nights per year, dry air, and are at
high elevations. At high elevations, the Earth's atmosphere is thinner thereby minimizing the effects of atmospheric turbulence and resulting in better astronomical "seeing".