An optical observatory is equipped to study the visible light from stars, planets, and other objects. Some optical observatories are also used to study infrared radiation from these objects. The ideal location for an optical observatory on the ground is a site that has clear, dry, calm, and stable air, and that lies far away from cities and other sources of scattered light that might interfere with viewing. Some of the best sites that have been found for observatories are island mountaintops and remote mountaintops in relatively dry regions.
The most important piece of equipment in the optical observatory is the telescope with its attachments and mounting. Major observatories often contain several telescopes. The principal telescope in most major observatories is a type of telescope called a reflector.. Among the common attachments used with a telescope are interferometers, photometers, and spectrographs.
A telescope mounting must bear the entire weight of the telescope while allowing the instrument to move easily. The mounting is usually built with a very precise, motor-powered drive mechanism designed so that the telescope can be automatically moved to compensate for the rotation of the earth and thus keep an object centered in the field of view over a period of time.
The telescope in most optical observatories is housed in an unheated metal dome. The dome has a slit, extending from the base to the top along one side, covered by movable metal doors. The slit is wide enough to allow an unobstructed view of the heavens through the telescope. The entire dome can be turned mechanically or by hand.
In addition to the telescope, many observatories contain a computer department, photographic laboratory, machine shop, and library, and offices for the observatory staff. Some also contain sleeping quarters for astronomers.