A place from which astronomical observations are made; sometimes the name is also used for an administration building completely separate from the observing site. An observatory contains instruments for detecting electromagnetic radiation from celestial objects at various wavelengths. Before the invention of the telescope, observatories such as those of Ulugh Beg and Tycho Brahe had only simple equipment for naked-eye sightings of positions. The development of optical telescopes in the seventeenth century led to the need for a permanent protective building, usually with a dome that could be opened to view the sky. To ensure the best possible observing conditions, astronomers began in the late nineteenth century to site their telescopes on high mountains, above the densest parts of the atmosphere. This trend has continued by placing telescopes in orbit, where they can also observe wavelengths such as X-rays, ultraviolet, and infrared that do not penetrate the atmosphere. Radio telescopes, developed after World War II, are not usually housed in observatory buildings, because they are too large and also because they do not need as much protection from the elements as optical telescopes.