The function of the astronomical observatory is centered around the telescope. In addition to visual and photographic observations of astronomical bodies and phenomena, perhaps the most valuable use of the telescope is in connection with the spectroscopic study of starlight. The total light from a star is separated into its various wavelengths (see spectrum), and the intensity of each is measured. The temperature and chemical composition of stars can be obtained by this method, as well as information about stellar motion and magnetic fields. Using computers, astronomers can measure the spectra digitally recorded by spectrographs and photometers. Observatories specializing in solar astronomy usually have coronographs and spectroheliographs. Atmospheric limitations on telescopic observations include weather conditions, air turbulence, air glow, pollution, and any source of extraneous illumination. To minimize such conditions optical observatories are generally located at high altitudes in sparsely populated areas.
See articles on specific observatories.
Early civilizations, such as those of Babylon, China, and Egypt, recognized the regular and periodic nature of heavenly motions and established primitive observatories to maintain astronomical records. The main purposes of these early observatories were to regulate the calendar and predict the changes of season. Because it was believed that unusual occurrences, such as comets and eclipses, foretold future events on earth, the early observatories also served a religious function, and most of the ancient astronomers were priests. Later observatories were established to compile accurate star charts and an annual ephemeris that would be of use to navigators in determining longitude at sea. For some 600 years, beginning in the 13th cent., Roman Catholic churches included solar observatories to measure the movements of the sun and so determine the correct date for Easter.
The instruments in use before the invention of the telescope include the sextant, quadrant, astrolabe, and armillary sphere. These are all calibrated sighting devices for determining the angular positions of stars and planets. The armillary sphere was the most sophisticated of these instruments. It was composed of a number of rings corresponding to great circles on the celestial sphere and was used to determine both the right ascension and the declination of a star. The last great observatory of the pretelescopic era was built by Tycho Brahe at Uranienborg, on the island of Ven, Denmark.
The invention of the telescope in the early 17th cent. revolutionized observational astronomy in two ways. First, the positions and motions of celestial bodies could be measured much more accurately with telescopes than with the earlier instruments. Such data provided a source of precise time signals. Second, the telescope could be used to analyze the physical nature of celestial bodies themselves. Until the 19th cent., telescopic images were inspected visually by highly trained observers who made drawings of what they saw. The development of dry-plate photography, which permitted long exposure times, however, offered a much more sensitive method of recording images. In the late 20th cent., electronic digital detectors utilizing charge-coupled devices (CCDs) superseded the use of film; a CCD can detect the arrival of a single photon of light. A recent development is the extension of astronomical observations to wavelengths outside the visible spectrum. Most important has been the development of radio astronomy, the study of radio waves emitted by celestial bodies.
Because the atmosphere interferes with astronomical observations from the ground, the ideal location for an observatory is beyond the earth's atmosphere. Since the late 20th cent., there has been an increasing emphasis on space-based observatories (see observatory, orbiting). Several artificial satellites have been equipped with telescopes for infrared, visible, ultraviolet, and X-ray observations. The International Ultraviolet Explorer (IUE) satellite, launched in 1978, is an 18-in. (0.45-meter) space telescope for ultraviolet studies. Launched in 1983, the Infrared Astronomy Satellite (IRAS) discovered some 246,000 infrared sources, as well as several stars around which planetary systems appear to be forming. Skylab was a manned orbiting space observatory (see space exploration). The largest space-based observatory is the Hubble Space Telescope, launched in 1990. Other observatories include the Compton Gamma-Ray Observatory, launched in 1991, and the Chandra X-ray Observatory, launched in 1999. ROSAT [ROentgen SATellite], a joint German-U.S.-British project launched in 1990, studies both X-ray and ultraviolet wavelengths never before imaged from space. It has detected a new class of bright stars that shine only in the ultraviolet part of the spectrum. The Cosmic Background Explorer (1989-93) studied microwave background radiation that no star or other known object could emit—it is believed to have come from the creation of the universe (see cosmology).
The computer age has also impacted observatories in several ways. Instead of being cast in one piece, reflecting telescope mirrors can be constructed of numerous small segments that move under computer control to focus the light and create an image equivalent to that of a much larger telescope. This has made it economically possible for observatories to reach further and further into the cosmos. Computers have also made it easier to construct and retrieve information from archival data bases. Computer networks are under construction that will make it possible for these archival databases to be made available to other observatories so as to create a virtual observatory, with gains in productivity and cost-effectiveness for the observatories that participate in it.
The U.S. Orbiting Solar Observatory (OSO) program, comprising eight satellites launched between 1962 and 1971, was one of the earliest series of orbiting observatories; it studied the sun's atmosphere and the sunspot cycle. Also beginning in 1962 and extending through 1979 were the launches of the six satellites in Great Britain's Ariel program, which concentrated on solar ultraviolet and X radiation. The Orbiting Geophysical Observatory (OGO) program consisted of six satellites, launched between 1964 and 1969, that provided data on the earth's atmosphere, ionosphere, and magnetosphere and on the solar wind. The Orbiting Astronomical Observatory (OAO) program comprised four satellites, launched between 1966 and 1972, that studied astronomical phenomena at ultraviolet and X-ray wavelengths inaccessible to earthbound equipment.
In the following years, a large number of satellites were launched to study solar and galactic radio waves, X rays, gamma rays, and ultraviolet rays. In addition to the United States a number of countries participated, among them the Netherlands with ANS-1 (1974-76), which studied soft and hard X radiation; India with Aryabhata (1975), which returned atmospheric data for only four days before being silenced by a power failure; Japan with Hakucho (1979-85) and Tenma (1981-84), both of which studied X radiation; and the European Space Agency (ESA) with Exosat (1983-86), an X-ray observatory. This period also saw the first cooperative efforts, such as the International Ultraviolet Explorer (IUE), a joint effort of the United States, ESA, and Great Britain (1978-96), which returned data on ultraviolet radiation for 18 years.
ROSAT [Roentgen Satellite] (1990-99), a joint German-U.S.-British project, studied both X-ray and ultraviolet wavelengths never before imaged from space. It detected a new class of bright stars that shine only in the ultraviolet part of the spectrum and X-ray emissions from comets. The Cosmic Background Explorer (1989-93) studied microwave background radiation that no star or other known object could emit—it is believed to have come from the creation of the universe (see cosmology). The Infrared Space Observatory (ISO; 1995-98), launched by ESA, found water in the Orion nebula and in the atmospheres of the giant planets and Titan, found fluoride molecules in interstellar space, and studied the "cool" galaxies first seen by the Infrared Astronomy Satellite (IRAS) in 1983. Another European-built orbiting observatory, the Solar and Heliospheric Observatory (SOHO), was launched by NASA in 1995. After reaching a position about 950,000 mi (1.5 million km) from the earth, where the gravitational attraction of the earth and the sun are in balance (called a Lagrangian point), SOHO initiated a program of solar physics studies, such as the solar wind and solar plumes.
To fully explore the cosmos it is necessary to collect and analyze radiation emitted by phenomena throughout the entire electromagnetic spectrum. Toward that end, NASA proposed the concept of great observatories, a series of four orbiting observatories designed to conduct astronomical studies over many different wavelengths. An important aspect of the program was to overlap the operations phases of the missions to enable astronomers to make concurrent observations of an object at different spectral wavelengths. The first member of the program and the largest orbiting observatory is the Hubble Space Telescope (HST), which was deployed by a space shuttle in 1990 and repaired in orbit in 1993. Subsequent servicing missions added capabilities to the HST, which observes the universe at ultraviolet, visual, and near-infrared wavelengths. The second great observatory, the Compton Gamma-Ray Observatory, was launched and deployed by a shuttle in 1991; it collected data on gamma-ray bursts, which are some of the most violent physical processes in the universe. The third great observatory, the Chandra X-ray Observatory, formerly called the Advanced X-ray Astrophysics Facility, was deployed from a shuttle and boosted into a high earth orbit in 1999; it focuses on such objects as black holes, quasars, and high-temperature gases throughout the X-ray portion of the electromagnetic spectrum. The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility, represents the fourth and final element in the great observatory program; launched in Aug., 2003, it fills an important gap in wavelength coverage not available from earthbound telescopes.
In 2009 ESA launched the Herschel Space Telescope, with a 138-in. (3.5-m) mirror, the largest sent into space so far. Positioned some 930,000 mi (1.5 million km) from earth on a mission expected to last three years, it is designed to observe wavelengths from the infraread to the submillimeter and study the formation of galaxies in the early universe and the formation of stars as well as objects in the solar system. On the same Ariane rocket that carried Herschel, ESA also launched Planck, which observes the cosmic microwave background radiation.
An observatory is a location used for observing terrestrial and/or celestial events. Astronomy, climatology/meteorology, geology, oceanography and volcanology are examples of disciplines for which observatories have been constructed. Historically, observatories were as simple as containing an astronomical sextant (for measuring the distance between stars) or Stonehenge (which has some alignments on astronomical phenomena).
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 other similar structure in order to protect the delicate instruments from the elements. Telescope domes contain a slat or other opening in the roof that can be opened during observing and then closed when the telescope is not in use. In most cases, the entire upper portion of the telescope dome can be rotated in order 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 in order 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". Sites that meet the above criteria for modern observatories include the southwestern United States, Hawaii, the Andes Mountains region, Australia and the mountainous Sierra Negra in Mexico. Major optical observatories include Mauna Kea, HI, USA, Roque de los Muchachos Observatory in Spain, Paranal Observatory in Chile and Kitt Peak observatory in the USA.
Space-based observatories are telescopes or other instruments that are located in outer space, many in orbit around the Earth. Space-based observatories can be used to observe astronomical objects at wavelengths of the electromagnetic spectrum that cannot penetrate the Earth's atmosphere and are thus impossible to observe using ground-based telescopes. The Earth's atmosphere is opaque to ultraviolet radiation, X-rays, and gamma rays and is partially opaque to infrared radiation so observations in these portions of the electromagnetic spectrum are best carried out from a location above the atmosphere of our planet. Another advantage of space-based telescopes is that, because of their location above the Earth's atmosphere, their images are free from the effects of atmospheric turbulence that plague ground-based observations. As a result, the angular resolution of space telescopes such as the Hubble Space Telescope is often much smaller than a ground-based telescope with a similar aperture. However, all these advantages do come with a price. Space telescopes are much more expensive to build than ground-based telescopes. Due to their location, space telescopes are also extremely difficult to maintain. The Hubble Space Telescope can be serviced by the Space Shuttle while many other space telescopes can not be serviced at all.
The oldest proto-observatories, in the sense of a private observation post, include:
The oldest true observatories, in the sense of a specialized research institute, include: