Introduction to Observatory

Observatory, in astronomy, a building or group of buildings from which the heavens are studied. Observatories range in size from a single small building in an amateur astronomer's backyard to huge complexes where many professional astronomers work and live. Professional observatories are operated by universities, governments, and private research institutions. An orbiting observatory is an assembly of astronomical instruments installed in an artificial satellite.

There are two basic types of ground observatoriesoptical and radio.

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Optical Observatories

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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.

Radio Observatories

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A radio observatory is equipped to study the heavens with radio waves, either emitted by the objects being studied or transmitted from the earth and reflected from the moon or planets. Radio telescopes are much larger than optical telescopes. The greater size is necessary to focus radio waves, which are much longer than light waves. Two or more radio telescopes are often used together in arrays to study radio signals in greater detail. Radio telescopes generally are not housed inside special structures. Small ones are sometimes built on rooftops; large ones are built in open fields.

Radio observatories are best located in valleys surrounded by mountains. Light, clouds, and turbulent air have virtually no effect on radio telescopes. However, radio signals from broadcasting stations and electrical machinery interfere with observations in the same way that stray light from earth interferes with optical observations. Mountains or hills surrounding a radio observatory tend to block out stray radio signals.

Like optical observatories, radio observatories have computer departments, libraries, offices, and machine shops. The main difference is in the auxiliary instrumentation related to making and recording observations. Radio waves are not visible. In most cases immediate knowledge of what is being observed comes from radio receivers that present the amplified signals from space as graphical records. These records are made by devices such as an automatic pen that traces a line on a sheet of paper. The pen makes a wavy line, the height of the wave corresponding to the strength of the radio signal.

Radio observatories can operate 24 hours a day in all types of weather. Unlike optical equipment that can receive a variety of wavelengths of light simultaneously, radio equipment must be tuned to one wavelength (or band of wavelengths) at a time. Tuning sometimes can be done automatically at high speed to give a composite result in several wavelengths.

History of Observatories

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Astronomical observatories were built long before telescopes were invented. About the third century B.C. an astronomical observatory was built in Alexandria, Egypt. No Greek observatories are known, but some Greek astronomers, including Ptolemy, worked at Alexandria. Throughout Roman times and the decline that followed, there was little interest in astronomy and no noteworthy observatories were built. Astronomy was revived by the Arabs and in the ninth century A.D. a caliph, al-Mamun, built a large observatory at Baghdad.

In the New World, several observatories had been built by the 11th century. The earliest observatory in Europe, built at Nuremberg, was started in 1472.

The greatest of the pre-telescope observatories, called Uraniborg (Castle of the Heavens), was built in the 1570's by the Danish astronomer Tycho Brahe. The observatory had many instruments to help an observer accurately measure the positions of heavenly bodies.

After the telescope was invented in the early 17th century, observatories built for telescopes began to replace the earlier observatories. The French national observatory in Paris was established in 1671. Four years later the Royal Greenwich Observatory in England was established to aid in the determination of positions of celestial bodies for navigation. Observatories were built in almost all of the European countries. Many were part of major universities. The first observatory in the United States was built at the University of North Carolina in 1830.

Until the latter part of the 19th century, the work done at observatories was primarily determinations of positions and movements and the discovery of new celestial bodies. However, with the development of photography and the invention of instruments such as the spectroscope and the interferometer, it became possible to study both the composition and the motion of stars. For this work, larger telescopes that gathered more light were built.

The foundations of radio astronomy were laid in the 1930's, when it was discovered that radio signals were coming from space. Although some radio telescopes were built earlier, most of the large-scale building of radio observatories began in the 1950's.

The atmosphere strongly interferes with observations of some types of radiation, in particular ultraviolet radiation and X rays. A number of satellites have been placed in orbit to study stars (including the sun) and other sources of these types of radiation. One of the first such satellites was the first Orbiting Solar Observatory (OSO-1), which was launched in 1962. Others include the High Energy Astronomy Observatories launched in the late 1970's. The Hubble Space Telescope, a large optical telescope, was launched in 1989. Observations showed that the telescope's primary mirror was flawed, and in 1993 astronauts installed a corrective optical system.