Observatory

I

Introduction

Observatory, building from which astronomers observe celestial objects such as planets, stars, and galaxies. The main instrument in an astronomical observatory is usually a telescope, a device that gathers light from distant objects and makes them appear larger than they do with the naked eye. See also Astronomy.

Scientists keep sophisticated instruments such as telescopes, cameras, computers, and other electronic devices inside observatories to protect them from moisture, sudden temperature changes, and other dangers caused by outside weather. Astronomers use this equipment to analyze light beams coming from space and celestial bodies. This light helps them study the physical and chemical properties of the objects.

People also use the word observatory to refer to the complex of buildings where astronomers go to carry out research. One such complex, Kitt Peak National Observatory west of Tucson, Arizona, is a complete mountaintop city. At Kitt Peak, astronomers from around the world have access to dozens of telescopes, electronic and machine shops, and laboratories. Kitt Peak also includes a library, dormitories, a cafeteria, and even has its own water supply, electrical generators, and fire-fighting equipment.

Astronomers build observatories in places where Earth’s atmosphere creates the least amount of interference between the telescope and space. Interference occurs when particles in the atmosphere, such as water molecules, reflect and distort light. Astronomers look for sites where the weather is clear and the air is calm and dry. Most modern observatories are on high mountaintops far from cities. At high altitudes, Earth’s atmosphere is thinner, allowing astronomers to see the universe more clearly. It is also important that the nighttime sky at an observatory site be free from city lights or interference from human-made radio sources. Artificial light and radio sources can pollute telescope observations with unwanted signals.

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Astronomical instruments are also carried into orbit around Earth, where the atmosphere does not interfere with their observations. Observatories such as the Hubble Space Telescope orbit far above Earth and can see the cosmos more clearly than any ground-based instrument in operation. These observatories are usually operated remotely by scientists on the ground. See also Space Telescope.

Other fields of science, such as the study of volcanoes (volcanology), the study of earthquakes (seismology), and the study of Earth’s weather (meteorology), also use buildings called observatories to house their equipment and to carry out research. However, the term observatory usually means a place of astronomical observations.

II

Equipment

Astronomy covers a broad range of research, and astronomers use many different types of equipment. The most familiar kind of observatory holds an optical telescope designed to look at visible light, but many other types of observatories and telescopes exist. Astronomers who wish to study other types of light need special telescopes and equipment, and the design of observatories reflects the equipment they contain.

A

Optical Telescopes and Domes

In a normal optical observatory, the most prominent instrument is the telescope. A typical modern telescope is a huge tube with a giant concave (dish-shaped) mirror or collection of mirrors at one end. This mirror collects light from space and focuses it to a point. The telescope takes this gathered light and usually sends it into a camera or other electronic instruments for analysis.

The dome, or housing, protects the telescope and other equipment from the elements. Domes open to allow the telescope access to the sky. Most domes have a single slit down one side. When astronomers begin their work at dusk, they open the dome slit and allow the telescope to peer outward into the universe.

During the night Earth rotates, so as observed from Earth, the stars seem to move in the opposite direction. The telescope must take this into account and turn in the opposite direction of Earth’s rotation to remain focused on a stationary celestial object. The dome of an observatory also turns so the slit in the dome always allows the telescope to see out. Many telescopes stand on special mounts that have one axis parallel to Earth’s axis (the line around which Earth rotates) and the other at right angles to the Earth’s axis (or parallel to the direction in which the stars seem to be moving). This type of mount is called an equatorial mount. The axis parallel to Earth’s axis is called the polar axis. The perpendicular axis is called the declination axis. If an astronomer locks the declination axis in place after the telescope is pointed at an object of interest, a small motor can drive the telescope slowly around its polar axis, following the motion of the object in the sky as Earth turns. Equatorial mounts are expensive and complicated. Newer telescopes often stand on simpler, less expensive mounts and rely on computers to control the motor and position the telescope correctly.

Modern astronomers seldom actually look through a telescope eyepiece. Instead they operate the telescope and dome from comfortable control rooms—or even remotely from their offices far away from the observatory—where computers and video screens show exactly what happens at all times.

B

Equipment for Different Wavelengths

The visible light viewed through optical telescopes is just one part of the electromagnetic spectrum, the range of electric and magnetic waves that spans radio waves, visible light, X rays, and all the types of radiation in between. Astronomers are also interested in observing electromagnetic waves with wavelengths longer or shorter than those of visible light. These waves, invisible to the human eye, are also invisible to the telescopes designed to gather visible light, so astronomers need special telescopes and detectors to study the rest of the spectrum. See also Electromagnetic Radiation.

The coldest objects in the universe emit radio waves. Radio waves have the longest wavelength of any electromagnetic waves. Astronomers use instruments called radio telescopes to gather and study radio waves that come from space. Radio telescopes are usually huge metal dishes, set into the ground or perched on supports above the ground. Radio waves from space hit the dish and bounce off. The dish is shaped in such a way that the radio waves bounce off to a single point above the dish, no matter where they first hit the dish. Detectors at this point, called the focal point, send the signals to computers that process the information. The largest radio telescope is at Arecibo Observatory in Puerto Rico. The Arecibo radio telescope is set in a natural depression in the earth and measures 305 m (1,000 ft) across. See also Radio Astronomy.

On the electromagnetic spectrum, infrared radiation falls between radio waves and visible light. Most objects in the universe produce some amount of infrared radiation, but infrared observatories are especially useful for studying cooler objects. Infrared telescopes work much like optical telescopes; however, the water in Earth’s atmosphere blocks most infrared radiation from space. Infrared telescopes are also very sensitive to heat—they produce the clearest images when cooled to very low temperatures. Therefore, astronomers place infrared observatories atop high, dry mountains, or even in orbit around Earth. The Infrared Space Observatory (ISO) of the European Space Agency (ESA) was one such orbiting infrared observatory. It studied the infrared sky from 1995 to 1998. See also Infrared Astronomy.

Ultraviolet radiation, X rays, and gamma rays have shorter wavelengths than visible light has. These types of radiation tell astronomers about the hottest and most violent phenomena in the universe. Earth’s atmosphere blocks most of this radiation, so astronomers must send their observatories above the atmosphere aboard balloons, rockets, or satellites. Ultraviolet telescopes are much like visible light telescopes, but X-ray telescopes must have special nested cylindrical mirrors to prevent X rays from passing right through the telescope. Gamma-ray observatories often carry several telescopes because combining data from different telescopes makes it easier for astronomers to find the region of the sky where the gamma rays originated. See also Ultraviolet Astronomy; X-Ray Astronomy; Gamma-Ray Astronomy.

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