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.

C.

Cameras and Other Detectors

Modern astronomers use cameras and other electronic instruments to record and analyze radiation. Such instruments can detect light not visible to the human eye and make more accurate measurements than human eyes can.

In the late 1970s, electronic detectors called charge-coupled devices (CCDs) began replacing traditional cameras in most observatories. A CCD is a rectangular array of tens of thousands, or even millions, of tiny light-sensitive cells known as pixels. When a CCD is exposed to light, each pixel builds up an electric charge. A computer then reads the charges and constructs an image from the information. CCD images can reveal detail and color not visible to the human eye.

Astronomers use other electronic light detectors to learn more about a source of radiation. Two of the most common detectors are photometers and spectrographs. A photometer is a device that measures the brightness of an object in different wavelengths. A spectrograph uses a prism or diffraction grating to break starlight into its spectrum of colors. Astronomers can photograph and analyze this spectrum in detail and learn things such as the object’s temperature, chemical composition, magnetic field, and speed toward or away from Earth. If an object has a close, dim neighbor, its spectrum can reveal the presence of the companion object. By examining an object’s spectrum, astronomers can also tell whether the object is spinning on its axis. See also Photometry; Spectroscopy.

D.

Other Instruments and Computers

Many other types of instruments add to the information that observatories gather. Some of the most common and most useful tools are image tubes, fiber optics, and lasers. Astronomers use computers throughout the observing process to control telescopes and detectors. They also use computers to manipulate images and to analyze data.

An image tube is a device that electronically amplifies faint images. Light enters the tube, then reacts with a special phosphorescent substance inside the tube. This reaction causes more particles of light (called photons) to be released, multiplying the amount of light gathered by the telescope. Image tubes are less sensitive than CCDs, but they can create clearer images because they are better at distinguishing between real light and electronic noise.

Optical fibers are tiny, flexible glass rods that can carry light from one end of the fiber to the other, even around corners, with very little distortion. In observatories, astronomers use fiber optics to increase the efficiency of instruments. For example, if a telescope is pointed at an area of sky that contains many galaxies, astronomers can attach a special template with a hole over each galaxy to the telescope. Optical fibers connect to the holes, so the light of each galaxy is carried by a separate fiber. Each fiber can go to a separate instrument (such as a spectrograph), so many galaxies can be studied at once.

Turbulence in the atmosphere distorts light from astronomical objects as it travels to a telescope. Astronomers use lasers to counteract this effect. They shine a powerful laser beam from the observatory in the general direction of the star to be observed. Some of the laser light reflects back through the atmosphere to the observatory, where special detectors analyze the reflection and determine how the atmosphere distorted the beam. The detectors send a signal to small computer-controlled motors that bend and twist the shape of the telescope’s mirror. Deforming the mirror makes up for the distortion caused by the atmosphere. This technique, called adaptive optics, produces remarkably sharp images.

None of this efficient electronic imaging would be possible without modern computers. Modern observatories depend on computers for controlling telescopes, for allowing astronomers to use telescopes from far away, for analyzing data, and for processing images. Computers allow telescopes to follow complicated paths across the sky. With electronic links to universities and other institutions, astronomers no longer have to travel to a distant observatory to use a specific telescope. They can control the telescope and dome remotely by communicating with the observatory’s computer. The ability to remotely control a telescope is especially important for observatories in space. Computers collect data and analyze it rapidly for the astronomer—sometimes while he or she is still observing the object—enabling the astronomer to make changes in the observing program without delay. Astronomers also use computers to process images. Computers allow astronomers to isolate particular wavelengths and look at the levels of that wavelength emitted by different parts of the sky. Imaging software also allows astronomers to clean up images from the telescope, removing electronic noise, adjusting the contrast of objects and background, and adding artificial color to make specific features more noticeable.