Observatory - astronomy.

Observatory - astronomy. 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 firefighting 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. 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. 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 computercontrolled 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. III LOCATION The mission of an observatory helps determine where its builders locate it. Other factors that influence the choice of location include the wavelength that the telescope will study, the location of other telescopes that the observatory plans to cooperate with, and the ease of moving heavy equipment and many people. A High Altitude The motion of convection cells (regions of moving air) in the atmosphere distorts incoming light, and water molecules in the air absorb and scatter some wavelengths of light. Therefore, telescopes are often located in places with a thin atmosphere and with little moisture in the air. At high altitudes on Earth, there is less atmosphere between the telescope and space. The atmosphere at high altitudes also contains less moisture. Modern optical observatories are always constructed at a high altitude above sea level, often higher than 1,800 m (6,000 ft). The highest observatory in the world is located on Mauna Kea, an extinct volcano in Hawaii. Because of its prime location at 4,200 m (13,800 ft) above sea level, Mauna Kea Observatory is home to several huge telescopes. The two largest optical telescopes in the world, the twin Keck telescopes, are part of the Mauna Kea Observatory. B Combining Telescopes Astronomers sometimes combine the light from two or more individual telescopes to create a particularly clear image. This technique is called interferometry. The telescopes are connected to a device called an interferometer that manipulates the light that the telescopes gather to produce a very sharp image. The images of such a pair of telescopes have the clarity of a single telescope as large as the distance between the telescopes. For example, two telescopes 30 m (100 ft) apart simulate the sharpness of a single telescope with a mirror 30 m (100 ft) in diameter. Astronomers have been using radio telescopes for interferometry since the 1960s, but were not able to develop optical interferometers until the early 1990s. To get the clearest results, the cables connecting the telescopes to the interferometer must be almost, but not quite, the same length. They should differ by exactly one-half of the length of the light waves that the telescopes gather. By traveling exactly one-half wavelength difference in distance, the combined signals interfere with each other in a certain way. This interference produces the clearest possible image. Radio waves are much longer than waves of visible light, so it was easier for builders to measure the connecting cables to the correct length. Radio astronomers use long rows of radio telescopes for radio interferometry. The largest of these is the Very Large Array (VLA) near Socorro, New Mexico. The 27 radio telescopes of the VLA cover 63 km (39 mi). Optical interferometry has a much smaller scale. The wavelength of visual light is less than 1 micrometer (1/1,000,000 of a meter, or 1/25,400 of an inch), so cables for interferometry must be measured to an accuracy of at least 0.5 micrometers. The accuracy required makes it difficult to connect telescopes that are far apart. However, a few long-distance interferometry telescopes started coming into use in the late 1990s. In a joint effort, the United States, Chile, Great Britain, Brazil, Canada, and Argentina are building two interferometry telescopes, the Gemini telescopes. One is being built in the Atacama desert in Chile, and the other will be at Mauna Kea Observatory in Hawaii. Observation with the Gemini telescopes is scheduled to begin in 2001. In a test of optical interferometry in the mid-1990s, a team of American astronomers produced an image of a binary star system so clear that it was equivalent to being able to see a car on the Moon. Producing such stunning clarity requires tremendous computer power, and creating images can take many hours or days of telescope time to complete. C Space Observatories Observatories high above Earth's turbulent atmosphere make it possible for astronomers to see the universe much more clearly than with ground-based instruments, as well as study wavelengths of radiation that are blocked by the air. Some of the many space observatories now in Earth-orbit are part of NASA's Great Observatories Program. The first of the series was the Hubble Space Telescope (HST), launched in 1990, followed one year later by the Compton Gamma-Ray Observatory (CGRO). The Chandra X-Ray Observatory was placed in orbit in 1999, and the Spitzer Space Telescope was launched in 2003. IV HISTORY The telescope is a relatively new invention, but astronomical observatories are not. Even in ancient times, sky watchers built and used observatories. Records that are 5,000 years old show that the Babylonian civilization, in what is now Iraq, observed the movement of celestial objects. About 4,500 years ago, Egyptians developed a calendar based on the movement of the Sun, showing that ancient Egyptians watched the movement of heavenly bodies. Most scientists believe that the stone pyramids of the Maya civilization, mostly in southern Mexico and dating from 2,500 years ago, have astronomical significance. One of the most mysterious ancient observatories is Stonehenge, an arrangement of stones surrounded by a circular earthwork in southern England. Stonehenge, which dates from the late Stone and early Bronze ages, may have been used by prehistoric peoples to predict eclipses and to tell time. Most historians believe that Dutch spectacle maker Hans Lipperhey built the first known telescope in 1608. The Dutch used telescopes as military tools to spot ships approaching from a distance. In 1609 Italian astronomer Galileo Galilei built a telescope to study the sky. Although his telescope could magnify only 20 times, Galileo discovered many things, including the cratered surface of the moon, the constantly moving moons of Jupiter, the changing phases of Venus, and the countless stars that make up the wispy light of the Milky Way. Galileo's observations helped overturn the idea that Earth was the center of the universe, and opened the door to the true science of astronomy. The first telescopes used glass lenses to gather and bend light to a focus. These refracting telescopes could be fairly powerful, but were very long, and were hard to steady and balance. Near the end of the 18th century, English scientist Sir Isaac Newton built a telescope that used a mirror instead of a lens. The first of these mirrors were made of speculum metal (a mixture of copper and tin), but glass mirrors soon replaced the metal. Because of the better balance of refractors and because large mirrors are lighter than large lenses, telescope makers could make larger reflecting telescopes than refracting telescopes. The large reflectors could see much farther into the universe and reveal many more mysteries. Soon some telescopes were so large they could be moved only by complex pulley systems. Eventually they grew so large that astronomers needed buildings to protect them from the weather. Many of these buildings were constructed in the shape of domes with moveable slits that could be opened to the outside. Through the centuries, telescopes continued to grow. Eventually the mirrors became so large and heavy that it was almost impossible to build telescopes sturdy enough to hold them, and yet light enough to point the mirrors at different areas of the sky. Instead of using single large mirrors, in the 1970s and 1980s telescope makers began building telescopes with multiple, or "segmented," mirrors. Advances in computer technology meant that observatories no longer needed complicated telescope mountings--the telescope could stand on a simpler mount and the computer could determine the exact movement needed to follow an object across the sky. At this time, observatories began to take on unconventional shapes. Some observatories built in the later part of the 20th century were large square buildings, instead of the usual cramped round buildings. The new buildings improved the flow of air around the telescopes, preventing distortion from regions of different temperature. In the 1990s, the largest observatories in the world opened to the sky. The mirrors of the two Keck telescopes on Hawaii's Mauna Kea are each about 10 m (33 ft) across. The Very Large Telescope (VLT) of the European Southern Observatory uses four telescopes--each 8.2 m (27 ft) across--joined together to create the lightgathering power of a single instrument 16 m (52 ft) in diameter. In less than five decades, telescope light-gathering power has more than tripled. As the 21st century opens, half a dozen more telescopes--each more than 6.5 m (21 ft) across--are being constructed in joint projects by countries such as the United States, Great Britain, Canada, Brazil, Argentina, Italy, Chile, and Japan. Contributed By: Dennis L. Mammana Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.