OBSERVATORIES, ASTRONOMICAL. The dark-adjusted human eye has a pupil size of only about
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five millimeters, a biological apparatus with limited light-gathering capability and only for the visual spectrum. By contrast, modern astronomical observatories offer views that are millions of times more powerful and for a wide range both above and below the visible electromagnetic spectrum. These instruments extend human sight even farther by an increasingly complex assortment of measurements and astrophotography.
History of New World Observatories
New World astronomy began with the early observations made and recorded by indigenous peoples, notably evidenced by the stone observatories and calendar inscriptions of the Mayans of Central America, built between a.d. 300 and 900, and the Incas of South America, between a.d. 1200 and 1533, among others. These stone instruments allowed them to predict agricultural seasons as well as such celestial events as lunar and solar eclipses. Early Europeans in the New World used astronomical instruments for navigation and exploration, and Amerigo Vespucci cataloged southern stars from the coast of South America in 1499 and 1501. In time, early English Colonials displayed keen interest in the science of the day, including astronomy. Their observations came in the form of temporary platforms such as the one that David Rittenhouse constructed to view the 1769 transit of Venus.
However, not until the decades following the Revolutionary War (1775–1783) did Americans see a serious observatory, constructed through the efforts of Ferdinand R. Hassler, director of the U.S. Coast Survey, with the support of President John Quincy Adams. Between 1830 and the Civil War (1861–1865), occasional private individuals, such as William Mitchell of Nantucket, Massachusetts, and Lewis M. Rutherfurd in New York City, pursued serious astronomical observations. With an increasing availability of astronomical apparatus, schools began to include practical as well as theoretical astronomy; both private and public funds built some twenty-five observatories complete with refracting telescopes—instruments that use lenses to focus incoming light.
In 1842, the federal Depot of Charts and Instruments founded the U.S. Naval Observatory in Washington, D.C. Congress established the Nautical Almanac Office in 1849 and used the work of the observatory to collect data for both navigators and astronomers. The observatory's primary function was to provide astronomical data for safe navigation at sea, in the air, and in the twentieth century, in space. The facility also provided standardized time for local fire and police stations. By the 1880s, both telegraphs and railroads used the observatory's time. Eventually, Washington's light pollution—that is, excessive light diffracted into the atmosphere and degrading astronomical observations—forced the Naval Observatory to build a second facility in Flagstaff, Arizona, in 1955.
After the Civil War, observatories became a common addition to colleges and universities, which increasingly undertook serious astronomical research. Harvard's Henry Draper became the first to photograph stellar spectra in 1872 and began, with the help of Williamina P. Fleming and others, to develop a catalog of the stars. Nineteenth-century air and light pollution led to a growing number of observatories in rural areas and in the high desert plains and mountains of the American West, as well as in South America and Africa.
The second half of the nineteenth century also saw a movement to build bigger and better telescopes. Alvan Clark and Sons of Cambridgeport, Massachusetts, successfully produced ever larger refractors than any previous ones: 18.5 inches (1863), Dearborn Observatory in Chicago (later relocated to Evanston, Illinois, at Northwestern University); 26 inches (1872), U.S. Naval Observatory; 30 inches (1883), Pulkovo Observatory, near St. Petersburg, Russia; 36 inches (1887), Lick Observatory, near San Jose, California; and 40 inches (1897), Yerkes Observatory, at Williams Bay, Wisconsin. American astronomer Percival Lowell (1855–1916) established the Lowell Observatory in Flagstaff, Arizona, in 1894. Lowell used his facility to make significant observations of the planets and to predict the discovery of Pluto, which astronomers first observed in 1930 from the Lowell Observatory.
Refracting telescopes began to give way to reflecting telescopes; these instruments use mirrors rather than lenses to focus incoming light, allowing for larger construction and more accurate observations and measurements. As a result, the first half of the twentieth century saw the construction of a new generation of large-aperture telescopes: 60 inches (1908), Mount Wilson, California; 100 inches (1918) Mount Wilson; and 200 inches (1948), Palomar Mountain, California.
The World's Largest Optical Observatories
Since astronomical observatories tend toward the maxim of "bigger is better," consequent construction has often required increasing collaboration among nations in order to fund the scale of more modern, sophisticated installations. What follows are descriptions of the more noteworthy observatories constructed during the twentieth and early twenty-first centuries.
The Mount Wilson and Palomar observatories were jointly named Hale Observatories in December 1969, in honor of pioneering astronomer George Ellery Hale (1868–1938). The complex focuses on solar observations and studies, and astronomers from around the world use the facility for astrophotography as well as spectroscopic research. The equatorially mounted 60-and 100-inch telescopes on Mount Wilson routinely serve long-exposure celestial photography. The observatory's 60-inch instrument ventured the first measurements of the Milky Way galaxy and determined the distance to M31, the great galaxy in Andromeda. In 1928, the Rockefeller Foundation agreed to fund construction of a 200-inch (or 5.1 meters, since larger telescopes are generally measured in meters) reflecting telescope, which the California Institute of Technology, together with the observatory staff, super-vised. World War II (1939–1945) delayed completion of
the project, and the great telescope, formally called Hale Telescope, finally saw its dedication twenty years later, in 1948. Because of the encroaching population growth of Los Angeles, Mount Palomar served as the location for later astronomical constructions, although the two facilities continued to operate as one research installation.
Kitt Peak National Observatory, southwest of Tucson, Arizona, began operating in 1960 and is administered by the Association of Universities for Research in Astronomy. The installation contains many telescopes, including the McMath solar telescope, the largest of its type, with a 1.5-meter diameter. The observatory's largest reflecting telescope is a 4-meter instrument completed in 1973. The National Radio Astronomy Observatory operates an 11-meter radio telescope here as well. The facility hosts other telescopes operated by The University of Arizona, the University of Michigan, the Massachusetts Institute of Technology, and Dartmouth College.
Whipple Observatory, located on Mount Hopkins south of Tucson, Arizona, has a number of telescopes, including a ten-meter dish made up of 248 hexagonal-shaped mirrors, installed in 1968. This instrument observes gamma rays from space interacting with the atmosphere. The facility also houses two conventional reflecting telescopes installed in 1970, one with a 1.52-meter mirror and the other with a mirror of twelve inches (30 cm). The largest instrument, completed in the late 1970s, was a multiple-mirror telescope consisting of six 72-inch mirrors with a combined area equivalent to a 4.5-meter mirror. This instrument made both optical and infrared observations of the sky. The six mirrors, however, were replaced in 1997 by a single 6.5-meter mirror.
Mauna Kea Observatory sits atop Hawaii's dormant volcano Mauna Kea and has the nighttime advantage of a minimum of light pollution. Founded in 1967, the observatory operates under the University of Hawaii but houses several internationally sponsored instruments. The United States, Canada, and France sponsor a 3.58-meter optical and infrared reflector, placed in operation in 1979. The United Kingdom and the United States operate a 3.8-meter infrared reflector as well. A 15-meter British-Dutch paraboloid telescope operates in the ultrashort wave band. This instrument, built of 200 individual mirror panels, was completed in 1987. Mauna Kea additionally houses a 3-meter infrared reflector and a 2.24-meter optical and infrared reflector.
Mauna Kea is also home for the Keck Observatory, completed on Mauna Kea in 1990 and housing two of the world's largest optical telescopes. Keck I, completed in 1993, has a ten-meter primary mirror consisting of thirty-six separate hexagonal segments. The telescope produced detailed and significant images of Jupiter when fragments of Comet Shoemaker-Levy 9 bombarded the planet in July 1994. Keck II, completed in 1998, also possesses a similar ten-meter mirror array.
The McDonald Observatory in Fort Davis, Texas, jointly operated by the University of Chicago and the University of Texas, houses the Hobby-Eberly Telescope. The instrument was installed in 1998 with an eleven-meter diameter, making it one of the largest single-mirror telescopes in the world.
One of the most ambitious telescope projects of the late twentieth century occurred as a joint European and Chilean venture atop Chile's Cerro Paranal, where four side-by-side observatories became operational in September 2000. Each of the 8.2-meter telescopes of the ESO Very Large Telescope at Paranal can operate independently or in cooperation through a process known as interferometry, whereby images are made to blend together to create viewing equal to a combined mirror surface of more than 210 square meters.
Nonoptical and Space-Based Telescopes
Of the nonoptical telescopes—that is, those operating outside the visible spectrum of light—the largest is the Very Large Array, which consists of twenty-seven movable radio telescopes constructed on tracks extending some seventeen miles apart, near Socorro, New Mexico. Each parabolic dish is twenty-five meters in diameter, but when the telescopes are spread fully apart, the array can receive signals equivalent to a telescope of seventeen miles in diameter.
Of the space-based observatories, two are particularly prominent. Launched in 1990, the Hubble Space Telescope required corrective optics that astronauts from the space shuttle Endeavor installed in 1993. The instrument's 2.4-meter telescope can study wavelengths from the near-infrared through the ultraviolet. NASA's Chandra X-ray Observatory, launched and deployed by space shuttle Columbia in 1999, is a sophisticated X-ray observatory built to observe X-rays from high-energy regions of the universe, such as the remnants of exploded stars. NASA's premier X-ray observatory was named in honor of the late Indian American Nobel laureate (1983) Subrahmanyan Chandrasekhar, known to the world as Chandra (which means "moon" or "luminous" in Sanskrit). Harvard University and the Smithsonian Institution jointly operate the observatory's research center.
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