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The two largest refracting telescopes are the 36-inch instrument at Lick Observatory of the University of California, located on Mount Hamilton, and the 40-inch telescope of the Yerkes Observatory of the University of Chicago, located in Williams Bay, Wis. The focal length of the Yerkes Observatory refractor is 63 feet.

An obstacle to building ever larger telescopes is the distortion of large lenses and mirrors caused by gravity. In 1978 an innovative reflector called the Multiple Mirror Telescope (MMT) began operation at the Smithsonian Astrophysical Observatory in Arizona. Instead of one large mirror, the MMT features six mirrors arranged to focus together. The six-mirror combination acts like a single mirror 21 feet in diameter.

Similarly, the Keck Telescope on Mauna Kea in Hawaii, completed in 1991, has a 33-foot series of mirrors forming a mosaic of hexagons. Astronomers operating the New Technology Telescope of the European Southern Observatory in La Silla, Chile, use a special computer system that frequently pushes and tugs on the mirror to keep it from sagging under its own weight.

Other telescopes under construction in the early 1990s that are based on innovative mirror designs include the Columbus Telescope on Mount Graham in Arizona and the Very Large Telescope in Chile.

The Nordic Optical Telescope in the Canary Islands has the thinnest and lightest mirror of any comparably sized telescope in the world.

A telescope’s resolution is its ability to delineate distant objects that appear close in the sky–increases proportionally to the diameter of the objective. A 6-inch telescope theoretically can resolve stars 0.6 second of arc apart. (A second of arc is a tiny unit of measure; for example, a penny must be 2.5 miles away before it appears as small as 1 second of arc.) This resolving power limits useful magnification to 60 power for every inch of the objective’s diameter.

Infrared, UV, and X-Ray Telescopes

Orbiting telescopes are used to observe the ultraviolet (UV), far infrared, and X-ray portions of the electromagnetic spectrum. The Infrared Astronomical Satellite, placed in orbit in 1983, carried a 22.5-inch infrared telescope. Because all matter emits infrared radiation if warm, technologists had to cool the telescope to near absolute zero with liquid helium so its internal heat radiation would not mask radiation it was collecting from deep space objects. Among its many discoveries was a disk of gas surrounding a star, from which planets may be condensing.

The Hubble Space Telescope, launched aboard the space shuttle Discovery on April 24, 1990, has special infrared-, UV-, and X-ray-sensitive instruments for the study of structures and systems too faint to be seen clearly with ground-based telescopes. Because the telescope orbits miles above the Earth and its distorting atmosphere, scientists hoped it would be able to capture and magnify light from about 20 billion light-years away. Just days after the launch, however, NASA engineers discovered major flaws in the telescope’s mirrors. Despite this setback, the telescope remained operational and sent back, among other things, evidence of a black hole and information about very young stars to engineers on Earth.

For shorter wavelengths, those in the X-ray region of the spectrum, ordinary mirrors will not work. X rays tend to penetrate conventional mirrors rather than be reflected by them.

Only if X rays are bounced off mirrors at a small, glancing angle can they be focused. X-ray satellites, such as Einstein, launched in 1978, and Exosat, launched in 1983, carried telescopes with deeply concave metal mirrors shaped so that they could focus X rays onto detectors

Radio Telescopes

The first radio telescope was built in 1937 by Grote Reber, an American electrical engineer. It looked a little like the reflector of an optical telescope, but it was much bigger: 31 feet in diameter. Its reflector was made of wire screen instead of polished glass or metal. A much larger one, 250 feet in diameter, was built at Jodrell Bank, England, in 1957, and a 328-foot radio telescope began operating in West Germany in 1971. One such telescope, 1,000 feet across, was constructed in the 1960s at Arecibo, Puerto Rico, and fills an entire valley. Although it cannot move, its focal point can be scanned on large cranes.

Radio telescopes made vast new regions of the universe observable on Earth because radio waves penetrate dust and gas that obscure light. For long-wavelength radio waves, however, even the largest telescopes have

resolutions not much better than the unaided eye, though they have enormous power to detect weak or distant radio emitters.

To overcome this drawback, astronomers developed a new type of telescope that concentrated signals picked up by physically separate telescopes. Such interferometers work by reconstructing the shape of emitted radio waves, which are “sampled” by radio telescopes at various points. The resolution of such interferometers is comparable to that of a single radio telescope whose diameter is equal to the separation between the individual telescopes that make up the array.

One such array, constructed in the 1970s, is the Very Large Array (VLA) in New Mexico. The VLA consists of 27 radio telescopes, or antennas, spread over 24 miles. Each antenna is an 82-foot-wide dish mounted on a large pedestal, which is in turn attached to a transporter that moves the 200-ton antennas on rails laid out in a Y shape. The entire array can point to any part of the sky and, by changing the locations of the antennas, view a large object in the sky or focus at higher resolution on a small one. The maximum resolution of the VLA is about 1 arc second, which is comparable to that of optical

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