Telescope

V

Ultraviolet Telescopes

Some of the hottest and most energetic stars in the universe are visible in the ultraviolet region of the spectrum (see Ultraviolet Astronomy). However, this light is largely blocked by Earth’s atmosphere and so can only be studied from space. In the 1980s and 1990s a series of highly successful Earth-orbiting observatories explored the ultraviolet universe, most notably the International Ultraviolet Explorer (IUE), the Extreme Ultraviolet Explorer (EUVE), the ASTRO space shuttle observatory, and the Hubble Space Telescope (HST).

Ultraviolet telescopes are similar to optical reflecting telescopes, but their mirrors have special coatings that reflect ultraviolet light very well. Ultraviolet telescopes provide much information about interstellar gas, young stars, and the gaseous areas of active galaxies.

VI

X-Ray Telescopes

X-ray astronomy was developed in the early 1960s when simple X-ray detectors were mounted on high-altitude rockets. Astronomers were surprised to discover X rays streaming from many energetic astronomical objects. Space-based X-ray astronomy was pioneered in 1970 by the U.S. Explorer 42 (Uhuru) satellite, which mapped the sky for X rays. Two important new X-ray telescopes were launched in 1999: the National Aeronautics and Space Administration’s (NASA) Chandra X-ray Observatory and the European Space Agency’s X-ray Multimirror mission (XMM).

Some X-ray telescopes are built like optical reflecting telescopes. The main mirror of these telescopes must be nearly cylindrical, as opposed to the dish-shaped optical mirrors. X rays from a targeted object hit the mirror at such a shallow angle that they just graze it in order to be reflected into the detector. Some X-ray telescopes are just X-ray detectors that can be pointed at sources. To block out X rays not coming from the target, most detectors are surrounded by a cylinder of X-ray absorbing lead.

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VII

Gamma-Ray Telescopes

Gamma rays are electromagnetic radiation with wavelengths even shorter than X rays. Some of the most catastrophic events in the universe, such as neutron star collisions and black holes, blast high-energy gamma rays across space. Since gamma rays cannot penetrate Earth’s atmosphere, they must be observed from space. In the early 1990s the Compton Gamma Ray Observatory (GRO) found that mysterious gamma-ray bursts are evenly distributed across the sky. Because of their even distribution, astronomers believe that these bursts are extraordinarily powerful events that occur in normal galaxies. Many astronomers believe collisions between two neutron stars or between a neutron star and a black hole produce these bursts.

Gamma-ray telescopes consist of two or more gamma-ray detectors in a line. A detector is triggered by any gamma ray that passes through it, no matter what direction the gamma ray is traveling. To observe gamma rays from a particular source, then, at least two detectors are placed in a line pointing to the source. Only a gamma ray from the targeted source will pass through both detectors.

VIII

History

The fundamental optical principles of the telescope were first described in the 13th century by English scientist Roger Bacon. Dutch spectacle-maker Hans Lippershey is credited with inventing the first telescope in the year 1608, when he discovered that a distant object appeared to be much closer when viewed though a concave lens and a convex lens held in front of each other. He mounted the lenses in a tube to make the first crude refracting telescope.

Early telescopes were not used to explore the heavens; rather, they were employed for military purposes, to detect advancing armies or ships. News of the telescope’s invention spread rapidly through Europe. Glass grinding and polishing techniques, which had been developed since the 13th century, made it easy for the telescope design to be constructed and improved.

Science historians credit Italian scientist Galileo with the first use of the telescope for scientific observations of astronomical objects. In 1609, using a homemade telescope that could magnify objects to 20 times the size seen by the naked eye, Galileo discovered four moons orbiting the planet Jupiter. By the end of the following year, he had used his telescope to resolve the Milky Way Galaxy into countless stars, see dark spots on the Sun, and map the face of the Moon. The rapid rate of these discoveries and the extraordinary new insights they offered are unique in the history of astronomy.

Telescope technology took a giant leap forward in the 17th century. In 1663 Scottish astronomer James Gregory first conceived the reflecting telescope. A major departure from the refractor, Gregory’s design brought light rays to a focus by bouncing them off a curved mirror rather than bending them. English mathematician and scientist Isaac Newton was the first to build this new type of telescope, in 1688. Scientists quickly found that reflecting telescopes produced optically better images than refractors because the mirrors built for reflecting telescopes could be made much larger than the lenses needed for refracting telescopes.

Early reflecting telescope mirrors were made of speculum metal (a copper-tin mixture). Soon, larger and larger reflecting telescopes were being made. By the mid-1800s Irish astronomer William Parsons built a 72-in (180-cm) reflecting telescope in Ireland that enabled him to study details in nebulas, fuzzy patches of light scattered across the sky that contain clues to a far vaster and more complex universe than imagined in his time.

Parson’s telescope remained the largest telescope in the world until the construction of the 100-in (254-cm) Hooker telescope on Mount Wilson (see Hale Observatories) in 1917. It was powerful enough to resolve stars in neighboring galaxies, providing conclusive proof that the Milky Way was just one such group of stars in a universe filled with galaxies.

In 1950 the Hale Telescope went into operation and remained the best in the world for nearly half a century. It was used to refine measurements of the rate of the expansion of the universe and discovered new phenomena, such as quasars.

IX

New Developments

Telescope technology continues to advance in all fields of astronomy. Several new optical telescopes designed for interferometry are being built. Georgia State University’s Center for High Angular Resolution Astronomy (CHARA) began construction of five 3-ft (1-m) telescopes at the Mount Wilson Observatory in California in 1995. The telescopes should become operational in 2000. The United States, the United Kingdom, Canada, Chile, Argentina, and Brazil joined forces to build two 26-ft (8-m) telescopes, one in Mauna Kea, Hawaii, and one in Cerro Pachón, Chile, in 1996. The telescope in Mauna Kea, called Gemini North, became operational in 1999, while the telescope in Chile, called Gemini South, became operational in 2000.

A team of scientists from the University of Arizona, Ohio State University, and German and Italian astronomical research institutions cast the largest single-piece mirror ever in 1997 for the Large Binocular Telescope (LBT). The LBT was dedicated in October 2004 at the Mount Graham Observatory in Arizona. Only one of its two 27.6-ft (8.4-m) mirrors had been installed, however. When fully complete, the LBT will provide an image comparable to that of a single 75-ft (23-m) telescope.

The launch of Japan’s Space Observatory Program satellite in 1997 enhanced the radio astronomy program called the Very Long Baseline Interferometer (VLBI), creating a radio telescope larger than Earth. The satellite and about 40 Earth-based radio telescopes combine signals to produce radio images about three times clearer than was previously possible.

The Chandra X-ray Telescope, launched by NASA in July 1999, began sending images in January 2000. In its first few months of operation, it revealed numerous black holes in the centers of galaxies, including a particularly low-temperature black hole at the center of the Andromeda galaxy, a neighbor of the Milky Way.