Telescope, device used to form a magnified image of a distant object.

The telescope was invented in Holland, but some controversy exists over the actual inventor. The invention is usually ascribed to Hans Lippershey, a Dutch spectacles maker, about 1608. In 1609 the Italian astronomer Galileo exhibited the first telescope on record. The German astronomer Johannes Kepler discovered the principle of the astronomical telescope with two convex lenses. This idea was employed in a telescope constructed by the German Jesuit astronomer Christoph Scheiner about 1630. Because of the difficulties caused by irregularities in the curvature of the lenses, early astronomical telescopes had to be of considerable focal length-some of them up to 61 m (200 ft).

The invention of the achromatic, or colorless, object glass in 1757 by the British optician John Dollond and the improvement of optical flint glass, which began in 1754, soon permitted the construction of improved refracting telescopes. Dollond's lenses were only 7.5 to 10 cm (3 to 4 in) in diameter, however, so these telescopes all had modest dimensions. Methods of making large disks of flint glass were discovered in the late 18th century by Pierre Louis Guinand, a Swiss optician who became associated with the German physicist Joseph von Fraunhofer. Guinand's discovery permitted the manufacture of telescopes as large as 25 cm (10 in) in diameter.

The next successful manufacturer of telescope lenses was the American lens maker and astronomer Alvan Clark, who gradually achieved the highest rank as a maker of telescope lenses. With his son, Alvan Graham Clark, he constructed the lenses not only for the leading American observatories, but also for the Imperial Russian Observatory in Pulkovo, and for other European institutions.

A concave mirror is used to form an image in the reflecting telescope. Numerous varieties of this telescope have been devised, and many of the most important astronomical discoveries have been made with it. Early in the 17th century, an Italian Jesuit, Niccolo Zucchi, was the first to use an eye lens to view the image produced by a concave mirror, but the Scottish mathematician James Gregory first described a telescope with a reflecting mirror in 1663. The English mathematician and physicist Sir Isaac Newton constructed the first reflecting telescope in 1668, but viewing was difficult in this type of telescope because the eyepiece and the head of the observer cut off a large portion of the incident, or incoming, rays. Gregory removed this difficulty in his design by interposing a second concave mirror, which reflected the rays to the eyepiece. Henry Draper, one of the few early American astronomers to construct a reflecting mirror, successfully used a prism that reflected all light instead of a flat mirror.

The French physician and astronomer Giovanni D. Cassegrain invented a telescope about 1672 that used a convex mirror instead of a concave one. The English astronomer Sir William Herschel successfully tilted the mirror in his telescope and placed the eyepiece so that it did not block the incident rays. Herschel's mirrors were as large as 122 cm (48 in) in diameter, with a tube about 12 m (about 40 ft) in length. The mirrors for reflecting telescopes were usually made of speculum metal, a mixture of copper and tin, until the German chemist Baron Justus von Liebig discovered the method of depositing a film of silver on a glass surface. Silvering mirrors became generally adopted because it not only facilitated construction of the mirror but made possible its resilvering at any time without destruction of its configuration. Silvering has been superseded by aluminum coating, which lasts much longer.

In 1982 Canadian physicist Ermanno Borra helped revive interest in building reflecting telescopes with mirrors created by spinning pure liquid mercury in a concave dish. This technique had been proposed as early as the 17th century, but the technology required to spin the dish without shaking it was not present until the late 20th century. The movement of the dish forces the mercury up against the sides, forming a perfect, smooth paraboloid , even if the surface of the dish is not perfectly smooth or perfectly shaped. Liquid mercury mirrors can be made much larger than other mirrors without jeopardizing the quality of the reflective surface, but liquid mercury mirrors cannot be tilted as far as other mirrors, making observations of objects on the horizon impossible. The technique of spinning liquids to form paraboloids has been adopted by makers of conventional mirrors. Molten glass is spun into shape until it cools and hardens in a process called spin casting, then smoothed and polished.

In 1931, the Russian-born German optician Bernhard Schmidt invented a combination reflecting-refracting telescope that can accurately photograph large areas of the sky. The Schmidt telescope contains a thin lens at one end and a concave mirror with a correcting plate at the other end. The largest Schmidt telescope, with a 134-cm (53-in) lens and a 200-cm (79-in) mirror, is at Karl Schwarzschild Observatorium in Tautenberg, Germany.

At present the largest reflecting telescope in the world is the 982-cm (387-in) Keck telescope at Mauna Kea Observatory in Hawaii. The list of reflectors more than 254 cm (100 in) in diameter also includes the 600-cm (236-in) instrument at Russia's special Astrophysical Observatory near Zelenchukskaya; the 508-cm (200-in) Hale Telescope at Palomar, California; the 419-cm (165-in) telescope at Observatorio del Roque de los Muchachos in Las Palmas, Canary Islands; the 401-cm (158-in) instrument at Cerro Tololo Inter-American Observatory near La Serena, Chile; the 389-cm (153-in) telescope at Anglo-Australian Observatory near Coonabarabran, Australia; the 381-cm (150-in) instrument at Kitt Peak National Observatory near Tucson, Arizona; and the 381-cm (150-in) telescope at Mauna Kea.

The Keck telescope incorporates an important design innovation. The surface of the instrument's mirror consists of 36 individual hexagonal segments, each of which can be positioned by three actuator pistons. Electronic techniques keep the segments aligned with one another. Segmentation not only reduces the weight of the instrument, it also makes polishing the giant mirror a much easier task. A second telescope at Mauna Kea, Keck II, is planned to begin operation in late 1996.
Another important innovation in telescope design is the multiple-mirror telescope (MMT), the first of which was completed in 1979 on Mount Hopkins, south of Tucson, Arizona . The MMT employs an array of six 183-cm (72-in) concave mirrors (which are to be replaced by a single 650-cm/256-in mirror in early 1997) operating in unison to achieve the light-gathering effectiveness of a single 450-cm (177-in) reflector.

The Hubble Space Telescope has the advantage of being above the earth's distorting atmosphere. Launched in 1990 with multiple mechanical and electronic problems, the telescope was repaired in December 1993. Even before its repair, however, the space telescope was providing some images that were better than had been obtained from earthbound instruments.

In the late 1980s a group of astronomers at the Mullard Radio Astronomy Observatory at Cambridge University in England began applying radio astronomy interferometry techniques to optical astronomy. In interferometry, two or more beams of electromagnetic radiation (radio waves or light waves, for example) are sent along slightly different paths, then combined so that their interference patterns can be studied . Optical interferometry requires that the apparatus be shielded from air movement, changes in temperature, and all vibrations; technology that can produce such an environment only became available in the last part of the 20th century, even though the idea of optical interferometry in astronomy was proposed in the 19th century. The Cambridge astronomers used three fairly small telescopes to gather light from the double star Capella, which consists of a very bright star and a dim star that orbit each other so closely that not even the Hubble Space Telescope can produce an image showing two separate stars. By combining the beams of light from the three telescopes and analyzing the interference patterns, the astronomers produced an exceptionally clear image that shows both stars.
 
 

The Very Large Array (VLA) radio telescope is located about 80 km (50 mi) west of Socorro, New Mexico. There, 27 mobile, steerable antennae with diameters of 25 m (82 ft) are arranged along 21-km (13-mi) arms shaped like a Y. By combining the signals from all 27 of the dishes through interferometry, the VLA has a much better resolving power than any single dish has. The Very Long Baseline Array (VLBA) is an array of 10 dishes positioned across North America from Hawaii to the Virgin Islands of the United States. Completed in 1993, the VLBA uses the same principles as the VLA to combine the signals of its ten dishes to create signals sharper than any other radio telescope.

The largest steerable radio telescope, with a 100-m (328-ft) dish, is located at the Max Planck Institute for Radio Astronomy near Bonn, Germany. The largest stationary radio telescope with a spherical reflecting surface is operated by Cornell University and is located in a natural bowl-shaped hollow in the mountains near Arecibo, Puerto Rico. The detecting device at the focus of the telescope is suspended over the reflector from three steel supports. The telescope, 1000 ft (305 m) in diameter, was completed in 1963.
 

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