TELESCOPE

TELESCOPE

TELESCOPE, a lens, mirror, or other reflecting surface used to form an image of a distant object, together with a microscope, for the observer, a photographic camera, or some form of spectroscopic apparatus.

History.

The telescope was invented in Holland, but some controversy exists over the actual inventor. The invention is usually ascribed to Hans Lippershey (158-1619), a Dutch spectacles maker, in about 1608. Galileo exhibited, in 1609, the first telescope on record. The German astronomer Johannes Kepler discovered the principle of the astronomical telescope with two convex lenses. This idea was actually employed in a telescope constructed by the German Jesuit astronomer Christoph Scheiner (1579?-1650) in about 1630. Because of the difficulties caused by spherical aberration, astronomical telescopes had to be of considerable focal length--some of them up to 200 ft (up to 61 m).

The invention of the achromatic object glass in 1757 by the British optician John Dollond (1706-61), and the improvement of optical flint glass, which began in 1754, soon permitted the construction of improved refracting telescopes. Dollond’s lenses were only 3-4 in. (7.5-10 cm), 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 (1748-1824), a Swiss optician, who became associated with the German optician and physicist Joseph von Fraunhofer. Guinand’s discovery permitted the manufacture of telescopes as large as 10 in. (25 cm) in diameter.

The next successful manufacturer of telescope lenses was the American lens maker and astronomer Alvan Clark (1804-87), who gradually achieved the highest rank as a maker of telescope lenses. With his son, Alvan Graham Clark (1832-97), 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 (1586-1670), was the first to use an eye lens to view the image produced by a concave mirror, but James Gregory (1638-75), a Scottish mathematician, 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 rays. Gregory removed this difficulty in his design by interposing a second concave mirror, which reflected the rays to the eyepiece. Henry Draper (183-82), one of the few early American astronomers to construct a reflecting mirror, successfully used a total reflection prism instead of the plane mirror.

The French physician and astronomer Giovanni D. Cassegrain (1625-1712) invented a telescope in about 1672 that employed a convex mirror instead of a concave one. The British astronomer Sir William Herschel successfully tilted the mirror in his telescope and placed the eyepiece below the axis of the instrument so that it did not block the incident rays. Herschel’s mirrors were as large as 48 in. (1.22 m) in diameter, with a tube about 40 ft (about 12.2 m) 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 of mirrors eventually was 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 1931 the Russian-born German optician Bernhard Schmidt (187-1935) 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 53-in. (1.34-m) lens and a 79-in. (2-m) mirror, is at Karl Schwarzschild Observatorium in Tautenberg, Germany.

The first equatorial mounting, in which one axis, called the polar axis, is parallel to the axis of the earth, is ascribed to the British astronomer William Lassell (1799-1880). In England, telescopes were mounted by having the polar axis supported at each end, but the German system, in which the mounting is in the center and the weight of the telescope is balanced by counterpoises, is not generally used for large refractors, and a modified form is employed for reflectors. The great telescope of the Yerkes Observatory in Williams Bay, Wis., is the largest refracting telescope with an equatorial mounting.

At present the largest reflecting telescopes in the world are the 387-in. (9.82-m) Keck telescopes--Keck I Kea Observatory in Hawaii. The list of reflectors more than 100 in. (more than 2.54 m) in diameter also includes the 236-in. (6-m) instrument at Russia’s Special Astrophysical Observatory near Zelenchukskaya; the 200-in. (5.08-m) telescope at Palomar Observatory in California; the 165-in (4.2-m) telescope at Observatorio del Roque de los Muchachos in Las Palmas, Canary Islands; the 158-in. (4-m) instrument at Cerro Tololo Inter-American Observatory near La Serena, Chile; the 153-in. (3.89-m) telescope at the Anglo-Australian Observatory near Coonabarabran, Australia; the 150-in. (3.81-m) instrument at Kitt Peak National Observatory near Tucson, Ariz.; and the 150-in. (3.8-m) telescope at Mauna Kea. The historically famous U.S. telescope, the 100-in. (2.5-m) Hooker telescope at Mount Wilson Observatory in Pasadena, Calif., which was shut down from 1985 to 1992, has been revamped with new electronic technology and new accessories, such as the speckle interferometry system.

The Keck telescopes incorporate an important design innovation. The surface of the instrument’s mirror consists of 36 individual hexagonal segments, each repositionable by means of 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. Among the accessories built for the Keck telescopes is the low resolution imaging spectrograph, which has captured images from some of the most distant known celestial objects.

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, Ariz. Its original array of six 72-in. (1.83-m) concave mirrors operating in unison to achieve the light-gathering effectiveness of a single 177-in. (4.5-m) reflector was replaced by a 256-in. (6.5-m) mirror in the 1990s.

The idea of MMT was further advanced in the mid-1980s with the building of the New Technology National Telescope (NTNT) for the National Optical Astronomy Observatories, also located near Tucson. The NTNT consists of four mirrors with the capacity to act as a 590-in (15-m) telescope, with ten times the light-gathering capability of the Palomar Observatory telescope.

Radio Telescopes.

The largest and most sensitive radio telescope, the Very Large Array, is located about 80 km (about 50 mi) west of Socorro, N.Mex. There, 27 mobile, steerable antennae with diameters of 25 m (82 ft) are arranged along three 21-km (13-mi) arms shaped like a Y. 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, 305 m (1000 ft) in diameter, was completed in 1963. Much larger telescopes based on the interferometer concept of the Very Large Array are now being planned or built.