TELESCOPE

device used to form magnified images of distant objects. In the first telescopes, visible light was focused by refraction through lenses; in later instruments, the same result was obtained by reflection from curved mirrors. Since the 1930s, radio telescopes have been used to form images from radio waves emitted by celestial objects. More recently other wavelength bands, notably the infrared and X-ray regions, have been exploited by appropriately designed telescopes. The optical telescope was apparently invented in the Netherlands in 1608. Among the earliest of the optical systems were the Galilean telescopes, so called because they were modeled after the simple instruments built by Galileo. These telescopes consisted of two lenses mounted in a tube whose length was the difference between the focal lengths of the two lenses. In 1611 the German astronomer Johannes Kepler proposed an improved version of the Galilean telescope, which became the basis for modern refracting instruments. The Keplerian telescope employed a convex eyepiece positioned in back of the focus, a feature that significantly enlarged the field of view. It also increased the possibilities for magnification by 1,000 times or more. Later developments in the refracting telescope have been mainly matters of detail. By the end of the 19th century this form of telescope had reached its apogee in the 1-metre (40-inch) instrument built for Yerkes Observatory at Williams Bay, Wis. The reflecting telescope, in which light is gathered and focused by a mirror and then magnified by an eyepiece, came into its own after the British astronomer William Herschel used an instrument of this type to discover the planet Uranus in 1781. The reflector became the predominant astronomical instrument in the 20th century. The world's largest reflecting telescopes are the 6-metre (236-inch) instrument at the Special Astrophysical Observatory on Mount Pastukhov, Caucasus, Russia, and the 5-metre (200-inch) Hale Telescope on Mount Palomar in California, U.S. Another type of optical system, a catadioptric telescope, was invented in 1930 by Bernhard Schmidt of the Bergedorf Observatory in Hamburg for use as a wide-angle camera. In the Schmidt telescope, a weak lens, called a correcting plate, is placed in front of a spherically curved mirror to correct its aberration. There is no magnifying eyepiece. The result is an instrument with high light-gathering power, high resolution, and a wide field of view that is ideal for sky surveys. The Earth's atmosphere is transparent not only to visible light but also to radio waves extending from 1 mm (0.04 inch) to approximately 10 m (33 feet). It was not until 1931, however, that this radio window was opened to astronomical observations. Today, astronomers systematically study radio emissions from many kinds of celestial objects, including stars, galaxies, and quasars. The most familiar type of radio telescope is a radio reflector consisting of a large parabolic antenna that is commonly known as a dish. The largest single instrument of this kind is the 305-metre (1,000-foot) fixed dish of the Arecibo Observatory in Puerto Rico (see also radio telescope). Since the early 1960s increasing efforts have been made to study the celestial sphere at other wavelength bands of the electromagnetic spectrum. Instruments similar to optical telescopes but more sensitive to radiation of wavelengths somewhat longer than visible light have been installed on high mountain peaks, such as Mauna Kea on the island of Hawaii, to make infrared observations. Infrared telescopes also have been carried high above the atmosphere by Earth-orbiting satellites. Ultraviolet, X-ray, and gamma-ray observations can be made only from spacecraft, because the atmosphere is completely opaque to electromagnetic radiation of wavelengths less than about 3,000 angstroms. Ultraviolet telescopes resemble reflectors, but their optical surfaces require special coatings that provide high reflectivity. A good example of such an instrument is the one on the Hubble Space Telescope. X-ray telescopes, on the other hand, differ radically in design from conventional optical systems. Because of their extremely high energy, X-ray photons cannot be focused by lenses and will penetrate mirrors if they are arranged as in ordinary reflectors. Consequently, X-ray telescopes, such as the one on the HEAO-2 satellite (the so-called Einstein Observatory), are equipped with highly polished, cylindrical mirrors mounted so as to reflect the incoming photons at a shallow grazing incidence angle (i.e., an extremely low angle, usually less than 4) onto a focal plane; the image formed is recorded by an electronic detector. Similar grazing-incidence techniques are employed in gamma-ray telescopes. Such instruments are carried aboard orbiting satellites to observe neutron stars, supernova remnants, galaxy clusters, and other high-energy cosmic systems. device used to form magnified images of distant objects. The telescope is undoubtedly the most important investigative tool in astronomy. It provides a means of collecting and analyzing radiation from celestial objects, even those in the far reaches of the universe. Galileo revolutionized astronomy when he applied the telescope to the study of extraterrestrial bodies in the early 17th century. Until then, magnification instruments had never been used for this purpose. Since Galileo's pioneering work, increasingly more powerful optical telescopes have been developed, as has a wide array of instruments capable of detecting and measuring invisible forms of radiation, such as radio, X-ray, and gamma-ray telescopes. Observational capability has been further enhanced by the invention of various kinds of auxiliary instruments (e.g., the camera, spectrograph, and charge-coupled device) and by the use of electronic computers, rockets, and spacecraft in conjunction with telescope systems. These developments have contributed dramatically to advances in scientific knowledge about the solar system, the Milky Way Galaxy, and the universe as a whole. Additional reading Techniques of observation with the help of telescopes are detailed in P. Clay Sherrod and Thomas L. Koed, A Complete Manual of Amateur Astronomy: Tools and Techniques for Astronomical Observations (1981), including a discussion of telescope setup; Robert T. Dixon, Dynamic Astronomy, 5th ed. (1989), a comprehensive, well-illustrated text; and Jay M. Pasachoff, Astronomy, from the Earth to the Universe, 3rd ed. (1987). A highly readable and profusely illustrated discussion of radio telescopes is given in Gerrit L. Verschuur, The Invisible Universe Revealed: The Story of Radio Astronomy (1987). More technical accounts are offered in John D. Kraus et al., Radio Astronomy, 2nd ed. (1986); and W.N. Christiansen and J.A. Hgbom, Radiotelescopes, 2nd ed. (1985). Bernard Lovell, The Story of Jodrell Bank (1968), presents a dramatic story of the construction of the 76-metre Jodrell Bank Radio Telescope. Other modern types of telescopes are described in D.S. Finley et al., Design of the Extreme Ultraviolet Explorer Long-Wavelength Grazing Incidence Telescope Optics, Applied Optics 27(8):147680 (April 15, 1988); Adelaide Hewitt (ed.), Optical and Infrared Telescopes for the 1990s, 2 vol. (1980); and Joseph F. Baugher, The Space-Age Solar System (1988). Numerous articles on various types of telescopes are found in Sky and Telescope (monthly). Historical surveys tracing the development of the telescope from the earliest versions to those constructed in the late 1980s include Arthur Berry, A Short History of Astronomy (1898, reprinted 1961); Henry C. King, The History of the Telescope (1955, reprinted 1979); and Richard Learner, Astronomy Through the Telescope (1981). B.L. Klock Kenneth I. Kellermann Other types of telescopes Infrared telescopes Telescopic systems of this type do not really differ significantly from reflecting telescopes designed to observe in the visible region of the electromagnetic spectrum. The main difference is in the physical location of the infrared telescope, since infrared photons have lower energies than those of visible light. The infrared rays are readily absorbed by the water vapour in the Earth's atmosphere, and most of this water vapour is located at the lower atmospheric regionsi.e., near sea level. Earth-bound infrared telescopes have been successfully located on high mountaintops, as, for example, Mauna Kea in Hawaii. The other obvious placement of infrared instruments is in a satellite such as the Infrared Astronomical Satellite (IRAS), which mapped the celestial sky in the infrared in 1983. The Kuiper Airborne Observatory, operated by NASA, consists of a 0.9-metre telescope that is flown in a special airplane above the water vapour to collect infrared data. Much of the infrared data is collected with an electronic camera, since ordinary film is unable to register the low-energy photons. Another example of an infrared telescope is the United Kingdom Infrared Telescope (UKIRT), which has a 3.8-metre mirror made of Cer-Vit (trademark), a glass ceramic that has a very low coefficient of expansion. This instrument is configured in a Cassegrain design and employs a thin monolithic primary mirror with a lightweight support structure. This telescope is located at Mauna Kea Observatory. The 3-metre Infrared Telescope Facility (IRTF), also located at Mauna Kea, is sponsored by NASA and operated by the University of Hawaii. Ultraviolet telescopes These telescopes are used to examine the shorter wavelengths of the electromagnetic spectrum immediately adjacent to the visible portion. Like the infrared telescopes, the ultraviolet systems also employ reflectors as their primary collectors. Ultraviolet radiation is composed of higher-energy photons than infrared radiation, which means that photographic techniques as well as electronic detectors can be used to collect astronomical data. The Earth's stratospheric ozone layer, however, blocks all wavelengths shorter than 3000 angstroms from reaching ground-based telescopes. As this ozone layer lies at an altitude of 20 to 40 kilometres, astronomers have to resort to rockets and satellites to make observations from above it. Since 1978 an orbiting observatory known as the International Ultraviolet Explorer (IUE) has studied celestial sources of ultraviolet radiation. The IUE telescope is equipped with a 45-centimetre mirror and records data electronically down to 1000 angstroms. The IUE is in a synchronous orbit (i.e., its period of revolution around the Earth is identical to the period of the planet's rotation) in view of NASA's Goddard Space Flight Center in Greenbelt, Md., and so data can be transmitted to the ground station at the end of each observing tour and examined immediately on a television monitor. Another Earth-orbiting spacecraft, the Extreme Ultraviolet (EUV) Explorer satellite, which is scheduled to be launched in the early 1990s, is designed to survey the sky in the extreme ultraviolet region between 400 and 900 angstroms. It has four telescopes with gold-plated mirrors, the design of which is critically dependent on the transmission properties of the filters used to define the EUV band passes. The combination of the mirrors and filters has been selected to maximize the telescope's sensitivity to detect faint EUV sources. Three of the telescopes have scanners that are pointed in the satellite's spin plane. The fourth telescope, the Deep Survey/Spectrometer Telescope, is directed in an anti-Sun direction. Its function is to conduct a photometric deep-sky survey in the ecliptic plane for part of the mission and then to collect spectroscopic observations in the final phase of the mission.