An optical astronomical telescope is an instrument that is used to collect light from a celestial object, to bring the light to focus and produce an image, and to magnify that image. The two main types of telescopes are refractors, which use lenses, and reflectors, which use mirrors. The main lens or mirror that focuses the light is called the objective. Both telescopes have an eyepiece, or combination of small lenses, to magnify the image formed by the objective. In addition, the reflector telescope uses a small secondary mirror to reflect the light from the main, or primary, mirror to a convenient position for placement of the eyepiece or of auxiliary equipment, such as a photometer or spectrograph. In general, refractor telescopes are used for lunar and planetary studies, as well as for astrometric work involving precision measurements of double stars and stellar proper motions. Reflector telescopes are preferred for extragalactic studies, photography of faint objects, photometry, and spectrographic work. A third type of telescope--a catadioptric telescope--uses a combination of lenses and mirrors to obtain the advantages of both the refractor and the reflector. The most famous of this type is the \TSchmidt telescope\t, developed (1934) by the German optician Bernhard Schmidt. It uses a concave spherical mirror in combination with a thin glass corrector plate; the plate is precisely ground to obtain a very wide, uniform field of view. The Schmidt telescope, however, cannot be used for optical viewing; it is used instead as a large camera by exposing a photographic plate at the focus of the mirror. Smaller catadioptric instruments use secondary mirrors at the focus of the primary mirror to permit viewing. The optical instruments described in this article are used to observe and photograph objects in the visible region of the electromagnetic spectrum, ranging into the near-infrared and ultraviolet regions as well. For work in other regions of hte spectrum, different forms of devices are used to perform equivalent functions, such as the parabolic antennas of \Tradio astronomy\t (see \Tgamma-ray astronomy\t; \Tinfrared astronomy\t; \Tultraviolet astronomy\t; X-RAY \Tastronomy\t). HISTORY Tradition attributes the invention of the telescope to the accidental alignment of two lenses of opposite curvature and diverse focal length by Hans Lippershey in Holland in 1608. The principle, however, may have been known to Roger Bacon in the 13th century and to the early spectacle makers of Italy. Refractors \TGalileo Galilei\t constructed (1609) the first lens, or refracting, telescope for astronomical purposes. Using several versions, he discovered the four brightest Jovian satellites, lunar mountains, sunspots, the starry nature of the Milky Way, and the apparent elongation of Saturn, now known to be its rings. Galileo's simple lenses suffered from a variety of aberrations, or defects of image formation: chromatic aberration, or the variation of focal length with color; spherical aberration, the variation of focal length with distance of parallel rays from the lens axis; coma, the increasing blur of a point image with angle of the rays to the axis; and distortion, the imaging of straight lines in the object as curves (see \Taberration, chromatic\t; \Taberration, spherical\t; \Tcoma\t, optics). These aberrations were minimized in a variety of ways. Christiaan \THuygens\t constructed extremely long aerial telescopes in which the objective lens was mounted on a pole and connected to the eyepiece by only a taut wire. Despite extraordinary difficulties, such instruments achieved useful results, especially in lunar mapping. The 18th-century London optician John \TDollond\t combined (1757) a convex crown glass and a concave dense flint to make a compound lens that compensated for the chromatic aberrations at two wavelengths. Two-component lenses of increasing sophistication have since been developed, and today computer programs exist for the design of lens systems with multiple components. Two-component objectives are the rule for refractors, which have been used for charting sky, determining stellar parallaxes and proper motions, making photometric plates for analysis by diaphragm photometers, and undertaking double-star astrometry. The largest visual refractors are at \TLick Observatory\t (36 in/ 91 cm) and \TYerkes Observatory\t (40 in/102 cm). The size of refractors is limited by the difficulty of making large defect-free glass blanks and by the distortion caused by the sagging of the lens under its own weight. Refractors with lenses cut diametrically in two (Repsold heliometers) and sliding with respect to each other were used by Friedrich Bessel and Sir David Gill in the 19th century to measure angular separations for determination of solar and even stellar parallaxes. Triplet lenses have been used for wide-field sky mapping and observing asteroids. Reflectors In England in the 17th century, attention turned to mirror, or reflecting, telescopes, which usually consisted of two mirrors because of the mistaken belief that lens aberrations were irreparable. James \TGregory\t proposed (1663) a configuration in which a primary concave parabolic mirror converges the light to one focus of a concave ellipsoidal mirror. Reflection of light rays at its surface causes convergence to the ellipsoid's second focus, located behind the main mirror and reached through a central hole in it. The tube length is thus less than the sum of the focal lengths of the two mirrors. Because of construction difficulties, only a few Gregorians have been made; the largest, 50 in (130 cm) in diameter, is at Canberra, Australia. Isaac \TNewton\t proposed (1668) a telescope in which a primary parabolic mirror reflects light rays to an inclined flat (in small instruments, a totally reflecting prism) placed centrally in the tube; this flat in turn reflects the rays to an observer located at the side of the upper end of the tube. An original instrument built by Newton still exists, at the Royal Society of London. Many large- and medium-sized reflectors are Newtonian in design, although for diameters less than 1 m (3.3 ft) the weights of instrumentation capable of being carried far from the support axes are small, and for larger-diameter instruments elaborate carriages are needed to give an observer access to the focus. For large Newtonians the focal ratio--the ratio of aperture size to focal length--is typically near f/4. In some cases--the Hale 200-in (5.1-m) telescope at Palomar Observatory and the 120-in (3.0-m) telescope at Lick Observatory, for instance--only a single mirror is used, and access to the prime focus is from a central cage containing the observer. Some large telescopes, such as the 156-in (4-m) \TKitt Peak National Observatory\t and the 153-in (3.9-m) \TAnglo-Australian Telescope\t, carry observers at the Cassegrain focus. Except for work done at the prime focus, both old and recently built large telescopes use television viewing systems and electronic devices instead of human observers at the focus. Early reflectors had mirrors made of speculum metal--a brittle alloy consisting of copper and tin--which required repolishing and refiguring when it became tarnished. Sir William \THerschel\t used such mirrors for making telescopes in quantity, including one as large as 48 in (1.2 m) in diameter. The Herschelian configuration has the main mirror slightly tilted to feed an eyepiece mounted at one side of the upper end of the tube. William Parsons, 3rd Earl of Rosse (1800-67), built (1845) at Birr Castle in Ireland a 72-in (180-cm) reflector with a speculum-metal mirror and used it to observe galaxies. Glass was first used for the manufacture of telescope mirrors in 1856, by Carl August von Steinheil and Leon Foucault; such optical surfaces were silvered by chemical processes developed by Justus von Liebig and John A. Brashear. A silver coating on glass is now superseded by vacuum-deposited aluminum and sometimes other metals by means of a technique developed (1934) by J. D. Strong and R. C. Williams. Between the world wars, glass with a low coefficient of thermal expansion (Pyrex) came into use, but since World War II such glass has largely been replaced by quartz (fused or vapor-condensed) and in some cases by special ceramics (CERVIT or its Soviet equivalent, SITAL) noted for their low thermal expansion. During the 20th century several large reflecting telescopes were built on mountain sites in clear-air regions. Since World War II telescope sizes escalated, and instruments larger than 150 in (3.8 m) in diameter are now common. OPTICS AND MECHANICS Visual telescopes perform the functions of light gathering, magnifying, and increasing resolving power. Light Gathering The light-gathering power of a telescope is its ability to see faint objects as compared to the ability of the dark-adapted human eye. It is directly proportional to the square of the diameter of the telescope objective. For example, a star viewed by the giant Hale telescope at Palomar Observatory appears approximately one million times as bright as it does to the naked eye. Magnification Magnification is the increase in apparent angular size of an object when viewed through a telescope, as compared to the object's apparent size when viewed by the naked eye. Telescope magnification is computable as the ratio of the objective's focal length to the focal length of the telescope's eyepiece. Usually a telescope has several eyepieces for varying the magnification as desired. Resolving Power A telescope's resolving power is defined as the minimum angular distance it can determine between two stars of moderate brightness under ideal viewing conditions. This minimum distance, measured in \Lradian\ls, is equal to the ratio of the wavelength of the light to the diameter of the telescope's objective. Magnification must be sufficiently substantial to compress the emerging rays to the size of the pupil of the eye, but not so large that the apparent scale greatly exceeds the angular resolution. Lenses Simple telescopes of the Galilean type produce erect images, whereas astronomical-type telescopes produce inverted images. Terrestrial eyepieces containing several lenses also produce erect images. Two-lens eyepieces, such as those made by Carl Kellner, Jesse Ramsden, Christiaan Huygens, and Carl Ziess, improve the correction of aberrations and increase the field of view; the Ramsden eyepiece also allows for the insertion of a graticule or cross hairs in the plane of the primary telescope image. Barlow lenses, developed by Peter Barlow, produce increased magnification. In photographic telescopes the plate is inserted in the plane or curved focal surface of the optical system. Mountings Astronomical telescopes are mounted on two perpendicular axes in order to give access to any point in the sky. Telescopes are driven by electric motors to compensate for Earth rotation, so that the telescope continues to point toward a particular celestial object. Most telescopes use the equatorial system of mounting, in which one axis is parallel to the Earth's polar axis, and the other axis is used for adjustments in declination. The development of computer-controlled drive along both axes, however, has made altazimuth mountings--mountings on vertical and horizontal axes--desirable, because they are simpler and more economical. several large telescopes have such mountings, including the Multiple Mirror Telescope (MMT) on Mount Hopkins, Ariz., which operates in the visible and near-ultraviolet spectral regions and performs as an \Tinterferometer\t in the infrared. DESIGN INNOVATIONS The MMT represents one of several design innovations that have been adopted by telescope makers in the late 20th century to avoid the extreme technical problems of producing very large single mirrors and keeping them from sagging or being otherwise distorted by environmental effects. The MMT, which began operating in 1979, consists of six telescopes, each 72 in (1.8 m) in diameter, that are computer-guided to perform as a single light-gathering mirror 176 in (4.5 m) in diameter. The resolving power is limited to that of the individual mirrors, but when the MMT functions as an interferometer the resolving power is significantly increased. Other projects adopting the MMT approach include the proposed Columbus project, a joint effort of the University of Arizona, the University of Chicago, Ohio State University, and Italy's Arcetri Astrophysical Observatory. Employing two 316-in (8-m) mirrors, its binocular telescope (to be constructed on Mount Graham, Ariz., by the mid-1990s) will have a light-gathering capacity comparable to a telescope with a single 444-in (11.3 m) mirror. Similarly, the planned Very Large Telescope (VLT) of the European Southern Observatory is designed to consist of four 316-in telescopes that are aligned so as to function as a telescope twice as wide. Mirrors of a size larger than 200 in (5.1 m) still present major design problems, and one innovative approach to their construction is being used by an American astronomer, Roger Angel. His team, working at the University of Arizona, has developed a technique for spin-casting mirrors rather than starting with giant slabs of cooled glass. Molten borosilicate glass is placed in a mold that creates a lightweight honey-comb backing to the paraboloidal surface produced by spinning the mold. The cooled mirror surface thus produced requires only a fraction of the polishing that was needed for mirrors produced in the traditional way. The liquid-mirror concept is not new, in the sense that a mirror made of spinning liquid mercury was built and used to view the Moon as early as 1909; and, although liquid-mercury mirrors must remain fixed, they are attracting renewed interest as a low-cost approach to telescope design. Another innovation is the "active optics" proposed for the VLT; hundreds of servomotor-driven mounting points will support the VLT's 4 thin mirrors and counterbalance any warping caused by temperature change or gravity. Another innovation is seen in the segmented-mirror design of the Keck Telescope built on Mauna Kea by the California Institute of Technology and the University of California. The mirror, 400 in (10 m) wide, consists of 36 hexagonal mirrors each about 72 in (1.8 m) wide. The mirrors were placed under stress and ground and polished to a spherical shape. When the stress was relieved, the mirrors assumed their desired curvatures for the final, computer-controlled array. Computerization is, in fact, assuming a leading role in many modern telescope functions. For example, the Advanced Technology Telescope at Australia's Mount Stromlo and Siding Spring Observatories has a fully computerized control system, and a photoelectric telescope at Fairborn Observatory in Arizona is programmed so that it can function automatically for several years. The New Technology Telescope at La Silla, Chile, is computerized to have an image analyzer examine the light from a faint star at least once each hour, providing a check on the curvature of the primary mirror and the positioning of the secondary. David S. Evans Bibliography: Asimov, Isaac, Eyes on the Universe (1975); Barr, L. D., et al., eds., Advanced Technology Optical Telescopes (1986); Cornell, James, and Carr, John, eds., Infinite Vistas (1985); Fischer, Daniel, "A Telescope for Tomorrow," Sky & Telescope, September 1989; Rutten, Harrie, and van Venrooij, Martin, Telescope Optics (1988); Sinnott, R. W., "The Keck Telescope's Giant Eye," Sky and Telescope, July 1990. See also: \Tobservatory, astronomical\t.