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telescope, traditionally, a system of lenses, mirrors, or both, used to gather light from a distant object and form an image of it. Traditional optical telescopes, which are the subject of this article, also are used to magnify objects on earth and in astronomy; other types of astronomical telescopes gather radio waves (see radio astronomy), X rays (see X-ray astronomy), or infrared or ultraviolet radiation.
Types of Optical Telescopes
Images Produced by Optical Telescopes
Resolving and Magnifying Power
The resolution of the telescope is a measure of how sharply defined the details of the image can be. The laws of diffraction make a certain amount of blurring unavoidable, because of the wave nature of light. If two stars are very close, a given telescope may not be able to separate them into two distinct points. The smallest angular separation that can be unambiguously distinguished is called the resolving power of the telescope and is proportional to the ratio of the wavelength of light being observed to the diameter of the telescope. Thus, the larger the diameter, the smaller the minimum angular separation and the higher the resolving power.
The magnification, or power, of the telescope is relevant only when an eyepiece, or ocular, is used to magnify the image for visual inspection. The angular size of the virtual image seen by the observer will be larger than the actual angular size of the object. The ratio of these two sizes is the magnifying power and is equal to the ratio of the focal lengths of the objective and ocular. Any desired magnification can be obtained with a given telescope by the use of an appropriate ocular, but beyond a point determined by the resolving power, higher magnification will reveal no further details.
In addition to diffraction, other defects limit the performance of real optical systems. The most serious of these for lenses is chromatic aberration. Other defects include coma, astigmatism, distortion, and curvature of field. In general, it is easier to eliminate these faults in the reflector than in the refractor.
Arrangement of Mirrors in a Reflector
The Schmidt Telescope and Other Innovations
Mounting the Telescope
Evolution of Telescopes
Because a lens can be supported only at its edge, the weight of the lens itself produces unavoidable distortion in the shape. Because a mirror can be supported from behind, it can be much more massive without incurring distortion, and mirrors many feet in diameter have been constructed. The first reflecting telescope, built by Isaac Newton in 1672, had a mirror made of a metal alloy. When techniques for depositing metal films on glass surfaces were developed, reflecting telescopes became comparable in precision to refractors. An important advantage of the reflecting telescope is the absence of chromatic aberration. Because only one surface must be ground to an exact shape, the reflector is also easier to manufacture. Although increasingly larger mirrors provide increasingly greater light-gathering ability, the cost increases even more rapidly. Several innovations were introduced toward the end of the 20th cent. to achieve the goal of increasing light gathering more economically.
One of these innovations is the use of segmented or multimirror reflectors and of computer programs to control their optics. The first, and among the largest of these telescopes, are the twin W. M. Keck telescopes at the Mauna Kea Observatories, Hawaii. Each has a segmented primary mirror, composed of 36 separate hexagonal pieces. Each segment is about 72 in. (1.8 m) across but only 3 in. (76 mm) thick, creating a 394-in. (10-m) diameter primary mirror. The position of each 880-lb (400-kg) segment is computer controlled to a tolerance of less than one millionth of an inch. The 430-in. (11-m) primary mirror array of the Hobby-Eberly telescope at the McDonald Observatory, Tex., is made of 91 250-lb (113-kg) hexagonal segments. The revolutionary design, which resulted in an effective aperture, after upgrades, of 394 in. (10 m), enabled it to be constructed at 20% the cost of similar telescopes. The largest segmented mirror telescope is the Great Canary Telescope at the Roque de los Muchachos Observatory on the island of Palma in the Canary Islands. Active optics and its 36 hexagonal mirrors create a 410-in (10.4-m) primary mirror.
Another technique for compensating for smaller mirrors is called optical interferometry. The signals from two or more smaller telescopes at separate locations are combined so that the resulting image is equal to that which would be received from a very large telescope, or virtual telescope. The largest of these installations is at the European Southern Observatory in Chile. Completed in 2003, it comprises four 315-in. (8-m) fixed telescopes and several movable 72-in. (1.8-m) auxiliary telescopes, the images from which can be combined to provide the total resolving capability of a 630-in. (16-m) conventional reflecting telescope. Similar approaches to solving the problem of building a large reflector were the multiple-mirror telescope (MMT) at the Fred Lawrence Whipple Observatory, Ariz., the COAST (Cambridge Optical Aperture Synthesis Telescope) system at the Univ. of Cambridge observatory, England, and the CHARA (Center for High Angular Resolution Astronomy) Array at the Mount Wilson Observatory, Calif. The MMT, which became operational in 1979, consisted of six 72-in. (1.8-m) telescopes on a common mounting and having a resolving capability equal to that of a 176-in. (4.5-m) reflector of conventional design. It was replaced by a conventional 256-in. (6.5-m) single-mirror telescope in 1999. The COAST system, which became operational in 1996, combines light from a trio of small telescopes spaced about 20 ft (6 m) apart. The twin Keck telescopes, in domes a few hundred feet apart, have adaptive optics that make them equivalent in resolving power to a telescope with a mirror 280 ft (85 m) across. The CHARA Array, fully operational in 2002, consists of six 39-in. (1-m) aperture telescopes arranged in a Y-shape and contained in a 1,300-ft (400-m) diameter circle; the combined signals from the six telescopes provide the equivalent of the resolving capability of a telescope 1,080 ft (330 m) wide.
The largest single-mirror reflecting optical telescopes are the 327-in. (8.3-m) Subaru telescope, formerly called the Japanese National Large Telescope, at the Mauna Kea Observatories, the four Unit Telescopes (each 323 in./8.2 m) of the European Southern Observatory's Very Large Telescope at Cerro Paranal, Chile, and the Gemini Observatory's Gemini North telescope (319 in./8.1 m), also at Mauna Kea, and its twin, the Gemini South telescope, at Cerro Pachon, Chile. Other large conventional optical telescopes include those at the Special Astrophysical Observatory near Zelenchukskaya, in the Caucasus (236 in./6 m), the world's largest solid-mirror optical telescope; Palomar Observatory (see under Palomar Mountain), Calif. (200 in./5 m); the Cerro Tololo Inter-American Observatory, Chile, and the Kitt Peak National Observatory, Ariz. (158 in./4 m each); the European Southern Observatory, Chile (142 in./3.6 m); Lick Observatory, Calif. (120 in./3 m); and McDonald Observatory, Tex. (107 in./2.7 m). Large Schmidt telescopes are at Palomar, Siding Spring Observatory, Australia, and the European Southern Observatory. The Hubble Space Telescope is a 94.5-in. (2.4-m) reflector.
See H. G. J. Rutten and M. A. M. Van Venrooij, Telescope Optics: Evaluation and Design (1988); R. N. Wilson, Reflecting Telescope Optics: Basic Design Theory and Its Historical Development (1996); R. Moore, Eyes on the Universe: The Story of the Telescope (1997); S. F. Tonkin et al., Amateur Telescope Making (1998); J. B. Zirker, An Acre of Glass: A History and Forecast of the Telescope (2005).
An instrument used to collect, measure, or analyze electromagnetic radiation from distant objects. A telescope overcomes the limitations of the eye by increasing the ability to see faint objects and discern fine details. In addition, when used in conjunction with modern detectors, a telescope can “see” light that is otherwise invisible. The wavelength of the light of interest can have a profound effect on the design of a telescope. See Electromagnetic radiation, Light
For many applications, the Earth's atmosphere limits the effectiveness of larger telescopes. The most obvious deleterious effect is image scintillation and motion, collectively known as poor seeing. Atmospheric turbulence produces an extremely rapid motion of the image resulting in a smearing. On the very best nights at ideal observing sites, the image of a star will be spread out over a 0.25-arcsecond seeing disk; on an average night, the seeing disk may be between 0.5 and 2.0 arcseconds.
The upper atmosphere glows faintly because of the constant influx of charged particles from the Sun. The combination of the finite size of the seeing disk of stars and the presence of airglow limits the telescope's ability to see faint objects. One solution is placing a large telescope in orbit above the atmosphere. In practice, the effects of air and light pollution outweigh those of airglow at most observatories in the United States.
There are basically three types of optical systems in use in astronomical telescopes: refracting systems whose main optical elements are lenses which focus light by refraction; reflecting systems, whose main imaging elements are mirrors which focus light by reflection; and catadioptric systems, whose main elements are a combination of a lens and a mirror. The most notable example of the last type is the Schmidt camera.
Astronomers seldom use large telescopes for visual observations. Instead, they record their data for future study. Modern developments in photoelectric imaging devices are supplanting photographic techniques for many applications. The great advantages of detectors such as charge-coupled devices is their high sensitivity, and the images can be read out in a computer-compatible format for immediate analysis.
Light received from most astronomical objects is made up of radiation of all wavelengths. The spectral characteristics of this radiation may be extracted by special instruments called spectrographs.
As collectors of radiation from a specific direction, telescopes may be classified as focusing and nonfocusing. Nonfocusing telescopes are used for radiation with energies of x-rays and above (x-ray, gamma-ray, cosmic-ray, and neutrino telescopes). Focusing telescopes, intended for nonvisible wavelengths, are similar to optical ones (solar, radio, infrared, and ultraviolet telescopes), but they differ in the details of construction. See Cerenkov radiation
The 5-m (200-in.) Hale telescope at Palomar Mountain, California, was completed in 1950. The primary mirror is 5 m in diameter with a 1.02-m (40-in.) hole in the center.
The 4-m (158-in.) Mayall reflector at the Kitt Peak National Observatory was dedicated in 1973. The prime focus has a field of view six times greater than that of the Hale reflector. An identical telescope was subsequently installed at Cerro Tololo Inter-American Observatory, in Chile.
The mirrors for these traditional large telescopes were all produced using the same general methodology. A large, thick glass mirror blank was first cast; then the top surface of the mirror was laboriously ground and polished to the requisite shape. The practical and economical limit to the size of traditional mirror designs was nearly reached by the 6-m (236-in.) telescope in the Caucasus Mountains, Russia. Newer telescopes have been designed and built that use either a number of mirrors mounted such that the light collection by them is brought to a common focus, or lightweight mirrors in computer-controlled mounts.
The Keck Telescope on Mauna Kea, Hawaii, completed in 1993, is the largest of the segmented mirror telescopes to be put into operation. The telescope itself is a fairly traditional design. However, its primary mirror is made up of 36 individual hexagonal segments mosaiced together to form a single 10-m (386-in.) mirror. Electronic sensors built into the edges of the segments monitor the relative positions of the segments, and feed the results to a computer-controlled actuator system.
In 1989, the European Southern Observatory put into operation their New Technology Telescope. The 3.58-m (141-in.) mirror was produced by a technique known as spin-casting, where molten glass is poured into a rotating mold.
Worldwide efforts are under way on a new generation of large, ground-based telescopes, using both the spin-casting method and the segmented method to produce large mirrors. The Gemini project of the National Optical Astronomy Observatories included twin 8.1-m (319-in.) telescopes, Gemini North (see illustration) on Mauna Kea, Hawaii, and Gemini South on Cerro Pachon in Chile.
The Very Large Telescope (VLT), operated by the European Southern Observatory on Cerro Paranel, Chile, consists of four 8-m (315-in.) “unit” telescopes with spin-cast mirrors. The light from the four telescopes is combined to give the equivalent light-gathering power of a 16-m (630-in.) telescope. The last of the four telescopes began collecting scientific data in September 2000.
The ability of large telescopes to resolve fine detail is limited by a number of factors. Distortion due to the mirror's own weight causes problems in addition to those of atmospheric seeing. The Earth-orbiting Hubble Space Telescope (HST), with an aperture of 2.4 m (94 in.), was designed to eliminate these problems. The telescope operates in ultraviolet as well as visible light, resulting in a great improvement in resolution not only by the elimination of the aforementioned terrestrial effects but by the reduced blurring by diffraction in the ultraviolet. See Diffraction, Resolving power (optics)
Soon after the telescope was launched in 1990, it was discovered that the optical system was plagued with spherical aberration, which severely limited its spatial resolution. After space-shuttle astronauts serviced and repaired the telescope in 1993, adding what amounted to eyeglasses for the scientific instruments, the telescope exceeded its prelaunch specifications for spatial resolution. Subsequent servicing missions replaced instruments with newer technology.
What does it mean when you dream about a telescope?
Dreaming about a telescope can represent the need to take a closer look at something. Alternatively, it could represent exaggerating something, making it bigger than it actually is.