Telescope
   Device used to form a magnified image of a distant object. A very simple telescope can be made from two convex lenses: one (the object glass, or objective) forms an upside-down image of the distant object; the second is used simply as a magnifying glass to examine this image. Telescopes for terrestrial use have an extra lens to turn the image the right way up.

   Binoculars are twin telescopes that have extra optical components to "fold" the light rays' paths and make the instrument compact.

   A telescope in which the objective is a convex lens is described as a refractor; instruments that use a concave mirror as the objective are called reflectors. Large astronomical telescopes are always reflectors. In advanced astronomical work, the human eye plays little role-the image is reflected to a location where it can be analyzed by photographic cameras or electronic detectors. The telescope has to be moved continually to follow the apparent movement of celestial bodies across the sky and prevent smearing of the image. Observations continue over many hours, sometimes on successive nights, to accumulate the faint light of the objects viewed.

History of telescopes
   The telescope was invented in Holland, but some controversy exists over the actual inventor. The invention is usually ascribed to Hans Lippershey, a Dutch spectacle-maker, who is thought to have constructed a simple model in 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 constructed 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 spherical aberration, astronomical telescopes had to be of considerable focal length-some of them up to 200 ft (61 m).

   The invention of the achromatic 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 3 to 4 in (7.5 to 10 cm) in diameter, however, so these telescopes all had modest dimensions. Methods of making large discs 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 10 in (25 cm) 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 lenses not only for the leading American observatories, but also for the Imperial Russian Observatory in Pulkovo, and for other European institutions.

   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 Isaac Newton constructed the first reflecting telescope in 1668. In this type of telescope the light reflected by the concave mirror must somehow be brought to a convenient viewing position to the side of the instrument or below it-otherwise 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. The American astronomer Henry Draper successfully used a total-reflection prism instead of the plane 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 48 in (122 cm) in diameter, with a tube about 40 ft (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 a 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 spoiling its shape. Silvering has been superseded by aluminium-coating, which lasts much longer.

   In 1931, the Estonian-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 53-in (134-cm) lens and a 79-in (200-cm) mirror, is at the Karl Schwarzschild Observatory in Tautenberg, Germany.

   At present the largest reflecting telescope in the world is the 387-in (982-cm) Keck telescope at Mauna Kea Observatory in Hawaii. The list of reflectors more than 100 in (254 cm) in diameter also includes the 236-in (600-cm) instrument at Russia's Astrophysical Observatory near Zelenchukskaya; the 200-in (508-cm) Hale telescope at the Palomar Observatory, California; the 165-in (419-cm) William Herschel telescope at Roque de los Muchachos Observatory on La Palma, one of the Canary Islands; the 158-in (401-cm) instrument at Cerro Tololo Inter-American Observatory near La Serena, Chile; the 153-in (389-cm) Anglo-Australian Telescope at the Siding Spring Observatory near Coonabarabran, Australia; the 150-in (381-cm) instrument at Kitt Peak National Observatory near Tucson, Arizona; and the 150-in (381-cm) UK Infrared Telescope at Mauna Kea. One historically famous American telescope, the 100-in (254-cm) Hooker telescope at Mount Wilson Observatory in Pasadena, California, was shut down from 1985 to 1992 owing to financial pressures, new developments in technology, and a desire to simplify operations.

   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. Because the segments are smaller than a single equivalent mirror, they can be thinner, and hence lighter, without the risk of becoming deformed. 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, came into operation in 1996. The twin telescopes, 85 m (280 ft) apart, will combine their images using a technique that is the optical analogue of the interferometry that has long been carried out in radio astronomy.

   The Hubble Space Telescope has the advantage of being above the Earth's distorting atmosphere. Launched in 1990 with a misshapen primary mirror, 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.

   The Hubble Telescope will meet strong competition, however, from a new generation of ground-based instruments using adaptive, or active, optics. In these telescopes the shape of the primary mirror is constantly altered under computer control in order to compensate for the scintillation (twinkling) of star images caused by the turbulence of the atmosphere. Such a system is typified by the one that will be used by the VLT (Very Large Telescope) being built by the European Southern Observatory at Cerro Paranal in Chile. The VLT will consist of four 323-in (8.2-m) "unit" telescopes whose images will be combined interferometrically, making it the equivalent of a 646-in (16.4-m) instrument. The VLT will then be the largest optical telescope in the world. A separate image analyzer continually monitors the quality of the image from moment to moment by comparing it with that of an artificial "reference star", a spot of light in the sky created by a laser beam fired from the ground. A computer calculates the types and amounts of distortions that are present, and issues commands to motors in each of the unit telescopes; these change the shapes of the primary mirrors and the positions and orientations of the secondary mirrors.

Types of telescopes

Early Telescopes
   A Dutch optician, Hans Lippershey, probably designed the first telescope, in the first decade of the 17th century. Galileo was one of a handful of observers who turned it to the heavens. Galileo's telescope (top) was a refractor with a convex lens at the front and a concave eye-lens. The 18th-century instrument (middle) is also a refracting telescope, and therefore prone to chromatic aberration, the production of spurious color fringes in images. This was eventually overcome by combining lenses of different refractive indices. The bottom instrument is a reflecting telescope, which uses two mirrors and an eyepiece lens in an arrangement that eliminates the problems of long viewing tubes and color distortion.

Newtonian Reflecting Telescope

   The first reflecting telescope was designed by Isaac Newton in 1668. A reflecting telescope uses a curved mirror to focus the light. Light from distant objects such as stars and galaxies enters the telescope tube in parallel rays. These rays are reflected from the concave mirror to a diagonal plane mirror. The diagonal mirror reflects the light through a hole in the side of the telescope tube to a lens in the eyepiece. Reflecting telescopes can be made larger than refracting telescopes because the curved mirror can be supported along its entire surface, while a large lens can be supported only at its edges. Larger mirrors are advantageous because they can collect more light. Modern reflecting telescopes include the 508-cm (200-in) reflector at the Palomar Observatory in California and the 400-cm (158-in) reflector at Cerro Tololo Inter-American Observatory near La Serena, Chile.

Refracting Astronomical Telescope

   The simplest type of refracting astronomical telescope has two lenses. Both are convex-that is, thicker in the middle than at the edges. The lens closer to the object is called the objective lens. This lens collects light from a distant source and brings it to a focus as a "real" and inverted image within the telescope tube. The lens in the eyepiece magnifies the image formed by the objective lens. In an astronomical telescope the "virtual" image formed by the eyepiece lens remains inverted. Eyepieces often incorporate several lenses, but their action is essentially the same as a single convex lens. In a terrestrial telescope a third lens is inserted into the system to invert the image a second time, so that the viewer sees a distant object the right way up.

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