telescope

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An arrangement of lenses or mirrors or both that gathers visible light, permitting direct observation or photographic recording of distant objects.
2. n. Any of various devices, such as a radio telescope, used to detect and observe distant objects by their emission, transmission, reflection, or other interaction with invisible radiation.
3. v. To cause to slide inward or outward in overlapping sections, as the cylindrical sections of a small hand telescope do.
4. v. To make more compact or concise; condense.
5. v. To slide inward or outward in or as if in overlapping cylindrical sections: a camp bucket that telescopes into a disk.

Century Dictionary and Cyclopedia

1. n. An optical instrument by means of which distant objects are made to appear nearer and larger. It originated in the first decade of the seventeenth century, apparently earliest in Holland; but Galileo in 1609 independently invented the form which bears his name, published it to the world, and was the first to apply the instrument to astronomical observation. The telescope consists essentially of two members: one, the objective, a large converging lens, or a concave mirror (technically speculum), which forms an optical image of the object; the other, the eyepiece, a small lens or combination of lenses, which magnifies this image. The optical parts are usually set in a tube, and this is so arranged that the distance between the objective and the eyepiece can be adjusted to give the most distinct vision. Telescopes are classed as refracting or reflecting, according as the objective is a lens or a speculum. The simple refracting telescope has for an objective a large convex lens, A (fig. 1), of long focus, while the eyepiece, B, is also a convex lens, but of short focus, the two being placed at a distance slightly less than the sum of their focal lengths. The “real” inverted image of the object formed at m by the object-glass is viewed by the magnifying lens B, the magnifying power being equal to the ratio between the focal lengths of the lenses A and B. With this form of instrument the object is seen inverted. In the Galilean telescope the eye-lens is concave instead of convex, and intercepts the rays from the objective before they reach the focus, so that the object is seen erect. But the field of view is very restricted, and this form of instrument now survives only in the operaglass. The simple refracting telescope in any of its forms is a very imperfect instrument, owing to the fact that rays of different color are not alike refrangible, the focus being nearer the lens for the blue rays than for the red. By making the telescope very long in proportion to its diameter, the injurious effect of this chromatic aberration can be greatly reduced, and about 1660 Huygens and Cassini used instruments more than 100 feet long in their observations upon Saturn. About the middle of the eighteenth century it was discovered in England that, by combining lenses of different kinds of glass, objectives could be made nearly free from chromatic aberration, and all the refracting telescopes now constructed have achromatic object-glasses of some form. The usual construction is a double-convex lens of crown-glass combined with a (nearly) planoconcave lens of flint-glass, the focal lengths of the two lenses being proportional to their dispersive powers, and the curves so chosen that the spherical aberration is corrected at the same time. But other forms are possible and even preferable. Fig. 2 shows some of those most used. For many years after the invention of the achromatic telescope it was impossible to obtain suitable glass for lenses of more than 5 inches in diameter. The discoveries of Guinand about 1800 partially relieved the difficulty, and from about 1870 to 1890 a considerable number of instruments have been made with apertures exceeding 2 feet—the largest so far being the great Lick telescope (fig. 3), of 36 inches diameter and 57 feet in length, the object-glass by Clark of Cambridge, Massachusetts. The next in size is the Pulkowa telescope, 30 inches in diameter, the object-glass also by Clark. The achromatic objective constructed of flint- and crown-glass is, however, by no means perfect, and cannot be made so while these kinds of glass are used. When the correction for the rays of mean wave-length in the spectrum is the best possible, the extreme rays—the red and violet—refuse to coincide with the others, so that the image of a bright object is surrounded by a purple halo, which renders it somewhat indistinct. This “secondary spectrum,” as it is called, is not very obtrusive in small instruments, but is a serious defect in large ones, and unfits the ordinary achromatic refractor for photography. For this purpose it is necessary to use an object-glass specially corrected for the violet rays, and therefore practically worthless for visual observations. But while it is impossible to secure a perfect color-correction with any lens composed of ordinary crown- and flint-glass, there is no reason why kinds of glass may not be invented which will render it possible; and since 1880 experiments, under the auspices of the German government, by Professor Abbé at Jena, appear to have resulted in at least partial success. Lenses as large as 12 inches in diameter have been made of the new glass. If large disks of this glass can be obtained sufficiently homogeneous, and not corrosible under exposure to the air, the art of telescope-making will immediately make enormous progress. The reflecting telescope was invented between 1660 and 1670. independently by Gregory and Newton, by the latter as the result of his discovery of the decomposition of light by refraction, which led him to conclude (erroneously) that the faults of the refracting telescope were necessarily incurable. There are four different forms of the instrument, differing only in the method by which the rays reflected by the concave speculum which forms the objective are brought to the eyepiece. In the Gregorian telescope (fig. 4) the rays reflected from the speculum are a second time reflected by a small concave mirror in the center of the tube, and just beyond the focus. The large mirror is perforated, and the eyepiece, placed behind the perforation, receives the rays thus twice reflected. In the Cassegrainian the construction is precisely similar, except that the small mirror is convex, and is placed within the focus; this shortens the instrument a little, but restricts the field of view. In both these forms the observer looks toward the object just as with a refractor. In the Newtonian form, which is the most used, the small mirror is plane, and set at an angle of 45°, so that the rays are reflected out at the side of the tube. Finally, in the front-view or Herschelian form the small mirror is dispensed with, the speculum being slightly tilted so as to throw the image to one side of the mouth of the tube. This saves the loss of light due to the second reflection, but involves some injury to the definition. Although the reflecting telescope is free from chromatic aberration, it seldom gives as perfect definition as an achromatic instrument, and is much more subject to atmospheric disturbance; the image also is less brilliant than that given by a refractor of the same aperture; but the speculum is much easier and less costly to construct than an achromatic object-glass of the same size, so that the largest telescopes ever made have been reflectors. At the head of the list stands the six-foot “leviathan” of Lord Rosse, erected in 1845, and still in use: it is of the Newtonian form. The five-foot silver-on-glass Cassegrainian reflector of Mr. Common, erected in 1889, stands next, and there are in existence a number of instruments with apertures of 3 and 4 feet. Herschel's great telescope, erected in 1789, but long since dismantled, was 48 inches in diameter and 40 feet long. The magnifying power of a telescope depends upon the ratio between the focal length of the object-glass and that of the eyepiece. (See eyepiece.) It can therefore be altered at pleasure by merely exchanging one eyepiece for another. As a rule, the highest power practically available, with the best object-glasses and under the best circumstances, is from 75 to 100 to every inch of aperture. The illuminating power is proportional, other things equal, to the area of the object-glass or the speculum; so that a telescope of 12 inches aperture ought to give four times as much light as one with a 6-inch lens. Practically, however, the larger lenses, on account of the increase in the thickness of the glass, do not reach their theoretical performance. Reflecting telescopes vary greatly in their light-gathering power. A Newtonian reflector with a silver-on-glass speculum freshly polished is not very greatly inferior in light to an achromatic of the same aperture; but as a rule a reflector in its ordinary working condition has only about half the light of the corresponding refractor. Small telescopes for terrestrial purposes are usually unmounted, but the tube is ordinarily made in several sections which slide into one another, reducing the length of the instrument, and making it more portable, as in the common spyglass. Larger telescopes are mounted upon stands of some kind, and the practical efficiency of the instrument depends greatly on the firmness and convenient arrangement of the stand. At present telescopes for astronomical use are almost always mounted equatorially—that is, the telescope-tube is attached to an axis, which itself is carried by another axis with its bearings so arranged that it points toward the pole. This principal axis is called the polar axis, and a clockwork is usually arranged to make it turn at the rate of one revolution in a sidereal day. When the telescope is once pointed at a celestial object, the clockwork will keep it apparently stationary in the field of view for any length of time. By the help also of graduated circles attached to the two axes it is easy to “set” the telescope so as to find any object whose right ascension and declination are known. Fig. 5 represents diagrammatically the equatorial of the usual German form.
2. n. [capitalized] Same as Telescopium.
3. n. A telescope with its tube completely filled with water. Such an instrument was used by Airy at Greenwich, about 1870, as part of a zenith-sector, in order to settle by observation certain questions relating to the aberration of light.
4. To drive into one another like the movable joints or slides of a spy-glass: as, in the collision the forward cars were telescoped; to shut up or protrude like a jointed telescope.
5. To move in the same manner as the slides of a pocket-telescope; especially, to run or be driven together so that the one partially enters the other: as, two of the carriages telescoped.

Wiktionary

1. n. A monocular optical instrument possessing magnification for observing distant objects, especially in astronomy.
2. n. Any instrument used in astronomy for observing distant objects (such as a radio telescope).
3. v. To extend or contract in the manner of a telescope.

GNU Webster's 1913

1. n. An optical instrument used in viewing distant objects, as the heavenly bodies.
2. v. Recent To slide or pass one within another, after the manner of the sections of a small telescope or spyglass; to come into collision, as railway cars, in such a manner that one runs into another; to become compressed in the manner of a telescope, due to a collision or other force.
3. v. Recent To cause to come into collision, so as to telescope.
4. v. to shorten or abridge significantly.
5. adj. Capable of being extended or compacted, like a telescope, by the sliding of joints or parts one within the other; telescopic.

WordNet 3.0

1. v. crush together or collapse
2. v. make smaller or shorter
3. n. a magnifier of images of distant objects