telescope

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tel·e·scope

(tel'ĕ-skōp),
1. An optic instrument used to visualize or photograph distant or out-of-view objects through magnification of the image or angulation of the reflected light. In medicine, usually combined with endoscopy.
2. Enclosure such that smaller segments collapse into larger.
[tele- + G. skopeō, to view]

telescope 

An optical instrument for magnifying the apparent size of distant objects. It consists, in principle, of two lenses: (1) the objective, being a positive lens which forms a real inverted image of the distant object; (2) the eyepiece through which the observer views a magnified image of that formed by the objective. The eyepiece may be either positive (astronomical or Kepler telescope) or negative (galilean telescope). The magnification M of a telescope is given by the following formula
M = fo/fe
= Do/Dewhere fo is the second focal length of the objective, fe the first focal length of the eyepiece, and Do and De are the diameters of the entrance and exit pupils of the telescope (approximately equal to the diameters of the objective lens and the eyepiece).There are also some telescopes that do not use a lens (or lens system) as objective, as these are difficult to produce if large apertures and minimum aberrations are required. These telescopes use a concave mirror (usually parabolic) as the objective. They are called reflecting telescopes. Light from a distant object is collected by the large concave mirror and reflected onto a small mirror (positive in the Cassegrain telescope and negative in the gregorian telescope). This mirror is located on the optical axis and light is then transmitted through a central hole in the concave mirror onto the eyepiece. In the newtonian telescope the light collected by the large concave mirror is reflected onto a small plane mirror at a 45º angle to the optical axis, and transmitted to the eyepiece, which is at right angles to the optical axis (Fig. T1). See binoculars; eyepiece; telescopic magnification; objective.
astronomical telescope See telescope.
bioptic telescope A system of lenses forming a galilean or Kepler telescope which is mounted high on a plastic spectacle or carrier lens with the distance correction, so as to allow the patient to look through either the telescope, or below, by moving his or her head. It is used to magnify distant objects for patients with low vision. Syn. bioptic position telescope.
Cassegrain telescope See telescope.
Dutch telescope See galilean telescope.
galilean telescope A simple optical system that allows observation of far objects with a low magnification and without image inversion. It consists of a convex lens, which acts as the objective, and a concave lens as the eyepiece. Magnification of such a telescope rarely exceeds ✕ 5. This optical system is used in opera glasses and as a low vision aid (Fig. T1). Syn. Dutch telescope. See binoculars; minification.
gregorian telescope; Kepler telescope; newtonian telescope See telescope.
reflecting telescope A telescope that uses a concave mirror as the objective.
refracting telescope A telescope that uses a positive lens system as the objective.
reverse telescope See visual expander field.
terrestrial telescope A telescope that provides an erect image of a distant object. The image is usually erected by means of a lens system placed between the objective and the eyepiece. It does, however, make the terrestrial telescope relatively longer than an astronomical telescope. See binoculars; erector.
Fig. T1 Telescopes: A, galilean; B, Keplerenlarge picture
Fig. T1 Telescopes: A, galilean; B, Kepler
References in classic literature ?
But if he elects to attempt it, let him be warily careful of two things: chose a calm, clear day; and do not pay the telescope man in advance.
Before they had had time to traverse the "Corridor" and reappear, twilight was come, and the power of the telescope was at an end.
Is it against the law to use a telescope in Portsmouth?
This innocent magic, the fruit at the same time of child-like musings and of manly genius -- this patient untiring labour, of which Boxtel knew himself to be incapable -- made him, gnawed as he was with envy, centre all his life, all his thoughts, and all his hopes in his telescope.
Sometimes, whilst covering Van Baerle with his telescope, he deluded himself into a belief that he was levelling a never-failing musket at him; and then he would seek with his finger for the trigger to fire the shot which was to have killed his neighbour.
Maston, was wasting his time, while leaning over the gigantic telescope he watched the course of the moon through the starry space.
Fentolin, who was holding the telescope to his eye, touched Hamel on the shoulder.
Hamel held the telescope to his eye and steadied it upon the little tripod stand.
Emmy tried to look through the telescope over George's shoulder, but she could make nothing of it.
He clapped to the telescope and flung his arms round his mother.
When in town, Saxon carried her oilcloth-wrapped telescope in her hand; but it was so arranged with loops, that, once on the road, she could thrust her arms through the loops and carry it on her back.
Pickwick's mouth, as he applied his telescope to his eye.