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Telescope

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Telescope.jpg
The 50 centimeters refractor at Nice Observatory.
A telescope is an instrument composed of one or more lenses or mirrors to gather and focus electromagnetic radiation. Telescopes increase the observed angular size of objects, as well as their apparent brightness. The largest telescopes are used in astronomy. A telescope was first turned on the sky by Galileo Galilei, the Italian scientist. (Telescopes used for non-astronomical purposes may be called "transits," "monoculars," "binoculars," "camera lenses," or "spyglasses[?]".)

The word telescope alone usually refers to optical telescopes, but there are telescopes for most of the spectrum of electromagnetic radiation.

Radio telescopes are focused radio antennas, usually shaped like large dishes. The dish is sometimes constructed of a conductive wire mesh whose openings are smaller than a wavelength. Radio telescopes are often operated in pairs, or larger groups to synthesize large apertures. The current record is nearly the width of the Earth. Aperture synthesis[?] is now being applied to optical telescopes.

X-ray and gamma-ray telescopes have a problem because the rays go through most metals and glasses. They use ring-shaped "glancing" mirrors made of heavy metals, that reflect the rays just a few degrees. The mirrors are usually a section of a rotated parabola.

The rest of this article considers optical telescopes.

Telescopes in which the primary light-gathering surface is a lens are called refractive telescopes, or "refractors[?]," and those in which it is a mirror are reflective telescopes, or "reflectors." Refractors are similar in basic design and function to microscopes, and share a history with them.

The basic scheme is that the primary light-gathering element, called the "primary," focuses light to a focal plane where it forms a bright virtual image. An "eyepiece[?]" then magnifies the virtual image. Many types of telescopes fold the optical path with secondary or tertiary mirrors, usually to make the telescope more compact and reduce the width of its field of view.

The angular resolution of a telescope is determined primarily by the width of the objective, termed its "aperture." Recently, it has become practical to perform aperture synthesis[?] with optical telescopes. Increasingly, high-resolution optical telescopes are actually groups of widely-spaced smaller telescopes, linked together by carefully-controlled optical paths.

The sensitivity of a telescope is determined by both the area of its objective, and the sensitivity of the sensor. Larger objectives gather more light, and more sensitive imaging equipment can produce better images from less light.

Finally, the f-ratio of a telescope denotes how wide an angle the telescope can view. Low f-ratios indicate wide fields of view. Wide-field telescopes are used to track satellites and asteroids, for cosmic-ray research, and for surveys of the sky.

Nearly all large research-grade astronomical telescopes are reflectors. Some reasons are:

Names of types:

Table of contents

Telescope mountings

The classic telescope mounting is an altazimuth mount[?]. It is similar to that of a surveying transit. A fork rotates in azimuth, and bearings on the tips of the fork allow the telescoep to vary in altitude.

The major problem with using an altazimuth for astronomy is that both axes must be continuously adjusted to compensate for the Earth's rotation. Even if this is done, by computer control, the image rotates at a rate that varies depending on the angle of the star from the celestial pole. The last effect especially makes an altazimuth mount impractical for long-exposure photography with small telescopes.

The preferred solution for many small telescopes is to tip the altazimuth so that the azimuth axis is parallel with the axis of the Earth's rotation.

Very large telescopes typically use a computer-controlled altazimuth mount, and for long exposures, they have (usually computer-controlled) variable-rate rotating erector prisms at the focus.

Research Telescopes

Most large research telescopes can operate as either a cassegrainian (longer focal length, and a narrower field with higher magnification) or newtonian (brighter field). They have a pierced primary, a newtonian focus, and a spider to mount a variety of replaceable secondaries.

A new era of telescope making was inaugurated by the MMT, a synthetic aperture composed of six segments synthesizing a mirror of 4.5 meters diameter. Its example was followed by the Keck telescopes, a synthetic-aperture 10 meter telescope.

The current generation of telescopes being constructed have a primary mirror of between 6 and 8 meters in diameter (for ground-based telescopes). In this generation of telescopes, the mirror is usually very thin, and is kept in an optimal shape by an array of actuators (see active optics). This technology has driven new designs for future telescopes with diameters of 30, 50 and even 100 meters.

Initially the detector used in telescopes was the human eye. Later, the sensitized photographic plate took its place, and the spectrograph was introduced, allowing the gathering of spectral information. After the photographic plate, successive generations of electronic detectors[?], such as CCDs, have been perfected, each with more sensitivity and resolution.

Current research telescopes have several instruments to choose from: imagers, of different spectral responses; spectrographs, useful in different regions of the spectrum; polarimeters, that detect light polarization, etc.

In recent years, some technologies to overcome the bad effect of atmosphere on ground-based telescopes were developed, with good results. See tip-tilt mirror and adaptive optics.

Famous Telescopes

See also:

External links

wikipedia.org dumped 2003-03-17 with terodump