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Telescope
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Telescope

A telescope is perhaps the most important astronomical tool; such technology gathers (and focuses) electromagnetic radiation. Telescopes increase the apparent angular size of objects, as well as their apparent brightness. Galileo Galilei is credited with being the first to use a telescope for astronomical purposes in 1609. Shortly later, Johannes Kepler described the optics of lenses (see his books "Astronomiae Pars Optica" and "Dioptrice"), including a new kind of astronomical telescope with two convex lenses (a principle often called Kepler telescope).

Telescopes used for non-astronomical purposes are often referred to as transits, spotting scopes, monoculars, binoculars, camera lenses, or spyglasses.

The word "telescope" 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 "virtual" apertures that are similar in size to the separation between the telescopes: see aperture synthesis. The current record is nearly the width of the Earth. Aperture synthesis is now also being applied to optical telescopes.

X-ray and gamma-ray telescopes have a problem because these 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.

Table of contents
1 Telescope mountings
2 Research telescopes
3 Famous optical telescopes
4 See also
5 External links

Telescope mountings

A simple telescope mounting is an altazimuth mount. It is similar to that of a surveying transit. A fork rotates in azimuth (in the horizontal plane), and bearings on the tips of the fork allow the telescope to vary in altitude (in a vertical plane).

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 mount so that the azimuth axis is parallel with the axis of the Earth's rotation; this is known as an equatorial mount.

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.

There are mountings even simpler than altazimuth, typically for very old observatories or for specialised instruments. A few are: meridian transit (altitude only); fixed with movable plane mirror for solar observing; ball-and-socket (ancient and useless for astronomy).

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.

The phenomenon of optical diffraction sets a limit to the resolution and image quality that a telescope can achieve, which is the effective area of the Airy disc, which limits how close we may place two such discs. This absolute limit is called Sparrow's resolution limit. This limit depends on the wavelength of the studied light (so that the limit for red light comes much earlier than the limit for blue light) and on the diameter of the telescope mirror. This means that a telescope with a certain mirror diameter can resolve up to a certain limit at a certain wavelength, so if you want more resolution at that very wavelength, you have to build a wider mirror.

Famous optical telescopes

See also

External links