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 is credited with being the first to use a telescope for astronomical purposes. 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 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.

Telescope mountings

The simpliest 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 telescope 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, this is known as 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.

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 Telescopes

• The Hubble space telescope is in orbit outside of the Earth's atmosphere to allow for observations not distorted by refraction, in this way they can be diffraction limited, and used for coverage in the ultraviolet (UV) and infrared.
• The Very Large Telescope (VLT) is currently (2002) the record holder in size, with four telescopes each 8 meters in diameter. The four telescopes, belonging to ESO and located in the Atacama desert in Chile, can operate independently or together.
• There are many plans for even larger telescopes, one of them is the Overwhelmingly Large Telescope or OWL, which is intended to have a single aperture of 100 meters in diameter.
• The 200 inch Hale telescope at Mt. Palomar is a conventional research telescope that was the largest for many years. It has a single borosilicate (Pyrex (TM)) mirror that was famously difficult to construct. The mounting is also unique, an equatorial mount that is not a fork, yet permits the telescope to image near the north celestial pole.
• The 100 inch Mt. Wilson telescope was used by Edwin Hubble to discover galaxies, and the redshift. It is now part of a synthetic aperture array with several other Mt. Wilson telescopes, and is still useful for advanced research.
• The 0.91m Yerkes Telescope (in Wisconsin) is the largest aimable refractor in use.
• The largest refractor ever constructed was French. It was on display at the 1900 Paris Exposition. Its lens was stationary, prefigured so as to sag into the correct shape. The telescope was aimed by by the aid of a Foucault sid�rostat, which is a movable plane mirror of diameter 6.56 feet, mounted in a large cast-iron frame. The horizontal tube was 197 feet long and the objective had 4.1 feet in diameter. It was a failure.

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