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Telescope

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   50 cm refracting telescope at Nice Observatory.
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   50 cm refracting telescope at Nice Observatory.

   A telescope (from the Greek tele = 'far' and skopein = 'to look or
   see'; teleskopos = 'far-seeing') is an instrument designed for the
   observation of remote objects. The term usually refers to optical
   telescopes, but there are telescopes for most of the spectrum of
   electromagnetic radiation and for other signal types.

   An optical telescope gathers and focuses visible light and other
   electromagnetic radiation. Telescopes increase the apparent angular
   size of distant objects, as well as their apparent brightness.
   Telescopes work by employing one or more curved optical elements -
   lenses or mirrors - to gather light or other electromagnetic radiation
   and bring that light or radiation to a focus, where the image can be
   observed, photographed or studied. Optical telescopes are used for
   astronomy and in many non-astronomical instruments including
   theodolites, transits, spotting scopes, monoculars, binoculars, camera
   lenses and spyglasses.

   Single-dish Radio telescopes are focusing radio antennae often having a
   parabolic shape. The dishes are sometimes constructed of a conductive
   wire mesh whose openings are smaller than a wavelength. Multi-element
   Radio telescopes are constructed from pairs or larger groups of these
   dishes to synthesize large "virtual" apertures that are similar in size
   to the separation between the telescopes: see aperture synthesis. As of
   2005, the current record array size is many times the width of the
   Earth, utilizing space-based Very Long Baseline Interferometry (VLBI)
   telescopes such as the Japanese HALCA (Highly Advanced Laboratory for
   Communications and Astronomy) VSOP (VLBI Space Observatory Program)
   satellite. Aperture synthesis is now also being applied to optical
   telescopes using optical interferometers (arrays of optical telescopes)
   and Aperture Masking Interferometry at single 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. High
   energy particle telescopes detect a flux of particles, usually
   originating at an astronomical source.

History

   A group of Newtonian Telescopes at Perkins Observatory, Delaware, Ohio
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   A group of Newtonian Telescopes at Perkins Observatory, Delaware, Ohio
   A replica of the first telescope built and used by Galileo, on display
   at the Griffith Observatory
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   A replica of the first telescope built and used by Galileo, on display
   at the Griffith Observatory

   The first telescopes may have been Assyrian crystal lenses , but the
   Visby lenses tentatively suggest that the technology was known to the
   Arabs and Persians. Leonard Digges is sometimes credited with the
   invention in England in the 1570s, but usually credit for assembling
   the first telescope is given to an unknown Dutch spectacle maker in
   about 1608. Some name that person as Hans Lippershey (c. 1570 – c.
   1619), but Jacob Metius and Zacharias Jansen also claimed to have
   invented a telescope during the same period. Even if Lippershey did not
   make the first one, he publicized it. Galileo Galilei made his own
   telescope in 1609, calling it at first a "perspicillum," and then using
   the terms "telescopium" in Latin and "telescopio" in Italian (from
   which the English word derives). Galileo is generally credited with
   being the first to use a telescope for astronomical purposes. Galileo's
   telescope consisted of a convex object lens and a concave eye lens,
   which is universally called a Galilean telescope (used as a viewfinder
   in many simple cameras). 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 the Kepler telescope). Optical interferometer
   arrays and arrays of radio telescopes were developed much more
   recently.

Types of telescope

   Telescopes are frequently characterised by their design details, and
   usually named after the principal designer or designers of that type of
   telescope. For optical telescopes, especially optical astronomical
   telescopes, there are three main types:
     * The refracting telescope which uses solely an arrangement of
       lenses.
     * The reflecting telescope which uses solely an arrangement of
       mirrors.
     * The catadioptric telescope which uses a combination of mirrors and
       lenses.

   Where the telescope is used for direct viewing by the human eye, then
   very few telescopes are pure reflectors, as it is usual to use an
   eyepiece to view the image, and most, if not all eyepiece design use an
   arrangement of lenses. Much astronomy is performed using photographic
   film or digital sensors which are usually used without an eyepiece, so
   for mechanised operation more telescopes are used as pure reflectors.

   For more details see List of telescope types

Research telescopes

   Harlan J. Smith Telescope at McDonald Observatory, TX
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   Harlan J. Smith Telescope at McDonald Observatory, TX

   Most large research telescopes can operate as either a Cassegrain
   telescope (longer focal length, and a narrower field with higher
   magnification) or a Newtonian telescope (brighter field). They have a
   pierced primary mirror, a Newtonian focus, and a spider to mount a
   variety of replaceable secondary mirrors.

   A new era of telescope making was inaugurated by the Multiple Mirror
   Telescope (MMT), with a mirror composed of six segments synthesizing a
   mirror of 4.5 meters diameter. This has now been replaced by a single
   6.5m mirror. Its example was followed by the Keck telescopes with 10 m
   segmented mirrors.

   The largest current ground-based telescopes have a primary mirror of
   between 6 and 11 meters in diameter. 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.

   Relatively cheap, mass-produced ~2 meter telescopes have recently been
   developed and have made a significant impact on astronomy research.
   These allow many astronomical targets to be monitored continuously, and
   for large areas of sky to be surveyed. Many are robotic telescopes,
   computer controlled over the internet (see e.g. the Liverpool Telescope
   and the Faulkes Telescope North and South), allowing automated
   follow-up of astronomical events.

   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 the charge-coupled device (CCDs), have been perfected, each
   with more sensitivity and resolution, and often with a wider wavelength
   coverage.

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

   In recent years, some technologies to overcome the distortions caused
   by atmosphere on ground-based telescopes were developed, with good
   results. See adaptive optics, speckle imaging and optical
   interferometry.

   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 two such discs can be
   placed. This absolute limit is called the diffraction limit (or
   sometimes the Rayleigh criterion, Dawes limit or 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. If greater resolution is needed at that
   wavelength, a wider mirror has to be built or aperture synthesis
   performed using an array of nearby telescopes.

Imperfect images

   No telescope can form a perfect image. Even if a reflecting telescope
   could have a perfect mirror, or a refracting telescope could have a
   perfect lens, the effects of aperture diffraction could still not be
   escaped. In reality, perfect mirrors and perfect lenses do not exist,
   so image aberrations in addition to aperture diffraction must be taken
   into account. Image aberrations can be broken down into two main
   classes, monochromatic, and polychromatic. In 1857, Philipp Ludwig von
   Seidel (1821-1896) decomposed the first order monochromatic aberrations
   into five constituent aberrations. They are now commonly referred to as
   the five Seidel Aberrations.

The five Seidel aberrations

   Spherical aberration
          The difference in focal length between paraxial rays and
          marginal rays, proportional to the square of the aperture.

   Coma
          A most objectionable defect by which points are imaged as
          comet-like asymmetrical patches of light with tails, which makes
          measurement very imprecise. Its magnitude is usually deduced
          from the optical sine theorem.

   Astigmatism
          The image of a point forms focal lines at the sagittal and
          tangiental foci and in between (in the absence of coma) an
          elliptical shape.

   Curvature of Field
          The Petzval curvature means that the image instead of lying in a
          plane actually lies on a curved surface which is described as
          hollow or round. This causes problems when a flat imaging device
          is used e.g. a photographic plate or CCD image sensor.

   Distortion
          Either barrel or pincushion, a radial distortion which must be
          corrected for if multiple images are to be combined (similar to
          stitching multiple photos into a panoramic photo).

   They are always listed in the above order since this expresses their
   interdependence as first order aberrations via moves of the
   exit/entrance pupils. The first Seidel aberration, Spherical
   Aberration, is independent of the position of the exit pupil (as it is
   the same for axial and extra-axial pencils). The second, coma, changes
   as a function of pupil distance and spherical aberration, hence the
   well-known result that it is impossible to correct the coma in a lens
   free of spherical aberration by simply moving the pupil. Similar
   dependencies affect the remaining aberrations in the list.

The chromatic aberrations

   Longitudinal Chromatic Aberration
          As with spherical aberration this is the same for axial and
          oblique pencils.

   Transverse Chromatic Aberration (Chromatic Aberration of Magnification)

Famous optical telescopes

   The Hubble Space Telescope orbits above Earth.
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   The Hubble Space Telescope orbits above Earth.
     * The Hubble Space Telescope is in orbit beyond Earth's atmosphere to
       allow for observations not distorted by astronomical seeing. In
       this way the images can be diffraction limited, and used for
       coverage in the ultraviolet (UV) and infrared.
     * The Keck telescopes are currently ( as of 2005) the largest, but
       will soon be superseded by the Gran Telescopio Canarias and
       Southern African Large Telescope.
     * The Very Large Telescope array (VLT) is currently ( as of 2002) the
       record holder for total collecting area in an array of telescopes,
       with four telescopes each 8 meters in diameter. The four
       telescopes, belonging to the European Southern Observatory (ESO)
       and located in the Atacama desert in Chile, are usually operated
       independently for faint astronomical observations, but up to three
       telescopes can be operated together for aperture synthesis
       observations of bright objects.
     * The Navy Prototype Optical Interferometer is the optical telescope
       (array) that can currently ( as of 2005) produce the highest
       resolution images at visible wavelengths.
     * The CHARA (Centre for High Angular Resolution Astronomy) array is
       the telescope array that can currently ( as of 2005) produce the
       highest resolution images at near-infrared wavelengths.
     * There are many plans for even larger telescopes. One of them is the
       Overwhelmingly Large Telescope (OWL), which is intended to have a
       single aperture of 100 meters in diameter.
     * The 200-inch (5.08-meter) Hale telescope on Palomar Mountain was
       the largest conventional research telescope for many years. It has
       a single borosilicate ( Pyrex™) mirror that was famously difficult
       to construct. The mounting is a special design of equatorial mount
       called a yoke mount, which permits the telescope to be pointed at
       and near the north celestial pole.
     * The 100-inch (2.54-meter) Hooker Telescope at the Mount Wilson
       Observatory was used by Edwin Hubble to discover galaxies and the
       redshift. The mirror was made of green glass by Saint-Gobain. In
       1919, the telescope was used for the first stellar diameter
       measurements using interferometry. The telescope now has an
       adaptive optics system, and is still useful for advanced research.
     * The 72-inch Leviathan at Birr Castle (in Ireland) was the largest
       telescope in the world from 1845 until it was dismanlted in 1908.
       It was not exceeded in size until the construction of the Hooker
       Telescope.
     * The 1.02-meter Yerkes Telescope (in Wisconsin) is the largest
       aimable refracting telescope in use.
     * The 0.76-meter Nice refractor (in France) that became operational
       in 1888 was at that time the world's largest refractor. This was
       the last time the most powerful operational telescope in the world
       was located in Europe. It was exceeded in size one year later by
       the 0.91-meter refractor at the Lick Observatory.
     * 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 the aid of a Foucault sidérostat, which is a movable plane
       mirror with a 2-meter diameter, mounted in a large cast-iron frame.
       The horizontal tube was 60 m long and the objective had 1.25 m in
       diameter. It was a failure.
     * The Gran Telescopio CANARIAS ( Grantecan, also GTC), is a high
       performance segmented 10.4 meter telescope that is being installed
       in one of the best sites of the Northern Hemisphere: the Roque de
       los Muchachos Observatory (La Palma, Canary Islands, Spain).
     * The 1-meter refracting Swedish Solar Telescope (SST) on La Palma
       (Spain), is currently the highest-resolution solar telescope in the
       world.

Other famous telescopes

     * Arecibo Observatory
     * Atacama Large Millimeter Array
     * Very Large Array
     * Chandra X-ray Observatory
     * XMM-Newton
     * LIGO
     * IceCube Neutrino Detector
     * Isaac Newton Telescope
     * William Herschel Telescope
     * Hexapod-Telescope

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   with only minor checks and changes (see www.wikipedia.org for details
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