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Optical telescope

2007 Schools Wikipedia Selection. Related subjects: Engineering; Space
(Astronomy)

   An optical telescope is a telescope which is used to gather, and focus
   light, for directly viewing a magnified image, making a photograph,
   etc. The term is used especially for a monocular with static mounting
   for observing the sky. Handheld binoculars are common for other
   purposes.

   Light is made up of photons, and professional telescopes concentrate
   the light onto electronic detectors which collect the photons. There
   are three primary types of optical telescope: Refractors ( Dioptrics)
   which use lenses, Reflectors ( Catoptrics) which use mirrors, and
   Combined Lens-Mirror Systems ( Catadioptrics) which use lenses and
   mirrors in combination (for example the Maksutov telescope and the
   Schmidt camera).

How it works

   The basic scheme is that the primary light-gathering element, the
   objective ( objective lens (1) or concave mirror), focuses light from a
   distant object (4) to a focal plane where it forms a real image (5).
   This image is viewed through an eyepiece (2), which acts like a
   magnifying glass. The eye (3) sees a magnified virtual image (6) at a
   large distance.
   Keplerian telescope, schematic
   Enlarge
   Keplerian telescope, schematic

   Telescopes which employ two convex lenses cause the image to appear
   inverted. Terrestrial versions of such telescopes and binoculars employ
   prisms (e.g. Porro prisms) or a relay lens between objective and
   eyepiece to invert the image once more. Thus, an upright image appears
   in the eyepiece.

   Many types of telescope fold the optical path with secondary or
   tertiary mirrors. These may be integral part of the optical design (
   Cassegrain reflector and similar types), but also serve for making the
   telescope more compact and placing the eyepiece or detector at a more
   convenient position. On large telescopes these additional mirrors are
   often used to provide improved image quality over a larger field of
   view.

Angular resolution

   Ignoring blurring of the image by turbulence in the atmosphere (
   atmospheric seeing) and optical imperfections of the telescope, the
   angular resolution of an optical telescope is determined by the width
   of the objective, termed its " aperture" (the primary mirror, or lens.)
   The Rayleigh criterion for the resolution limit α[R] (in radians) is
   given by

          α[R] = 1.22λ / D,

   where λ is the wavelength and D is the aperture. For visible light (λ =
   550rmnm), this equation can be rewritten:

          α[R] = 138 / D.

   Here, α[R] denotes the resolution limit in arcseconds and D is in
   millimeters. In the ideal case, the two components double stars can be
   split even if separated by slightly less than α[R]. This is taken into
   account by the Dawes limit

          α[D] = 116 / D.

   Essentially; the larger the aperture, the better the angular resolution

   It should be noted that the resolution is NOT given by the maximum
   magnification (or "power") of a telescope. Telescopes marketed by
   giving high values of the maximum power often deliver poor images.

   For large ground-based telescopes, the resolution is limited by
   atmospheric seeing. This limit can be overcome by placing the
   telescopes above the atmosphere, e.g., space telescopes, balloon
   telescopes and telescopes on high-flying airplanes ( Kuiper Airborne
   Observatory, SOFIA) or by adaptive optics or speckle imaging for
   ground-based telescopes.

   Recently, it has become practical to perform aperture synthesis with
   arrays optical telescopes. Very high resolution images can be obtained
   with groups of widely-spaced smaller telescopes, linked together by
   carefully-controlled optical paths, but these interferometers can only
   used for imaging bright objects such as stars or measuring the bright
   cores of active galaxies. Example images of starspots on Betelgeuse can
   be seen here.

Focal length and f-ratio

   The focal length determines how wide an angle the telescope can view
   with a given eyepiece or size of a CCD detector or photographic plate.
   The f-ratio (or focal ratio, or f-number) of a telescope is the ratio
   between the focal length and the aperture (i.e., diameter) of the
   objective. Thus, for a given aperture (light-gathering power), low
   f-ratios indicate wide fields of view. Wide-field telescopes (such as
   astrographs) are used to track satellites and asteroids, for cosmic-ray
   research, and for surveys of the sky. It is more difficult to reduce
   optical aberrations in telescopes with low f-ratio than in telescopes
   with larger f-ratio.

Light-gathering power

   The light-gathering power of an optical telescope is directly related
   to the diameter (or aperture) of the objective lens or mirror. Note
   that the area of a circle is proportional to the square of the radius.
   A telescope with a lens which has a diameter three times that of
   another will have nine times the light-gathering power. Larger
   objectives gather more light, and more sensitive imaging equipment can
   produce better images from less light.

Research telescopes

   Nearly all large research-grade astronomical telescopes are reflectors.
   Some reasons are:
     * In a lens the entire volume of material has to be free of
       imperfection and inhomogeneities, whereas in a mirror, only one
       surface has to be perfectly polished.
     * Light of different colors travels through a medium other than
       vacuum at different speeds. This causes chromatic aberration.
     * There are technical difficulties involved in manufacturing and
       manipulating large-aperture lenses. One of them is that all real
       materials sag in gravity. A lens can only be held by its perimeter.
       A mirror, on the other hand, can be supported by the whole side
       opposite to its reflecting face.

   The size of optical telescopes increased steadily in the 20th century,
   doubling between the 1910s and the 1940s, and doubling again between
   the late 1940s and the 1990s. The largest current telescopes are the
   11m SALT and Hobby-Eberly telescopes and the 10.4m Gran Telescopio
   Canarias.

   In the 1980s a number of technological improvements were made which
   created a new generation of telescopes. These advances included the
   creation of multi-mirror telescopes and the invention of cheap personal
   computers which could control the mirrors. Another major advanced was
   the invention of rotating furnaces in which centrifugal force would
   shape a telescope mirror to close to its final shape.

   Names of types:
     * Binoculars are just two monoculars mounted side-by side with
       adjustments to let both be used. A major practical advantage of
       these telescopes is not magnification, so much as a brighter field
       of view at dusk and dawn. Monoculars and binoculars with built-in
       compasses are used by army artillery units and ships to navigate by
       triangulating from topographic (shore) features. Hand-held
       telescopes are limited by hand-shaking to about 7 power. The
       brightest-field, best-magnifying practical monocular is about 7x50.

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