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Telescope Dictionary March 18, 2010

Posted by aquillam in Astronomy, MichiganAstro, telescope beginners guide.
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Telescope Dictionary

Below is an alphabetical list of terms related to amateur telescopes. References are to other posts in this blog.

I will continue to update this as I think of other terms that I haven’t included. If I missed something, let me know!

  • Alt-Az A type of coordinate system or mount.  See Alt-Az Mounts.
  • Altitude height above the ground.  See Alt-Az Mounts.
  • Aperture Generally, the width of a telescope. It is usually the width of the objective lens or primary mirror, however some telescopes, especially older ones, may have an aperature stop that permantly limits the telescope aperture.
  • aperature stop

    A permanant aperture stop behind the objective lens in a finder 'scope

  • Aperture stop A mask that cuts down on the ammount of light the enters a telescope
  • Acromatic A lens that corrects for chromatic aberration.See Refracting Telescopes.
  • Apocromatic A lens that corrects for chromatic and spherical aberation. See Refracting Telescopes.
  • astrophotography Photographing celetial objects. CCDs were invented for professional observers, and are now common in digital cameras.
  • Azimuth Direction relative to north. See Alt-Az Mounts.
  • Barlow A special lens used in conjunction with an eyepiece to increase magnification. May also be used to mount a camera for photography with more magnification.
  • C8 An 8″ schmidt Cassegrain telescope made by Celestron. The “orange tube” was one of the first high quality inexpensive amateur telescopes. (see also, C8 maintance and care)
  • Cassegrain See Reflecting Telescopes.
  • Catadioptric A system that uses both lenses and mirrors. Usually something like a schmidt cassegrain. See Reflecting Telescopes.
  • CCD Charge-coupled Device – the chip that makes up the ehart of a digital camera.
  • Clock drive A device that drives an telescope on an equatorial mount to keep it tracking on an object.  Technically,  a clock drive is mechanical, but some people use the name to refer to any drive.
  • Corrector or Corrector plate A sort of lens in a reflecting telescope that corrects problems like spherical aberation. See Reflecting Telescopes.
  • Declination or Dec The coordinate that measures the distance of an object from the equator.  See Equatorial Mounts
  • Diagonal A prism or mirror that redirects the direction of light to make it easy for the observer to look through the telescope.
  • Drive A device that keeps a telescope tracking on an object.  This is easiest with an equtorial mount, but a computer controlled drive can also drive a telescope on an alt-az mount.
  • Dobsonian See Reflecting Telescopes.
  • Dobzilla A very large Dobsonian Telescope.
  • Equatorial Mount See Equatorial Mounts
  • Eyepiece the lens the observer looks though.
  • Eye relief The distance back from the eyepiece where the observer can focus and get the full field of view.  A long eye relief is especily important if you need to wear eyeglasses.
  • Exit pupil The diameter of light coming out of the eyepiece. A wider exit pupil is easier to observe with.
  • Field of View The area visible through the telescope.  See Basic Telescope Properties.
  • Finder ‘scope A small telescope with low magnification and wide field of view attached to a larger telescope to help the observer find objects.
  • Focuser The thing that alows the observer to focus the image.
  • F-ratio The ratio of the focal length to the aperture. See Basic Telescope Properties.
  • Galilean See Refracting Telescopes.
  • Light gathering power A measure of the amount of light a telescope can collect.  See Basic Telescope Properties.
  • Magnification A measure of how much bigger an object appears.  See Basic Telescope Properties.
  • Meridian a reference line that runs through north and south.  Astronomers usually  refer to the local meridian, which passes directly overhead.  When an object crosses the meridian (transits), it is a its highest altitude. See  Alt-Az mounts.
  • Newtonian See Reflecting Telescopes.
  • Objective lens The large lens at the front of a refracting telescope. The lens that points at the object.
  • Observer The person looking through the telescope, a person who regularly looks through a telescope, or an astronomer  who observes as opposed to a theorist.
  • Pier A permanent mount for a telescope. See Alt-Az Mounts and Equatorial Mounts.
  • Primary mirror The mirror in a reflecting telescope responsible for collecting the light.
  • Reflector A telescope that uses a mirror to collect light. See Reflecting Telescopes.
  • Refractor A telescope that uses a lens to collect light.  See Refracting Telescopes.
  • Resolution A measure of the detail visible through the telescope.
  • Reticle Wires or  markings in an eyepiece to help with centering or measurements. Commonly found in the eyepiece of a finder ‘scope.
  • Right Ascention or RA The coordinate that corresponds to longitude on the sky, and measures position east or west of the vernal equinox. See Equatorial Mounts
  • Schmidt A type of corrector used in reflecting telescopes. See Reflecting Telescopes.
  • Secondary The small mirror in a reflector that re-directs the light to a position where it can be observed. See Reflecting Telescopes.
  • Shower cap A cover for the end of a telescope similar to a shower cap to protect it from dirt (or sometimes, actually a shower cap placed over the end of a telescope)
  • Sidereal Time Time measured by the stars. A sidereal day is 24h, 56 minutes, and the  local sidereal time  matches the local solar time (the time on your watch) on the autumnal equinox. The RA of the meridian is equal to the sidereal time.
  • Star trails The long curved lines that show up in long photographs of the night sky.
  • Sucker hole A gap in the clouds big enough to convince you to get your telescope out, even though it will probably be gone by the time you are set up.
  • Tripod A three legged mount. See Alt-Az Mounts.
  • Wedge A device that attaches to a tripod or pier that tilts the telescope at an angle, usually to make it an equatorial mount.


1. Amber - March 29, 2010

How about a separate entry differentiating instantaneous field of view (IFOV) from field of view? I find this gets confused frequently.

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