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Achromatic - Objective lens designed to correct for two wavelengths of light.  Inexpensive design but yields false color.  Apochromatic lenses (see below) are better corrected.

Afocal - Afocal imaging uses an eyepiece to project a magnified image into a camera.  This type of imaging is used for small targets such as the planets.

Alt-Azimuth Mount - A mount in which the axes are aligned with altitude (up and down) and azimuth (left and right), as opposed to an equatorial mount.  Alt-az mounts are much simpler than equatorial mounts but have limitations for long-exposure astrophotography or CCD imaging since they suffer from field rotationSee also Fork Mount.

Altitude - An object's height above the horizon.  The horizon has an altitude of 0° and directly overhead (zenith) has an altitude of 90°.  An object's position can be described by its altitude and azimuth (see below), although any celestial object's altitude and azimuth coordinates are constantly changing due to Earth's rotation.

Angstrom - A unit of measure used in reference to wavelengths of light.  An Angstrom is one ten-billionth of a meter, or 1/10th of a nanometer.  The visible spectrum (what the human eye sees) covers the wavelength range from 4000-7000 Angstroms.  Angstrom is abbreviated with the symbol Å.

Anti-Reflective Coatings - AR coatings reduce the amount of light that is reflected by a glass surface, thereby increasing light transmission through the glass.  Used on telescopes, binoculars, camera lenses, and filters.

Apparent Field - An eyepiece term used to designate how wide the inherent design of the eyepiece is.  Apparent field, together with magnification, determine the true field of view of an eyepiece/telescope combination.  Wide-field eyepieces have large apparent fields (usually 70-82°) compared to standard eyepieces (40-50°).

Aperture - Aperture is the diameter of a telescope's or binoculars' optics.  Aperture determines light-gathering ability and is therefore one of the most important aspects of a telescope design.  In a binocular or refractor telescope, this is the diameter of the objective lens.  In many reflecting telescopes, such as Newtonians, this is the primary mirror diameter.  In some designs, the mirror is oversized and the aperture is determined by a corrector lens (as in a Schmidt-Cassegrain).

Aperture Mask - An aperture mask is used to stop down a telescope's aperture to a smaller diameter.  This is often done on large telescopes when viewing bright objects (for which the extra aperture is less important) in order to decrease the amount of air being viewed through and therefore improve the sharpness of the view.  Aperture masks also decrease off-axis aberrations, although at the cost of reduced light-gathering ability.

Apochromatic - Objective lens designed to correct for at least three wavelengths of light.  The best designs eliminate false color.  "Apo" lenses and better corrected but more expensive than achromatic lenses (see above).

Arcminute - A measure of angular size, equal to 1/60th of a degree.  Deep sky objects such as nebulae and galaxies are usually measured in arcminutes.

Arcsecond - A measure of angular size, equal to 1/60th of an arcminute, or 1/3600th of a degree.  Planets, planetary nebulae, and double stars are often measured in arcseconds.

Aspheric - Most optical lenses, and many telescope mirrors, are spherical in shape.  This is the easiest shape to manufacture.  But to eliminate certain optical aberrations, sometimes an aspherical (non-spherical) optical surface must be used.  Aspheric surfaces are harder to make and therefore more expensive.

Asteroid - Asteroids are large, rocky solar system bodies.  Most asteroids orbit the sun in the main asteroid belt between Mars and Jupiter.  Some asteroids have orbits that bring them closer to or farther from the sun, and their paths may cross the orbits of the planets, including Earth.  Asteroids appear in a telescope as faint starlike objects, and most move very slowly, just like the planets.  However, if an asteroid comes close to Earth, its motion may be visible over the course of just a few minutes or seconds.  See also, meteors and comets.

Astigmatism - An optical aberration.  Most telescopes are designed to eliminate this aberration and its presence can indicate a problem with the optics.  Some telescopes have inherent off-axis astigmatism (notably the Ritchey-Chrétien design) as a trade-off against another aberration.  Of course, the human eye, being an optical system, can also suffer from astigmatism which can degrade the view through a telescope if corrective optics are not used.

Astrometry - Astrometric measurements determine the position of an object in the sky.  These measurements are often used to determine the orbit of a comet or asteroid, or the motion of double stars, etc.  See also, photometry.

Azimuth - Azimuth indicates an object's position around the horizon, or it's cardinal direction.  North has an azimuth of 0°, east is 90°, south is 180°, and west is 270°.  An object's position can be described by its altitude (see above) and azimuth, although any celestial object's altitude and azimuth coordinates are constantly changing due to Earth's rotation.

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