achromatic lens - A lens which removes most of the false color (that is, chromatic aberration) from an image.
altitude - The vertical angle above the horizon at which a telescope is tilted.
alt-azimuth - Simplest of all telescope mounts. Allows a telescope to be turned in either the altitude or azimuth directions, hence its name..
apparent field of view - of an eyepiece is how big the view seems to be to your eye.
aperture - the diameter of a telescope’s objective.
apochromatic lens - A lens which removes virtually all chromatic aberration from an image. It is vastly superior to an achromatic lens.
astigmatism - An focusing defect resulting from an optical surface which is asymmetrical. See Understanding Optical Defects.
astrology - Primitive belief that the stars have influence over peoples lives.
astronomy - The science pertaining to the stars, planets, and other astronomical objects. Some people erroneously confuse this with astrology which is NOT a science.
astrophotography - The art of taking pictures of heavenly objects.
azimuth - Horizontal angle at which a telescope is turned.
Barlow lens - A special lens which doubles or triples the magnification of a given eyepiece.
Cassegrainian - A reflector telescope whose secondary mirror is convex and directs light out the rear of the telescope by folding the light path. Generally this telescope is more compact than the Newtonian.
catadiotropic - A hybrid telescope with a large lens in the front of the tube and an objective mirror of similar size in the back. The lens compensates for defects in the mirror.
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