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Aberration
Any optical defect and/or design error which causes any of the processed light to deviate from reaching the focal point, therefore reducing the quality of the image.
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Absolute Magnitude
The apparent brightness a star would have if placed at a distance of 10 parsecs from the earth.
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Achromatic Lens
A refractor lens, made of two or sometimes three separate lenses, which has the effect of bringing most of the viewed colours to a sharp focus, thus reducing chromatic aberration.
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Alt-azimuth
A simple mount that allows movement in altitude (up and down) and in azimuth (side to side).
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Antireflection Coating
A thin layer of film applied to an optical surface that reduces the loss of transmission of light.
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Aperture
The diameter of the primary mirror or lens.
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Asterism
A group of stars that appear to make an easily recognized shape, such as the "Big Dipper" or the "Coathanger".
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