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Declination
Similar to Latitude on the Earth's surface, it is the distance in degrees North or South of the Celestial Equator (the projection of the Earth's Equator onto the Celestial Sphere). The degrees can be sub-divided into minutes and seconds.
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Deep Sky
A name given by amateur astronomers to objects beyond our Sun and its planets.
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Dew Cap
A tube extending forward from the front lens of a telescope. It prevents dew from forming on the lens as it cools down, and acts as a sunshade to reduce reflections during the day.
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Diagonal
A mirror or prism system which changes the angle and orientation of the light rays coming from the telescope to the eyepiece.
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Double Star
Two or more stars that appear very close in position. True double stars are in orbit about one another, while optical doubles simply seem close from our point of view.
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