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Eclipse
The blocking of one astronomical body by another as seen from the earth. The most common of these events are Solar and Lunar eclipses.
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Equatorial Mount
A telescope mount with an axis parallel to the axis of the earth. This provides easy tracking of sky objects and for photography when combined with a clock drive.
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Exit Pupil
This is the diameter of the beam of light from the eyepiece which reaches the pupil of the eye. It is usually expressed in mm, and determined by dividing the diameter of the primary (in mm) by the Magnification. Knowing this value and the diameter of your dilated pupil allows you to choose the eyepieces which will work best for you with a specific telescope.
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Eye Relief
The distance between the eyepiece lens and the position in which the eye must be placed to see through the telescope. Telescope users who wear eyeglasses while observing, appreciate the benefits of longer eye relief.
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Eyepiece
Also called an ocular. This is a small tube that contains the lenses needed to bring a telescope's focus to a final image in the eye. Telescopes usually come with at least two eyepieces: one for low power and a second for a higher power view.
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