Exit Pupil

09-28-2011 | Resource

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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