exit pupil

The exit pupil is the minimum diameter of the light beam leaving an eyepiece though which all of the light from the eyepiece passes. It can be calculated in either of two ways: by dividing the diameter of the telescope's primary objective by the magnification, or by dividing the focal length of the eyepiece by the focal ratio (f-number) of the telescope. For example, a telescope with a 20-centimeter (8-inch) primary mirror using a magnification 38x results in an exit pupil of 5.3 millimeters. The observer' eye should be located at the exit pupil to see the full and brightest field of view.