Definitions for "Exit Pupil"
The exit pupil diameter is the number attained by dividing the objective lens diameter by the magnification. The exit pupil appears as the bright circular area inside the eyepiece. The larger the exit pupil, the better it is for use in poor light.
The diameter of the light beam leaving any eyepiece and traveling to your own eye's pupil. The exit pupil can be calculate by using ether of the two formulas below: Diameter of the telescope's primary objective divided by the magnification OR Eyepiece focal length divided by the telescope's focal ration (f-number) Example: A telescope has a 203mm (8 inch) primary mirror. Using a magnification 38x, the Exit pupil would be 203mm divided by 38x or 5.3mm.
The diameter of the cone of light exiting the eyepiece. This can be calculated by dividing the diameter of the objective by the magnification used.