The exit pupil is the image of the objective that is formed by the eyepiece. It’s where you place your eye to see the full field of view. You can calculate the diameter of the exit pupil by dividing the focal length of the eyepiece by your scope’s focal ratio or you can divide aperture by magnification.
Exit pupil (mm)= Fe ÷ f/# (N) | D ÷ magnification
Fe: Focal length of eyepiece
f/# (N): Focal ratio of telescope
D: Diameter of aperture (mm)
For reflector telescopes, it’s best to avoid exit pupils larger than 7mm or smaller than 0.5mm. Refracting telescopes have no upper limits on exit pupil sizes.
For the full field of view to be seen, the exit pupil should correspond with the dilation of the dark-adapted pupil, which is between 5 and 7mm. The dialation of our dark adapted pupil has to be larger than the exit pupil to see the full field of view. An exit pupil larger than 7mm means that some of the light will be lost outside of the eye’s entrance pupil, no matter what the distance of the eye from the eyepiece.
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