Magnification Factor vs. Exit Pupil

For visual observations it is important to know the magnification factor (M) and exit pupil (EP). The magnification factor describes the enlargement facor of the appearance of an object compared to the human eye and the exit pupil describes the diameter of the light beam that exits the eyepiece.

The magnification factor (M) can be calculated with the following formula:

fT = Focal length of the telescope (mm).

fO = Focal length of the ocular (mm).

The exit pupil (EP) can be calculated with the following formula:

fO = Focal length of the ocular (mm).

D = Aperture of the telescope (mm).

fT = Focal length of the telescope (mm).

On most occasions, the exit pupil should be between 6.5 and 1.0. If the calculated EP is bigger than 7.0 the human eye can no
longer adapt to it because the diameter of the human pupil has reached its maximum for complete darkness. It
is said that approximately 95% of the observable resolution of the telescope is achieved with an exit
pupil of 1.0 and this is what I call max reasonable magnification

. The maximum theoretical resolution of
the telescope is achieved with an exit pupil of 0.5. But in my opinion an exit pupil of 0.5 only works on rare
occasions when the seeing is excellent.

For planetary observations, I prefer to work with an exit pupil of 1.0 to 0.7.

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