Aperture and Field of View Calculator

Enter the pupil below or calculate it.
Pupil Calculator
Eyepiece focal length (mm) =
Mirror focal ratio =
Coma corrector magnification factor =

Enter the eyepiece's field stop and FOV or aperture.
Eyepiece field stop diameter (mm) = apparent field (deg) =
Field of view (degrees) =
Aperture (inches) =

This handy calculator demonstrates the relationship between pupil, field of view and aperture.
Get the eyepiece field stop from the manufacturer specifications.
Resolution is based our eye's ability to resolve 2 arc minutes.
Here's a graph showing the relationship between aperture and field of view for three eyepieces with different apparent field of views. The focal ratios are adjusted so that all eyepieces give the same exit pupil. You can see that the 100 degree Ethos eyepiece allows a greater aperture and field of view than the 82 degree Nagler eyepiece or the older Erfle if you can get down to F3.

For more, see my ZipDob article, my Richest Field Telescope article, and my sketches of rarely observed objects.

Mel Bartels