This calculator will help you find the true field of view of a telescope after an eyepiece is set. You will need to know three variables in order to calculate it. These are:
Eyepiece field of view – This is the field of view the eyepiece comes with. It refers to the area of the sky that can be seen with it. It is given in degrees and will go from 40° to 75°. It is also known as apparent field of view and can usually be found listed in the eyepiece specifications.
Telescope focal length – It’s the length of the path light has to travel from the front mirror of a telescope to the exit pupil. It is given in millimeters. If you don’t know the FL of your telescope, it can usually be found simply by googling “[model] focal length”.
Eyepiece focal length – Same as above, but for the eyepiece. It can generally be found printed on the side.
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Telescope Field of View Formula
The formula for the true field of view is Apparent Field of View / (Telescope focal length / Eyepiece focal length).
If you have checked out our magnification calculator you might have noticed that the bottom part of that equation is the same formula as the one for magnification, so the formula can be shortened as Apparent Field of view / Magnification.