How this calculator works
Magnification is the telescope focal length divided by eyepiece focal length. Dividing the eyepiece apparent field by that magnification approximates the true field on the sky. A measured field-stop diameter gives a more reliable value for wide-angle eyepieces with angular distortion. Circular sky area is computed as a spherical cap instead of treating square degrees as a planar square.
Formula
M = Fscope/Feye; true field ≈ apparent field/M. Ω = 2π[1-cos(true field/2)]; area in square degrees = Ω(180/π)².
Worked example
A 1000 mm telescope and 20 mm eyepiece give 50×. With a 60-degree apparent field, the approximate true field is 1.2 degrees, covering about 1.131 square degrees.
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Frequently asked questions
Why is the field approximate?
Eyepiece distortion means apparent field divided by magnification is not exact, especially for very wide fields.
Is field area in square degrees the same as steradians?
Both measure solid angle. One steradian is about 3282.806 square degrees.
What does a Barlow lens change?
It increases effective telescope focal length and magnification, reducing the true field for the same eyepiece.