Calculate telescope FOV

Calculate telescope magnification, exit pupil, and approximate true field from an eyepiece.

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ToolTelescope FOV Calculator
Input
Output
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Preview

How it works

Magnification is telescope focal length ÷ eyepiece focal length, exit pupil is aperture ÷ magnification, and true field is apparent field ÷ magnification. Arcminutes multiply degrees by 60.

  • All four optical inputs are explicit.
  • Field-stop limits and distortion are not modelled.

Worked example

8 inch reflector with a 25mm eyepiece
200mm aperture, 1000mm focal length, a 25mm eyepiece with a 52 degree apparent field
Input
											Telescope aperture mm: 200
Telescope focal length mm: 1000
Eyepiece focal length mm: 25
Eyepiece apparent field degrees: 52
Output
												Magnification: 40
Exit pupil mm: 5
True field of view degrees: 1.3
True field of view arcminutes: 78

When to use this

Observers choose eyepieces, telescope setups check exit pupil, and astronomy lessons compare magnification with field.

Edge cases

  • An exit pupil wider than a dark-adapted eye wastes light.
  • Very high magnification can exceed seeing and mount stability.
  • Apparent field divided by magnification is approximate when field stop limits the view.