Telescope Magnification Calculator
Magnification M = F/f | Exit Pupil = D/M | True FOV = AFOV/M
Dawes Limit (arcsec) = 116/D(mm) | Max useful magnification ≈ 2×D(mm)
Dawes Limit (arcsec) = 116/D(mm) | Max useful magnification ≈ 2×D(mm)
How to use Telescope Magnification Calculator
1
Enter the known values in the input fields.
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Results update instantly as you type — no button needed.
3
See the formula box for the equation being used.
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All calculations run in your browser — nothing is sent to any server.
Frequently Asked Questions
How do I calculate telescope magnification? ▼
Magnification = Objective focal length / Eyepiece focal length. A 1000mm telescope with a 25mm eyepiece gives 1000/25 = 40× magnification. Higher magnification requires shorter focal length eyepieces.
What is exit pupil? ▼
The exit pupil is the diameter of the beam of light leaving the eyepiece: Exit Pupil = Aperture / Magnification. Optimum exit pupil is 5–7mm for dark-sky viewing and 2–3mm for planets. Below 0.5mm, the image appears dim and fuzzy.
What is the maximum useful magnification? ▼
Approximately 2× the aperture in mm. For a 100mm telescope, maximum useful magnification is about 200×. Beyond this, atmospheric turbulence (seeing) limits detail regardless of optical quality.
Is this tool free? ▼
Yes, completely free. No signup needed.