Telescope Focal Ratio Calculator
Enter a telescope's focal length and aperture to find its focal ratio, resolving power and maximum useful magnification, plus eyepiece magnification and exit pupil.
How to use the telescope focal ratio calculator
- Enter the telescope's focal length and aperture in millimetres.
- Optionally enter an eyepiece focal length in millimetres.
- Read the f-ratio, Dawes limit, max magnification, eyepiece power and exit pupil.
Examples
150 mm f/8 reflector
focal length = 1200 mm, aperture = 150 mm
f/8, Dawes limit 0.77 arcsec, max useful magnification 300x
With a 10 mm eyepiece
focal length = 1200 mm, aperture = 150 mm, eyepiece = 10 mm
magnification 120x, exit pupil 1.25 mm
Frequently asked questions
What is a telescope's focal ratio?
The focal ratio, or f-number, is the focal length divided by the aperture. A short f-ratio gives a wide, bright field; a long f-ratio suits high power on planets.
What is the Dawes limit?
The Dawes limit estimates the finest detail a telescope can resolve, in arcseconds, as 116 divided by the aperture in millimetres. A larger aperture resolves finer detail.
What is the maximum useful magnification?
A common rule of thumb is about 2x the aperture in millimetres, so a 150 mm scope tops out near 300x before the image gets dim and fuzzy.
How do I work out eyepiece magnification?
Divide the telescope focal length by the eyepiece focal length. A 1200 mm scope with a 10 mm eyepiece gives 120x magnification.
What is exit pupil and why does it matter?
Exit pupil is the width of the light beam leaving the eyepiece, found by dividing aperture by magnification. Keep it under about 7 mm so no light is wasted.
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