Calculator
Telescope Magnification Calculator
Three numbers — telescope focal length, eyepiece focal length and any Barlow factor — determine almost everything about what you will see. This calculator derives the standard set of observing figures from them and flags the combinations that exceed what your aperture and the atmosphere can support.
Result
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Measure
How to read these numbers
- Magnification above roughly 2× the aperture in millimetres produces an image that is larger but not more detailed — the extra size comes at the cost of brightness and contrast.
- An exit pupil above about 7 mm wastes light, because the pupil of a dark-adapted eye is rarely that wide. Below about 0.5 mm the image becomes dim and the eye's own floaters become visible.
- The limiting magnitude figure assumes a dark sky and a fully dark-adapted observer. Under suburban light pollution, subtract two to three magnitudes.
These are planning figures computed from standard approximations, not measurements of your equipment. Atmospheric seeing, optical quality, collimation and sky brightness move the real numbers, sometimes a lot.