Telescope Magnification Calculator
Magnification
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Exit Pupil
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Max Useful Magnification
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Min Useful Magnification
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How Telescope Magnification Works
Magnification = objective focal length / eyepiece focal length. A 1000mm scope with 25mm eyepiece gives 40x.
Exit pupil = aperture / magnification. For night viewing, 5-7mm is ideal (matching dark-adapted pupil). Below 1mm is dim.
Max useful magnification is about 2x aperture in mm. A 200mm scope maxes around 400x. Beyond that, atmosphere blurs the image.
Frequently Asked Questions
Magnification for planets?
150-250x for Jupiter, Saturn, Mars detail. Moon is great at 50-150x. Limited by atmospheric seeing.
Why does higher mag make images dimmer?
Same light spread over larger area. Doubling magnification reduces surface brightness by 75%.
What is exit pupil?
Light beam diameter exiting the eyepiece. aperture/magnification. 5-7mm ideal for night; 2-3mm for daytime.
How does focal ratio affect viewing?
Lower f/ (f/4-5) gives wider fields for deep-sky. Higher f/ (f/10-15) gives narrower fields for planets.