Telescope Eyepiece Magnification Calculator
Compare magnification, exit pupil and approximate angular fields using entered telescope and eyepiece dimensions.
YOUR CALCULATION
Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Multiply telescope focal length by the entered factor, divide by eyepiece focal length for magnification and divide aperture by magnification for exit pupil.
Magnification=effective telescope focal length/eyepiece focal length; exit pupil=aperture/magnification; approximate true field=apparent field/magnification
Worked example
A 1000 mm telescope with 100 mm aperture and 20 mm eyepiece at factor 1 gives 50× magnification, 2 mm exit pupil and approximate 1.2° true field from a 60° apparent field.
Assumptions and input details
Ideal geometric estimates, not an optical performance rating. Optional field-stop field uses 2 atan(stop/(2×effective focal length)); distortion and actual optical design can make estimates differ. No observer-pupil, resolving-power, useful-magnification or celestial-visibility recommendation is inferred.
References: Celestron: telescope and eyepiece magnification · Celestron: exit pupil definition
Unit definitions
Unit selection is independent of language. Each selector contains only units for its physical quantity. Calculations use defined base units internally.
Choose from 23 length definitions, including explicitly labelled regional measures. Area conversions square the length factor; geometric volumes cube it.
Regional measures are not interchangeable merely because their names look similar. This is a documented selection, not a claim to include every historical unit.
NIST measurement tables · Japanese traditional units · Mainland China and Hong Kong unit distinctions
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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