Your telescope.
Your view of the sky.
Discover what changes when you choose a different eyepiece or add a Barlow lens.
Your setup
Initial example: 150/750 telescope, 25 mm eyepiece and 50° apparent field of view.
Through the eyepiece
Enter the parameters, then run the calculation.
The result describes an optical combination. Image quality also depends on the sky, instrument adjustment and observer.
Understanding the results
Formulas used
Magnification = telescope focal length × Barlow factor ÷ eyepiece focal length.
Exit pupil = telescope aperture ÷ magnification.
Estimated true field of view = eyepiece apparent field of view ÷ magnification.
Native focal ratio = telescope focal length ÷ aperture.
Lengths are expressed in millimetres and fields of view in degrees. The true field is an approximation: the eyepiece’s field stop allows a more precise estimate.
Why more magnification is not enough
Atmospheric turbulence and the quality of adjustment limit visible details. A very small exit pupil gives a darker image; an overly large pupil may exceed your eye’s pupil.
The 0.5 mm and 7 mm guides used here are approximate. They are not universal limits or performance guarantees.
Methods: Celestron — choosing eyepieces and Travel Scope manual, calculating true field of view. References accessed on 18 October 2026. Calculations run on your device; no account required.
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