Telescope eyepiece planner
Compare magnification, exit pupil and approximate sky coverage across your eyepieces. Change the inputs to update the chart or diagram.
Your inputs
These editable values illustrate the calculation. Replace them with your own inputs. Results update as you edit.
Calculations run in your browser. Inputs are not saved or sent to analytics.
How it works
- Magnification = × / ; Exit pupil = / Magnification; Approximate true field ≈ / Magnification
- Worked example: 750 / 25 = 30×; 150 / 30 = 5 mm; 50° / 30 ≈ 1.66666667°
Input and formula key
Your result
Complete the inputs to see your result.
Assumptions and limitations
Use 1–20 eyepieces and millimetres for aperture and focal lengths. Barlow/reducer factor multiplies telescope focal length (0.1–10). True field is apparent field divided by magnification, an approximation that ignores field stops and distortion. Seeing, eye-pupil limits and equipment compatibility are not predicted. The headline uses the first eyepiece.
Sources
- Celestron — magnification formula Sources reviewed: 2026-10-08
- Celestron — eyepiece limits and exit pupil Sources reviewed: 2026-10-08
