One formula · your inputs
Telescope exit pupil calculator
Exit pupil is the diameter of the light bundle leaving the eyepiece in the idealised geometric model.
The calculator derives magnification first, then divides clear aperture by that value.
Formula
How it is calculated
exit pupil = clear aperture ÷ magnification = eyepiece focal length ÷ focal ratio- Use clear aperture rather than tube diameter.
- Enter effective telescope and eyepiece focal lengths.
- Read the geometric pupil without treating it as an equipment verdict.
What the result represents
Two equivalent ratios in ideal geometric optics.
The page keeps every decision-bearing quantity visible as an input. It does not look up a product catalogue, current market value or hidden recommended preset.
Limits of the model
The result is an idealised optical or geometric estimate from user-entered values. It does not model seeing, optical quality, central obstruction, transmission, distortion, focus error, tracking or sensor calibration.
Use the number to compare transparent scenarios. Verify critical observations, equipment combinations and coordinates independently.
Questions about this calculation
Does this exit-pupil calculator choose equipment for me?
No. It evaluates only the displayed formula with the numbers you enter and does not make a purchase, compatibility or safety decision.
Why can the observed result differ?
Real optics and observing conditions add obstructions, transmission losses, aberrations, seeing, focus and measurement uncertainty that are outside the arithmetic model.
Can I use the result for navigation or safety-critical work?
No. SkyClock tools are transparent planning estimates, not certified navigation, engineering or safety instruments.
Need a dated lunar calendar? Browse official USNO moon phases for 2026–2050.