Telescope Exit Pupil Calculator

What Is Exit Pupil?

The exit pupil is the diameter of the beam of light that emerges from the eyepiece and enters your eye. It determines how bright the image appears relative to your dark-adapted pupil.

Exit pupil (mm) = telescope aperture (mm) / magnification

Equivalently: exit pupil = eyepiece focal length / telescope focal ratio (f/number)

The Comfortable Range

A fully dark-adapted adult eye has a pupil diameter of about 5 to 7 mm (decreasing with age; people over 50 typically top out around 5 mm). An exit pupil larger than your adapted pupil wastes light because the beam is wider than your eye can accept.

For planetary viewing, small exit pupils (0.5 to 2 mm) give high magnification and good contrast. For deep-sky objects like nebulae and galaxies, larger exit pupils (3 to 7 mm) let in more light and give a wider field. For general scanning and star clusters, 3 to 5 mm is a good sweet spot.

Magnification

Magnification = telescope focal length / eyepiece focal length

For comfortable viewing, magnification above 50x per inch of aperture (about 2x per mm) tends to make images dim and unstable in average seeing conditions. Most telescopes have a practical maximum magnification of 1.5 to 2x per mm of aperture.

Limiting Magnitude

The naked-eye limit from a dark site is about 6.5. A telescope extends this by collecting more light: Limiting magnitude = 2.1 + 5 x log10(aperture in mm)

Each doubling of aperture adds about 1.5 magnitudes. A 100mm scope reaches about magnitude 12.1; a 200mm scope reaches about 13.6.