Telescope Exit Pupil Calculator
The exit pupil is the width of the beam of light leaving the eyepiece. Match it to your eye and target: enter aperture and magnification to find it.
Telescope Exit Pupil Calculator
Why use the Telescope Exit Pupil Calculator
The wrong magnification gives dim or wasteful views. Exit pupil helps you choose eyepieces that match your eye and target.
How to get your numbers
- Aperture in mm.
- The magnification you’re using.
- Aim for 2–3 mm for detailed views.
Recommended for this job
How to use it
- Enter your telescope’s aperture in mm.
- Enter the magnification you’re using.
- Aim for 2–3 mm for detail, up to ~7 mm for low-power deep-sky.
Frequently asked questions
How do I calculate exit pupil?
Divide the aperture (mm) by the magnification. A 150 mm scope at 50× gives a 3 mm exit pupil.
What exit pupil is best?
Around 2–3 mm is the sweet spot for detailed views; up to ~7 mm suits low-power, wide deep-sky views under dark skies.
Can the exit pupil be too large?
Yes — larger than your dark-adapted pupil (~7 mm) wastes light, and on some scopes you may see the shadow of the secondary mirror.
What exit pupil is best for deep-sky objects?
A larger exit pupil (around 5–7 mm) gives the brightest, widest views for faint nebulae and galaxies.
What exit pupil is best for planets?
A small exit pupil (roughly 0.5–1 mm) means high magnification, which suits the Moon, planets and tight double stars.
Why is my telescope view dim?
Often the exit pupil is too small — you are using too much magnification for the aperture. Switch to a longer eyepiece for a brighter image.
