Understanding telescope calculator
Match telescope and eyepiece to calculate magnification, exit pupil, and approximate field of view.
The idea behind it
Magnification depends on both telescope and eyepiece. Exit pupil describes the diameter of the light beam leaving the eyepiece. A wider true field lets you see more of the sky at once.
The relationship to remember
1,000 / 25 = 40×. Approximate true field: 60° / 40 = 1.5°. Dawes’ resolution estimate: 1.16 arcseconds.
Make your own discovery
Halve the eyepiece focal length. Magnification doubles and both exit pupil and true field shrink.
Try the interactive lab ↗What this model leaves out
True field ≈ apparent field / magnification. Dawes’ limit estimates resolution for double stars; atmosphere, optics, and target contrast affect actual results. Never view the Sun without a purpose-built front-mounted solar filter.
Check your understanding
Is higher magnification always better?
No. High power can make a view dimmer and amplify atmospheric blur. Aperture and observing conditions matter.
What does exit pupil mean?
It is the width of the light beam leaving the eyepiece, equal to aperture divided by magnification.
Continue with the formula reference or test yourself in space practice.