Astronomy without an account
Plan your optical system and record real observations
Telescope calculators, equipment catalogue, session planner and private observation log without an account.
Equipment, location labels and notes are stored locally in your browser.
Essential tools
Telescope magnification
Calculate telescope–eyepiece magnification with a Barlow or reducer.
Exit pupil
Check image brightness and practicality for a chosen eyepiece.
True field of view — approximation
Estimate sky coverage from apparent field and magnification.
True field from field stop
Calculate a more accurate field when the field-stop diameter is known.
Telescope focal ratio
Calculate the focal ratio before and after an optical factor.
Barlow or focal reducer
Compare effective focal length, magnification and focal ratio.
Solar observing safety
Never view the Sun through a telescope, binoculars or camera without a special-purpose solar filter secured over the front of the optics. Eclipse glasses are not telescope filters.
Read the full safety guidance