Telescope on a tripod under a starry sky, lit by a red light. — An 8-inch telescope set up for imaging in dark skies, Kickapoo Valley Reserve, WisconsinBrainandforce · CC BY 4.0 · original

Scope and camera numbers

Type in your telescope, eyepiece and camera numbers to see magnification, field of view, exit pupil, the finest detail your aperture can split and how long a fixed-tripod star photo can run.

Scope and eyepiece

Telescope aperture (mm)

Telescope focal length (mm)

Eyepiece focal length (mm)

Eyepiece apparent field (degrees)

Barlow lens

How long before stars trail (the 500 rule)

Lens focal length (mm)

Camera sensor

This page needs JavaScript to do the sums. Magnification is the telescope focal length divided by the eyepiece focal length. Exit pupil is the aperture divided by the magnification. The 500 rule gives the longest untracked exposure in seconds as 500 divided by the lens focal length times the crop factor.

Formulas used: magnification is telescope focal length over eyepiece focal length; true field is the eyepiece's apparent field over the magnification; exit pupil is aperture over magnification; the highest useful power is about twice the aperture in millimetres (about 50x per inch), reached only in steady air; the Dawes limit is 116 divided by the aperture in millimetres, in arcseconds; the 500 rule is 500 over focal length times crop factor. Sources: University of Sheffield and University of Portland course notes, Sky & Telescope, OASI and New Mexico Tech. These are rules of thumb; seeing, light pollution and your eyes change what you actually see.

Faintest star (limiting magnitude): Optics Trade, "Limiting magnitude calculations". Cross-check: University of Virginia, ASTR 313 telescopes lecture.