Dawes' Limit Calculator

About this calculator

This calculator finds a telescope's theoretical resolving power using Dawes' limit, 116/D arcseconds where D is the aperture diameter in millimetres (or 4.56/D with D in inches). Formulated by the astronomer William Rutter Dawes in 1867 from his own extensive observations of close double stars, it's an empirical companion to the diffraction-based Rayleigh criterion, giving the closest angular separation at which a telescope of a given aperture can still show two point sources of light — like a double star — as distinct rather than blurred together.

Amateur astronomers use it to judge how much detail a telescope can realistically resolve before buying one, double-star observers use it to check whether a given instrument can split a specific known pair, and telescope manufacturers reference it when specifying an instrument's performance. Enter the aperture diameter to get the theoretical resolving power in arcseconds.