Telescope Dawes Limit Calculator

The Dawes limit is an empirical measure of a telescope's resolving power: the smallest angular separation between two equal stars that the telescope can just distinguish. It was determined by the 19th-century observer William Rutter Dawes from visual double-star tests and is widely used to rate telescope aperture performance.

In arcseconds, the Dawes limit equals 116 divided by the aperture diameter in millimeters, or equivalently 4.56 divided by the aperture in inches. A larger aperture gives a smaller (finer) limit, meaning it can split closer double stars. The constant comes from Dawes's observations of equal-brightness pairs.

Both describe resolution, but the Rayleigh criterion is a theoretical diffraction limit (about 138 divided by aperture in mm for visible light) while the Dawes limit is a slightly tighter empirical value based on what a skilled observer can actually split. The Dawes limit gives a smaller number, reflecting that double stars can be detected slightly below the strict Rayleigh separation.

No. The Dawes limit is the telescope's intrinsic resolving power under ideal conditions. In practice, atmospheric turbulence (seeing) often limits ground-based resolution to one or two arcseconds regardless of aperture, so the theoretical Dawes value is reached only on nights of excellent seeing.

Enter the aperture diameter in millimeters; the calculator also accepts inches and reports the limit from both the millimeter and inch conventions for cross-checking. The output is the resolving limit in arcseconds, the standard unit for angular separation of close double stars.