Telescope resolving power calculator

Two limits, one telescope

The Dawes limit comes from observational data on double stars by William Rutter Dawes in the 19th century: two equal-brightness stars can just be resolved when separated by 116 / aperture in mm, measured in arcseconds. The Rayleigh criterion is a stricter theoretical limit at 138 / aperture mm.

For a 200 mm scope: Dawes ≈ 0.58", Rayleigh ≈ 0.69". Both assume an ideal optical system and perfect atmosphere.

Why bigger isn't always better at the eyepiece

A 300 mm scope has a Dawes limit of 0.39". But ordinary atmospheric turbulence — what astronomers call "seeing" — rarely lets you achieve better than 1.0" at sea-level sites, and 0.5" only at the best mountain observatories. A 200 mm and a 300 mm scope often deliver similar resolution on planets at most locations.

Beating the seeing

Lucky imaging — taking thousands of short exposures and stacking only the sharpest few percent — lets amateur scopes reach close to their theoretical limits. Adaptive optics on Keck, VLT and Subaru achieve diffraction-limited resolution down to about 0.04".