Dawes Limit & Telescope Resolution Calculator

How it works

Dawes limit (″) = 116 ÷ D (mm); Rayleigh (″) = 1.22 λ ÷ D × 206,265; Airy disc diameter = 2 × Rayleigh; effective resolution = max(optical limit, seeing)

Because light is a wave, a telescope images a star not as a point but as an Airy disc surrounded by faint rings, and two stars closer together than about the disc radius merge into one blob. The Rayleigh criterion puts that limit at 1.22 λ/D — about 0.9″ for a 150 mm aperture in green light — while Dawes’ empirical rule (116/D) is a little more optimistic because a trained eye can detect the dip between two equal stars before they are fully separated. Doubling aperture halves both figures. In practice the atmosphere blurs stars to 1–3″ on most nights, so apertures over about 120 mm are seeing-limited more often than not; large scopes still win on brightness and on the rare steady nights. Central obstructions in reflectors do not change resolution but push light from the disc into the rings, softening low-contrast planetary detail.

Worked example

A 150 mm reflector: Dawes limit 0.77″, Rayleigh 0.92″ at 550 nm, Airy disc 1.85″ across. In 2″ seeing the atmosphere is the limit — Izar (2.9″) and Porrima (3.3″) split, Zeta Boötis (0.8″) does not; about 300× is needed to see the diffraction pattern at all.

Frequently asked questions