Dawes' limit

The Dawes criterion describes the diffraction -limited resolution of a human observer when viewed close double stars through a telescope. It is named after the British astronomer William Rutter Dawes.

He gave the empirically found by him relationship between the diameter of the telescope aperture d in inches and the angular distance α in arc seconds at a so just to be separated as a binary star

With a telescope of two inches aperture ( 5 cm) Thus, a double star are perceived separately with 2.3 arcsec angular distance. For large telescopes, the formula is no longer valid, since the seeing limits resolution.

Most double stars are similar to sun yellow. Assuming for the wavelength 550 nm and forces from α in radians, then the Dawes criterion

Compared to the empirical Dawes criterion for human vision the formal Rayleigh criterion underestimated the resolution by a factor of 1.22: When Dawes criterion the two Airy discs overlap so strong that almost no indentation between the maxima can be seen, while in Rayleigh criterion, the recess is about 30%. Modern image processing enables the measurement of double stars at even greater overlap.

• Optics