Dawes' limit

An empirical formula devised by William Rutter Dawes (1799-1868) for the minimum angular separation of a close pair of star images detectable as double by a telescope of a particular aperture - its resolving power. The rule assumes that observing conditions are good and that the two stars are not of very different brightness. It gives a separation in arc seconds of 11.6/D, where D is the diameter of the telescope aperture in centimetres. If D is in inches, the formula is 4.6/D.