If two equally-bright stars are so close together that their Airy disks overlap, they will be seen as a single star, although perhaps as an elongated one. If, however, the Airy disk of one star falls in the first dark diffraction ring of the second, each star can be seen separately – not as two distinct points, but as two small disks of light touching and forming a Figure 8, in which the intensity of light between the two touching disks drops by a clearly visible 30%.
English astronomer William R. Dawes (1799-1868, and known as the “eagle-eyed” for his acute vision) determined that the smallest separation between two stars which shows this 30% drop is equal to 4.56 arc seconds divided by the aperture of the telescope in inches. The larger the telescope aperture, the smaller the separation that can be resolved.
This “Dawes’ limit” (which he determined empirically simply by testing the resolving ability of many observers on white star pairs of equal magnitude 6 brightness) only applies to point sources of light (stars). Smaller separations can be resolved in extended objects, such as the planets. For example, Cassini’s Division in the rings of Saturn (0.5 arc seconds across), was discovered using a 2.5” telescope – which has a Dawes’ limit of 1.8 arc seconds!
The ability of a telescope to resolve to Dawes’ limit is usually much more affected by seeing conditions, by the difference in brightness between the binary star components, and by the observer’s visual acuity, than it is by the optical quality of the telescope.
The resolving power of each of the telescopes on our website is shown as determined by the Dawes’ limit formula, as this is the standard measurement that all manufacturers use to specify the resolving powers of their telescopes.