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| f two equally-bright stars are so close together that their Airy disks overlap, they will be seen as one 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 - not as two distinct points, but as a Figure 8 as shown below, in which the intensity of light between the two touching disks drops by a clearly visible 30%. The smallest separation between two stars which shows this 30% drop was empirically determined by English astronomer William R. Dawes (1799-1868, and known as the "eagle-eyed" for his acute vision) to be 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. Dawes' limit (determined 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 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 illustration below simulates the visual appearance of a pair of equal magnitude stars separated by the Dawes' limit for the telescope being used to make the observation. As you can see, even though the telescope is resolving to Dawes' limit, the binary pair is not cleanly split into two separate stars, but appears as a Figure 8 in shape. |
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