Two points count as resolved when the centre of one Airy disc falls on the first dark ring of the other: separation = 1.22 λ/D. In green light (550 nm) this gives 138 ÷ aperture (mm) arcseconds, a little wider than the Dawes limit.
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Related terms
- Airy disc
Even perfect optics do not turn a star into a point but into a tiny disc surrounded by rings, because of diffraction. Its angular radius is 1.22 λ/D: the larger the aperture, the smaller the disc. About 84% of the light falls in the central disc.
- Dawes limit
A rule of thumb found in the 19th century by William Dawes from double star observations: minimum separation (″) = 116 ÷ aperture (mm). A 150 mm telescope thus splits two equal stars 0.77″ apart. It holds for two stars of similar brightness; if one is much fainter, a wider gap is needed.
- Resolving power
The smallest separation between two stars that an instrument can show as distinct, in arcseconds. It depends only on the aperture: about 1.2″ for 100 mm and 0.6″ for 200 mm. Seeing often prevents you from reaching it.
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