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.
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Related terms
- 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.
- Rayleigh criterion
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.
- Double star
Two stars very close together on the sky. Some are true pairs bound by gravity (binaries); others only line up by chance (optical doubles). Albireo in Cygnus shows a lovely orange and blue contrast from 30×; Mizar and Alcor split with the naked eye.
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