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.
ToolsUseful formulas
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.
- Star test
You point at a moderately bright star at high power and defocus it slightly on each side of focus. With good, well collimated optics you see identical concentric rings on both sides. It is the final judge of collimation and optical quality.
- Central obstruction
The fraction of the aperture blocked by the secondary mirror, as a percentage: a 50 mm secondary in front of a 200 mm primary is 25%. It does not make a hole in the image but slightly lowers the contrast of fine planetary detail. Visual Newtonians aim for 20 to 25%; Schmidt-Cassegrains often exceed 30%.
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