Telescopes · Optics
Telescope resolving power calculator.
Enter the aperture to see the closest double star your telescope can split in perfect conditions.
Inputs
Resolution
Enter an aperture.
How resolving power is worked out
Resolving power is the smallest angle between two points of light a telescope can show as separate. The Dawes limit is an empirical rule from double-star testing: R = 116 / D, with D in millimetres and R in arcseconds (equivalently 4.56 / D with D in inches). The Rayleigh criterion comes from diffraction theory: θ = 1.22 λ / D in radians, which the tool converts to arcseconds. Dawes is slightly tighter than Rayleigh.
Worked example. A 200 mm aperture: Dawes = 116 / 200 = 0.58 arcseconds. Rayleigh at 550 nm = 1.22 × 550×10-9 / 0.2 = 3.36×10-6 rad = 0.69 arcseconds.
Both assume perfect optics and equally bright stars. Seeing, collimation and tube currents usually matter more than the formula.
Frequently asked questions
What is the Dawes limit?
An empirical resolution limit found by W. R. Dawes from testing observers on close double stars of equal brightness: 116 / aperture in mm gives arcseconds.
What is the difference between Dawes and Rayleigh?
Rayleigh is a theoretical diffraction limit that depends on wavelength; Dawes is a practical observing rule. Dawes comes out slightly smaller.
What resolution does an 8 inch telescope have?
About 0.57 arcseconds by the Dawes formula (4.56 / 8 in) and 0.69 arcseconds by Rayleigh at 550 nm.
Why can I not split stars that close on most nights?
Atmospheric seeing usually blurs detail to around 2 to 3 arcseconds in backyard locations, far larger than the theoretical limit.
Does more magnification improve resolution?
No. Resolution is set by the aperture. Extra magnification only enlarges the blur beyond a useful point.
Sources
Checked
- Wikipedia: Dawes’ limit
- Wikipedia: Angular resolution (Rayleigh criterion)
- Astrobackyard: Understanding Image Scale in Astrophotography
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Scope
Theoretical limits for ideal optics. Education only.