Telescope Resolving Power Calculator

How it is calculated

You enter the diameter of the main lens or mirror, D, in millimetres. The tool returns two angular limits in arcseconds: Dawes = 116 ÷ D and Rayleigh = 138 ÷ D. Both fall as the aperture grows, so doubling the diameter halves the smallest separation you can split. The Dawes figure is an empirical rule from visual observation of equally bright double stars. The Rayleigh figure comes from the diffraction formula 1.22·λ ÷ D; the constant 138 corresponds to green light of about 550 nm, so no wavelength input is needed. Results are shown to three decimals. A zero or negative aperture gives no result. The numbers describe an ideal telescope under steady air; real seeing, optical quality, and the observer's eye usually make the practical limit worse.

Common mistakes

• Typing the aperture in inches. The field expects millimetres, so an 8-inch telescope is about 203 mm; entering 8 gives 14.5 arcseconds.
• Using the focal length instead of the aperture. Resolution depends on the diameter of the lens or mirror, not on how long the tube is.
• Entering the diameter of the eyepiece or the finder. Only the main objective counts.
• Expecting the calculated limit on every night. Atmospheric turbulence (seeing) often blurs the view to one or two arcseconds, which is worse than the limit for a mid-size telescope.
• Mixing up the two limits. The Rayleigh number is always about 19 percent larger than the Dawes number, so quoting the wrong one changes the claim about which close pairs can be split.