Calculate telescope resolving power: Dawes' Limit, Rayleigh Criterion, limiting magnitude, and light-gathering ratio.
Enter the aperture (diameter) of your telescope's primary mirror or lens in millimeters or inches.
The tool provides Dawes' Limit and the Rayleigh Criterion. These values indicate the smallest detail (in arcseconds) the telescope can theoretically resolve.
Atmospheric 'seeing' (turbulence) often limits resolution to about 1 arcsecond, regardless of how large your telescope is, unless you are in space or using adaptive optics.
Both estimate the closest double star a telescope can split. Dawes' limit (116 ÷ aperture in mm) is an empirical figure from visual double-star observations, while the Rayleigh criterion (138 ÷ aperture) comes from diffraction theory and is about 19% more conservative.
By Dawes' limit, 116 ÷ 200 = 0.58 arcseconds; by the Rayleigh criterion, 138 ÷ 200 = 0.69 arcseconds. In practice, atmospheric seeing of 1–2 arcseconds usually sets the real limit before the optics do.
It is the faintest star the telescope should show visually under dark skies, estimated as 2.1 + 5 × log10(aperture in mm). A 100mm scope reaches about magnitude 12.1, and doubling the aperture to 200mm gains another 1.5 magnitudes.
It compares the telescope's aperture with a dark-adapted 7mm eye pupil: (aperture ÷ 7)². A 200mm aperture collects about 816 times more light than the naked eye, which is what brings faint galaxies and nebulae into view.