What is the difference between the Dawes limit and the Rayleigh limit?

The Dawes limit is an empirical rule for splitting equal-brightness double stars, given by 116 divided by aperture in mm. The Rayleigh limit is the theoretical diffraction limit, 1.22 times the wavelength over aperture, and is slightly larger (a coarser number). Real telescopes typically perform between the two.

Why does a larger aperture resolve finer detail?

Resolution is set by diffraction, which scales inversely with aperture. A bigger mirror or lens has a smaller diffraction disk, so two close stars stay separated rather than blurring together. Doubling the aperture roughly halves the smallest angle you can resolve.

What units is the result in?

Resolution is given in arcseconds, where one arcsecond is 1/3600 of a degree. A double star with a separation larger than your telescope's limit can be split; one closer than the limit blurs into a single point.

Does atmospheric seeing affect this?

Yes. The figures here are the optical limits of the telescope itself. On a typical night, atmospheric turbulence (seeing) limits ground-based scopes to roughly 1 to 2 arcseconds regardless of aperture, so very large apertures rarely reach their full theoretical resolution visually.

Which wavelength should I use?

For visual observing, 550 nm (green-yellow) matches the peak sensitivity of the dark-adapted eye and is the standard reference. For red or blue light the diffraction limit is larger or smaller respectively, which is why the Rayleigh figure shifts with wavelength.

Telescope Resolving Power & Aperture Guide

The resolving power of a telescope is the smallest angular separation it can distinguish, set by the diffraction of light at its aperture. This tool computes both the empirical Dawes limit and the theoretical Rayleigh limit in arcseconds, so you can tell at a glance how tight a double star you can split.

How it works

Two standard formulas are used. The Dawes limit (empirical, for equal-brightness double stars) is:

Dawes (arcsec) = 116 / aperture_in_mm

The Rayleigh criterion (theoretical diffraction limit) is derived from the diffraction angle and converted from radians to arcseconds:

Rayleigh (rad)    = 1.22 × wavelength / aperture
Rayleigh (arcsec) = Rayleigh_rad × 206265

Here 206265 is the number of arcseconds in one radian. Wavelength and aperture must be in the same units, so the tool converts your aperture from mm and the wavelength from nm to metres before dividing.

Example and tips

A 100 mm refractor has a Dawes limit of 116 / 100 = 1.16 arcseconds and a Rayleigh limit of about 1.39 arcseconds at 550 nm. That means it can cleanly split the components of a double star separated by roughly 1.2 arcseconds on a steady night. Remember that atmospheric seeing usually caps real ground-based resolution near 1 to 2 arcseconds, so apertures above about 200 mm gain light-grasp and image scale faster than they gain visual splitting power.