Dawes' Limit

updated: 2022-10-04

The Dawes' Limit expresses the theoretical maximum resolving power of a telescope (or microscope) according to the formula:

R = 11.6/D

D in centimeters, R in arcseconds


D is the diameter of the main lens (aperture)

R is the resolving power of the instrument in arcsec

This formula is valid for wavelength of about 562nm. A more general formula that also takes into account the wavelength is called Rayleigh Criterion or Rayleigh Limit.

A telescope with an aperture of 90mm will therefor have a typical Dawes' Limit or angular resolution of 1.53" arcsec.

See: Dawes' limit - Wikipedia

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