Dawes' limit

diffraction pattern matching Dawes' limit

Dawes' limit is a formula to express the maximum resolving power of a microscope or telescope. ^[1] It is so named after its discoverer, William Rutter Dawes , ^[2] although it is also credited to Lord Rayleigh.

The formula takes different forms depending on the units.

R = 4.56/D	D in inches, R in arcseconds
R = 116/D	D in millimeters, R in arcseconds
where	D is the diameter of the main lens (aperture)
	R is the resolving power of the instrument

This formula agrees with the usual ${\displaystyle R=1.22\lambda /D}$ at a wavelength of about 460nm, somewhat bluer than the peak sensitivity of rod cells at c. 498nm.

See also

• Rayleigh criterion

References

1. Bob King (2017-03-01). "Pushing Limits: A Spring Sky Double Star Romp". Sky and Telescope magazine . Retrieved 2020-02-12. The most stringent test of telescopic resolution uses the criteria of Dawes' limit given by the equation R = 4.56/D, where R is the resolution in arcseconds and D is the telescope's aperture in inches.
2. Dawes, W.R., Catalogue of Micrometrical Measurements of Double Stars. In: Memoirs of the Royal Astronomical Society, Vol. 35, p.137 1867, Bibcode : 1867MmRAS..35..137D