Sparrow's resolution limit

[1] When a star is observed with a telescope, the light is diffracted or spread apart into an Airy disk.

Most astronomers say they can still distinguish the two stars when they are closer than Rayleigh's resolution limit.

Sparrow's Resolution Limit is reached when the combined light from two overlapping and equally bright Airy disks is constant along a line between the central peak brightness of the two Airy disks.

The same reasoning applies to the resolution of two wavelengths in a spectroscope, where lines of emission or absorption will have a diffraction induced width analogous to the diameter of an Airy disk.

Sparrow's resolution limit was derived in 1916 from photographic experiments with simulated spectroscopic lines and is most commonly applied in spectroscopy, microscopy and photography.