What is the Rayleigh criterion?

The Rayleigh criterion defines the minimum resolvable detail in an imaging process. Two images are resolvable when one’s diffraction center pattern is directly over the other image’s first diffraction pattern.

In order for two points to be resolvable, they have to be separated by a minimum angle that is defined as θ=1.22λ/D, where λ is the wavelength of the light, D is the diameter of the aperture through which the two points are being viewed and θ is the smallest angle between the two points. This criterion is useful in the design of imaging equipment, such as the Hubble Space Telescope.