Circular Aperture

In this case, the diffraction pattern has a central maximum whose angular width (determined by the radius of the first dark fringe) is given by

q_min= 1.22l/D

where D is the diameter of the circular aperture. An example of such an aperture is the input aperture of a telescope, hence this simple relation can be used to estimate the resolution of optical instruments. In this regard, the Rayleigh Criterion says that two distant objects can just be resolved if their diffraction patterns are separated by at least q_min .