The structure of an image formed by a circular aperture was first formulated by George Airy.1 In a refractor, the effect of diffraction on the image of a star in the focal plane is to produce a series of faint concentric rings around the central disk, called the Airy disk.
The diameter of the central peak of the Airy disk is:
Looking at a star, most of the light (84%) goes into this central disk inside the first dark ring. The intensity of the first bright ring is 7% of the total light contained within the star image. The second bright ring is only 3% of the total light, with the remaining 6% being distributed in the outer rings.
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