This animation shows two star images, i.e. Airy discs, closing and separating. The central discs are surrounded by rings, arising from diffraction at the aperture stop, and have been exaggerated for clarity. Normally it is very difficult to see these rings due to their low brightness relative to the that of the disc and a need for very accurate diffraction limited lenses.

The animation briefly pauses where the discs are roughly separated by the Rayleigh limit of resolution. The sampling and compression of the images has led to some profile distortions and the exactness in the original has been lost. However, you should be able to see that although there is a significant fall in brightness between two star images at the Rayleigh limit of resolution (see figure 2, page 2), and you might have thought an improvement possible, the ability to distinguish two images very soon disappears.

In microscopic circles there is some use of separations slightly reduced from the Rayleigh limit, but in the main most people stick with the Rayleigh criteria and its mathematical convenience.

© Animation prepared by Dr Ken Raine, formerly of the National Physical Laboratory, Teddington, London.