It is quite typical example for a text to mention Airy disc (a diffraction patten for a circular aperture), also in wiki http://en.wikipedia.org/wiki/Airy_disc. But what wiki confusing me is , in the mathematical details section, the intensity is given by J1(x)/x, where J1 is the first order Bessel function, it is ZERO around x=0. But the Airy disc has a maximum in the center, so how can on use J1(x)/x to describe an Airy disc? And I saw some introduction on using a circular slit (annulus) to produce a diffraction pattern and the it is said that annulus will produce a bessel beam on the screen. Any one know if this is true and where can I find some information on this?