 Uniform-disk spatial coherence

For a monochromatic uniformly bright incoherent disk of angular diameter $\delta$, the mutual coherence at baseline $b$ is proportional to $2J_1(\pi \delta b/\lambda )/(\pi \delta b/\lambda )$. The first zero is $b\simeq 1.22\lambda /\delta$. A nonuniform disk or finite spectral band changes the detailed curve.