 Broadband planetary coherence estimate

For an unresolved incoherent luminous disc of angular diameter $\alpha$, transverse optical coherence has scale $\lambda /\alpha$; a uniform circular brightness profile puts its first zero at $1.21967\lambda /\alpha$. A receiver transmitting a continuous frequency band of width $\Delta \nu$ instead has temporal coherence scale $1/\Delta \nu$ and longitudinal length $c/\Delta \nu$. Angular source size and spectral filtering control different correlations. Atmospheric phase fluctuations and instrument averaging can further reduce measured fringe visibility.