Generalized Van Cittert Zernike Theorem in Curved Spacetime - Some   Astrophysical Applications
  The {\em Van Cittert Zernike} theorem consists of a simple relation between the measured coherence of the radiation and the characteristics of the emitting stochastic source. In the present work, an extension of the theorem to partially correlated sources in curved spacetime is derived. The generalized theorem is used to evaluate the changes in the spectrum of light travelling through a gravitational lens. Some possible astrophysical applications are discussed, accompanied by a numerical estimation of the spectrum modification in the process of propagation through a gravitational lens. 
G. Hazak and R. Zamir