Coherence properties of the radiation from SASE FEL

We present a comprehensive analysis of coherence properties of radiation from a SASE FEL. The effective transverse correlation function is calculated by means of numerical simulations with the code FAST. This allows us to calculate the area and degree of transverse coherence. Evolution of these parameters is traced from the beginning of the undulator up to the deep nonlinear regime. It is shown that the degree of transverse coherence reaches maximum value at the saturation point. Then, it drops drastically because of poor transverse coherence of the radiation produced in the nonlinear regime.