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Abstract
Radiation from the SASE FEL operating in the linear regime holds properties of completely chaotic polarized light. Measurements of the SASE FEL gain curve allows to determine saturation length which is strictly connected with coherence time. Statistical analysis of the fluctuations of the radiation energies measured with different spatial apertures allows one to determine the number of the longitudinal and transverse modes. Thus, with these simple measurements it becomes possible to determine the degree of transverse coherence, coherence time, and photon pulse duration. In this report we present theoretical background and experimental results obtained at free electron laser FLASH.