Study of the transverse coherence at the TTF free electron laser

Double slits with different separations, crossed slits and circular apertures have been used to study the transverse coherence of the VUV light of the SASE Free Electron Laser at the TESLA Test facility at DESY. The resulting diffraction patterns are converted to visible light by a Ce:YAG crystal and imaged by a high-resolution CCD camera. The visibility of the diffraction patterns indicates a high degree of transverse coherence. Measurements have been taken at various operating modes and wavelengths of the FEL. A numeric FEL simulation code has been used to calculate the wavefronts of the light at the exit of the undulator. By propagating the wavefronts through the optical setup, the diffraction at the double slits is computed with the code GLAD. Good agreement with the measurements is found.