Wave-front observations at FLASH

During the first year of user operation at the Free-Electron Laser in Hamburg (FLASH) wave-front measurements were recorded in the vacuum-ultraviolet region using a Hartmann sensor (by Imagine Optic). The Hartmann principle is based on a hole array, which divides the incoming beam into a large number of subrays monitored in intensity and position of individual spots. The identification of the local slope of the incident wave front makes the aberrations from a perfect spherical wave front visible. Ray tracing in upstream direction accesses the beam path especially the focal spot in size and position. The intense and coherent vacuum-ultraviolet FEL beam leads to unique requirements for the wave-front sensor setup. We report an optimized setup to observe the metrology of flat and curved mirrors at FLASH beam lines. The use of wave-front measurements to provide reliable machine parameter is discussed. The wave-front sensor proved to be a valuable tool to observe the FEL beam quality and the performance of optical elements, filters and diagnostic tools.