Schematic setup: The spectrometer features two gratings, which are selectable under vacuum conditions and cover the whole wavelength region of the FLASH fundamental. Compared to alternative spectrometers based on the ionization of gases targets, grating spectrometers are free of space charge and non-linear effects. The dispersed FEL beam hits a Ce:YAG crystal and generates a fluorescence of visible light. The fluorescence is imaged and magnified via an out of vacuum lens system onto the detection area of the detector. [2] Reproduced from S. Palutke et. al., Rev. Sci. Instrum. 86, 113107 (2015), with the permission of AIP Publishing.