Characterization of the FERMI@Elettra's on-line photon energy spectrometer

FERMI@Elettra is a Free Electron Laser (FEL) under commissioning at Sincrotrone Trieste. It will provide an almost fully coherent and transform limited radiation with a very high brilliance in the VUV/Soft X-ray range. This article describes the working principles of the Variable Line Spacing diffraction gratings applied to the photon energy spectrometer as well as the design concept, ray tracing and efficiency simulations. Metrological results at various spatial frequencies of the optics involved and the first characterization results with FEL radiation will be shown.