• C. Behrens, C. Gerth
  DESY, Hamburg

The capability of the free-electron laser (FEL) user facility FLASH at DESY was expanded by several upgrades during the shutdown in 2009/2010. A key extension is the installation of a third-harmonic (3.9 GHz) RF system for the linearization of the longitudinal phase space in front of the bunch compressors. In order to control the bunch compression and make full use of the third-harmonic RF system, a new diagnostic section for the measurements of sliced bunch parameters directly in front of the undulators was designed and commissioned. In this paper, we describe the beam imaging systems and their optical performance. The achievable resolution of both time and energy is shown and compared to the design values. First measurements of the linearized longitudinal phase space with high resolution are presented.