## 3 X-ray Photon Beam Transports  
The X-ray optical systems that transport X-ray photons from the undulators to the experiment hall are located in long underground tunnels. From the source point of FEL radiation located within the last segments of the FEL undulators to the scientific instruments in the experiment hall these beam transport paths are up to 1 km long [38; 39]. The key optical elements of each transport system are three mirrors: Mirrors \"1\" and \"2\" create a horizontal offset of the X-ray FEL beam. This offset prevents unwanted background radiation, consisting of Bremsstrahlung and high-energy spontaneous radiation also produced in the long FEL undulator, to be transported into the experiment areas. The spontaneous radiation has a critical energy of typically 200 keV and is not reflected by the offset mirrors, but is rather absorbed by the first mirror or transmitted and then stopped by a massive tungsten beam stop. Only the desired X-ray FEL photons in the energy ranges of 3-25 keV (SAE1 and SASE2) and 0.25-3 keV (SASE3) can pass the offset mirror chicane. Mirror \"3\" (distribution mirror) can be optionally inserted to reflect the photons to the HED, FXE, or SCS instruments, while the undeflected beam passes to the MID, SPB, and SQS instruments, respectively (compare Figure 3). In the case of the FXE, MID and HED instruments, multi-bounce crystal monochromators (optional) are integrated in the beam transport. For SASE3 a grating monochromator has been integrated that can be used by the SCS and SQS instruments at this FEL source. The beam transport layout includes for each of the systems the possibility of integrating a third beamline to a third instrument. Such an extension is currently in preparation at SASE3. The distance from the source point to the first mirror is between \(\sim\)245 and 290 m, which is enough to expand the beam to a size filling the about 1 m long mirrors under grazing incidence angle and thereby reducing damage and heat load effects. In order to handle the demanding power densities, a surface coating with boron carbide is applied on the single crystalline silicon mirrors. A liquid indium gallium eutectic film, which is in contact with water-cooled copper blades, is used to remove excess heat from all mirrors of the beam transport system. Because the X-ray FEL radiation is close to the diffraction limit, its divergence is roughly proportional to the photon wavelength. To utilize the full length of the offset mirrors for all photon energies, their reflection angle can be varied from 1.1-3.6 mrad for the hard X-ray beam transports and from 6-20 mrad for SASE3. The reflectionangles define the energy cut-off and, thereby, the transport of higher harmonic radiation, too. The distribution mirror operates at a fixed angle, which is defined by the distance from the mirror to the experiment hall and the lateral distance (1.4 m) between the shutters of instruments operating at the same SASE beamline. To avoid over-illumination of the distribution mirror, the second offset mirror is bendable and can slightly focus the beam towards the distribution mirror. Alternatively, Be Compound Refracting Lenses (CRL) positioned upstream of the offset mirrors of the SASE1 and SASE2 beam transports can be used to collimate the beam or to produce a similar confocal beam situation with an intermediate focus behind the distribution mirror. The most crucial requirement to the X-ray mirrors is preservation of the almost perfect wave front created by the lasing process [40]. Source properties and source-to-mirror distances at the European XFEL lead to requirements of about 2 nm peak-to-valley shape error for all mirrors of the beam transport systems, corresponding to roughly 50 nrad rms in slope error. The mirrors were manufactured in Japan and Germany by deterministic polishing techniques, where material is iteratively removed on atomic length scales according to a very precise metrology map of the mirror before each polishing step.