## 3 Photon Beamlines
### X-ray Optics
In between both coatings a stripe of uncoated silicon serves as third reflecting area. It has the lowest critical energy of all coatings and may be used at low energies when high harmonic rejection becomes an issue. All mirrors are mounted in benders with two actuators, allowing bending radii from flat to \(\pm\)10 km. By this, the offset mirrors can produce a line focus at the experiment stations. Both beamlines include double crystal monochromators, each containing three pairs of crystals. Two pairs of silicon crystals, Si(111) and Si(311), are foreseen for standard and high resolution applications. An additional pair of InSb(111) crystals extends the wavelength range up to 7 A (1.77 keV). The crystals are mounted on a common Bragg-rotation axis that sets the Bragg angle for both crystals from 5\({}^{\circ}\) to 80\({}^{\circ}\). The translation of the second crystal perpendicular to its surface allows for a constant beam offset of 20 mm as well as for a variable beam offset in the harmonic rejection mode. The second crystals are long enough to omit the translation parallel to their surface and therefore the beam spot moves along the second crystal while the photon energy is scanned. As the average heat load on the crystals is quite low, a side cooling or even intrinsic cooling scheme is not required. Figure 6: Optical layout of the hard X-ray beamlines of the Aramis undulator. The insets show the setup for pink and monochromatic operation of the beamlines Aramis-1 (A,B) and Aramis-2 (C,D), respectively. However, for temperature stabilization the first crystals are mounted on a common water-cooled copper block. The second crystals are temperature stabilized via copper braids connected to this block. To allow for experimental flexibility both beamlines must be able to operate in both monochromatic and broad bandwidth (pink) beam modes with the same beam path downstream of the monochromator. The switching from monochromatic mode to pink beam mode is accomplished in two different ways as sketched in the insets of Figure 6. Aramis-1 has to retract the crystals as well as the HRMs, passing the beam untouched through both vessels, whereas Aramis-2 has to retract the crystals and instead insert the OMs into the beam path. Both methods differ in the number of optical elements that are in the beam at a time. Aramis-2 has always two optical elements in the beam, not counting the retractable refocusing KB mirrors. Aramis-1 utilizes two optical elements in pink beam and six optical elements in monochromatic mode. This rather large number of optical elements became necessary to improve the spectral purity in monochromatic mode. It is accomplished with the help of the two HRMs directly downstream of the DCM-1. Both HRMs are mounted in a fixed distance but the height of the first HRM as well as the beam offset in the DCM are variable. When setting a new deflection angle both mirrors are rotated. Due to the fixed separation of both mirrors, the height of the first HRM and the beam offset must be set to keep the beam with the new deflection angle in the center of the second (fixed height) HRM. A pair of retractable KB mirrors is foreseen for refocusing in both beamlines. They combine achromaticity with a high throughput. The KB mirrors with 500 mm optical length and B\({}_{4}\)C/Mo-coating are bendable and operate with two sets of deflecting angles. Above 4 keV a deflection angle of 8 mrad improves reflectance and below 10 keV a deflection angle of 12 mrad increases the acceptance. By this an acceptance of more than 5\(\sigma\) can be achieved down to 2.5 keV decreasing to 3.5\(\sigma\) at 1.8 keV. The surface quality of the mirrors must be extremely good to reflect the FEL beam without deteriorating the wavefront even at the shortest wavelengths. According to the Marechal criterion, the maximum allowed rms-profile error \(\sigma\) in a beamline with \(N\) mirrors and grazing incidence angles \(\theta\) must not be larger than \(\lambda/(2\times 14\sqrt{N}\sin\theta)\).