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Figure 3
(a) Schematic diagram of the spectrograph. For simplicity the horizontal components of the Montel mirrors are omitted. Photons scattered from a sample are collected and collimated by a Montel mirror (MC) before being further collimated (C) and dispersed (D) by two Ge(111) crystals with asymmetry angles α2 = −38.8° (b2 = −1/18) and α3 = −α2 (b3 = −18). The acquired energy dispersion of the beam is then mapped onto a position-sensitive detector using a focusing Montel mirror (MF). This effect is presented as red, green and blue lines hitting the detector at three different positions. Ray-tracing simulations follow the phase space of photons at positions marked with black lines: (b) non-dispersive photons exiting MC, (c) after the C-crystal, which introduces a dispersion rate [{\cal D}_{\rm{C}}] = 0.3 µrad meV−1, and (d) after the D-crystal, where the dispersion rate increases to [{\cal D}_{\rm{D}}] = 11 µrad meV−1.

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