Critical aspects in the development of the 1.49keV channel for the BEaTriX x-ray calibration facility

BEaTriX is a compact X-ray facility operated at the INAF-Brera Astronomical Observatory in Merate, Italy. It is designed and built to test the angular resolution and effective area of the SPO Mirror Modules, which form the grazing-incidence telescope of the NewAthena X-ray Space Observatory. BEaTriX has been conceived to accommodate two energy channels: the 4.51 keV beamline, which is already operational since 2022, and the 1.49 keV beamline, currently under installation. Both beamlines are based on the combination of symmetrically and asymmetrically cut Bragg crystals for monochromation and expansion of the beam; these, together with a paraboloidal grazing incidence mirror, provide a low-divergence (within a few arcsec) and wide (170 × 60 mm2) beam. However, the 1.49 keV channel presents more challenging aspects. As this beamline operates in the soft X-ray regime, the well-known silicon crystals used on the first beamline cannot be used, and Bragg crystals with larger d-spacings are needed. Accordingly, we adopted a system based on two asymmetrically cut Quartz (10-10) crystals for the monochromator and two asymmetrically cut ADP (Ammonium Dihydrogen Phosphate) (101) crystals for the horizontal beam expansion stage. The crystals were characterized at INAF-OAB, IMEM-CNR (Parma, Italy), and ESRF (Grenoble, France). A particular emphasis was placed on the measurement of the planarity of internal lattice planes, as the requirements are very tight (tens of kilometers of radius of curvature). Measurements performed at the diffractometer at IMEM-CNR and at the ESRF synchrotron revealed that the instrument’s capabilities were approaching their limits, thereby highlighting previously unidentified systematic errors. This paper reports on the characterization of the crystals and the impact on the BEaTriX X-ray imaging performance, simulated using both OASYS-SHADOW4 software and the IDL reference code. The characterization of the ADP crystals revealed that systematic errors were causing a misleading curvature. The correction of these errors resulted in the redesign of the initial bending system, which was conceived to compensate for the crystal planes curvature of a single crystal, into an alignment tilting system for both crystals.