Future large X-ray observatories will be equipped with very large optics obtained by assembling modular optical
elements. The final quality of the modular optic is determined by the accuracy in the assembly alignment, but also by the
compliance of the focusing elements to the nominal shape and the roughness tolerance in order to avoid excessive levels
of X-ray scattering. Because of the large number of modules, quality tests need to be routinely performed to assess the
technology readiness, and, in a later phase, to select the most performing stacked blocks to be integrated into the final
optic. Besides the usual metrology based on profile and roughness measurements, a direct, at-wavelength, focusing
measurement in X-rays would be the most reliable test. Synchrotron light beams are in general not sufficiently broad to
cover the aperture of a block without scanning it, which requires a focal spot reconstruction. To this end, we designed a
12 m long X-ray facility to be realized at INAF/ OAB, devoted to the functional tests of the focusing elements. A grazing
incidence parabolic mirror and an asymmetric Silicon crystal will produce a wide, parallel, and uniform beam of X-rays
to illuminate the entire aperture of the focusing elements. A X-ray camera at the focal distance from the mirrors directly
records the image. The tests will be performed at 4.5 keV, with the components operating in gaseous Helium to
minimize the absorption.
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