  Future large X-ray observatories like ATHENA will be equipped with very large
optics, obtained by assembling modular optical elements, named X-ray Optical
Units (XOU) based on the technology of either Silicon Pore Optics or Slumped
Glass Optics. In both cases, the final quality of the modular optic (a 5 arcsec
HEW requirement for ATHENA) is determined by the accuracy alignment of the XOUs
within the assembly, but also by the angular resolution of the individual XOU.
This is affected by the mirror shape accuracy, its surface roughness, and the
mutual alignment of the mirrors within the XOU itself. Because of the large
number of XOUs to be produced, quality tests need to be routinely done to
select the most performing stacked blocks, to be integrated into the final
optic. In addition to the usual metrology based on profile and roughness
measurements, a direct measurement with a broad, parallel, collimated and
uniform X- ray beam would be the most reliable test, without the need of a
focal spot reconstruction as usually done in synchrotron light. To this end, we
designed the BEaTriX (Beam Expander Testing X-ray facility) to be realized at
INAF-OAB, devoted to the functional tests of the XOUs. A grazing incidence
parabolic mirror and an asymmetrically cut crystal will produce a parallel
X-ray beam broad enough to illuminate the entire aperture of the focusing
elements. An X-ray camera at the focal distance from the mirrors will directly
record the image. The selection of different crystals will enable to test the
XOUs in the 1 - 5 keV range, included in the X-ray energy band of ATHENA
(0.2-12 keV). In this paper we discuss a possible BEaTriX facility
implementation. We also show a preliminary performance simulation of the
optical system.
