  The Beam Expander Testing X-ray facility (BEaTriX) is a unique X-ray
apparatus now operated at the Istituto Nazionale di Astrofisica (INAF),
Osservatorio Astronomico di Brera (OAB), in Merate, Italy. It has been
specifically designed to measure the point spread function (PSF) and the
effective area (EA) of the X-ray mirror modules (MMs) of the Advanced Telescope
for High-ENergy Astrophysics (ATHENA), based on silicon pore optics (SPO)
technology, for verification before integration into the mirror assembly. To
this end, BEaTriX generates a broad, uniform, monochromatic, and collimated
X-ray beam at 4.51 keV. [...] In BEaTriX, a micro-focus X-ray source with a
titanium anode is placed in the focus of a paraboloidal mirror, which generates
a parallel beam. A crystal monochromator selects the 4.51 keV line, which is
expanded to the final size by a crystal asymmetrically cut with respect to the
crystalline planes. [...] After characterization, the BEaTriX beam has the
nominal dimensions of 170 mm x 60 mm, with a vertical divergence of 1.65 arcsec
and a horizontal divergence varying between 2.7 and 3.45 arcsec, depending on
the monochromator setting: either high collimation or high intensity. The flux
per area unit varies from 10 to 50 photons/s/cm2 from one configuration to the
other. The BEaTriX beam performance was tested using an SPO MM, whose entrance
pupil was fully illuminated by the expanded beam, and its focus was directly
imaged onto the camera. The first light test returned a PSF and an EA in full
agreement with expectations. As of today, the 4.51 keV beamline of BEaTriX is
operational and can characterize modular X-ray optics, measuring their PSF and
EA with a typical exposure of 30 minutes. [...] We expect BEaTriX to be a
crucial facility for the functional test of modular X-ray optics, such as the
SPO MMs for ATHENA.
