BEaTriX: a facility for testing the modular elements of ATHENA

Top: conceptual design of the mirror structure - left, mirror assembly and accommodation - right, mirror modules and stacks (Credit ESA, Cosine and ACO Team) - Bottom: artistic view of Athena Telescope (Credit ESA).

BEaTriX (Beam Expander Testing X-ray facility) is a laboratory installed at INAF-OAB / Merate.
It is a unique pathfinder facility with a parallel (divergence ∼ 2 arcsec HEW), uniform, wide (170 x 60 mm2) and monochromatic (FWHM = 0.03 eV) X-ray beam, at the energies of 1.49 and 4.51 keV.
Currently, the first line at 4.51 keV is completed.
The beam is produced by a microfocus X-ray source, a paraboloid mirror, two symmetrical (monochromators) silicon channel-cut crystals, an asymmetrical crystal acting as beam expander, following the idea originally proposed by F. Christensen, A. Hornstrup, P. Frederiksen, S. Abdali, P. Grundsoe, H. Schnopper, Expanded beam x-ray optics calibration facility at the Daresbury Synchrotron, Proc. SPIE 2011, 540-548 (1994).
The beam propagates in vacuum (10-6 mbar). The small size (8x14 m^2) and modular design allow for short pump-down times. To avoid contamination, oil-free pumps are used, and the experimental chamber opens onto an ISO5 clean tent.
The main objective is to demonstrate that acceptance tests (PSF and effective area) of the optical components of the ATHENA telescope can be performed at their production rate. The system allows a certain flexibility to ensure the use also for X-ray optics with different dimensions and focal lengths.

ATHENA (Advanced Telescope for High-ENergy Astrophysics) is the second Large mission selected by ESA within the Cosmic Vision Program, with launch foreseen in early 2030s. The optics consists of a large aperture X-ray mirror with a diameter of 2.4 m, effective area of 1.4 m2 at 1 keV, and half-energy width (HEW) of 5 arcsec at 1 keV.
The ambitious requirements of the ATHENA mission can only be met with a novel X-ray optics technology.
The X-ray optics of this future space observatory are based on the Silicon Pore Optics (SPO) technology, developed at Cosine (, The Netherlands). To create the aperture of such a large X-ray telescope, a modular approach is foreseen, where each step of the process has to be followed by dedicated tests and calibration procedures.
In a typical SPO manufacturing process, 38 silicon plates with rectangular grooves are stacked, with dedicated robotic machines. Two stacks are aligned to form an X-ray Optical Unit (XOU), working in double reflection and so reconstruct the Wolter-I focusing geometry. Two XOUs are aligned and integrated in parallel to form an SPO Mirror Module (MM). The alignment is performed using synchrotron radiation at the XPBF 2.0 beamline at the BESSY II facility in Berlin, Germany. The MMs are, at present, characterized at XPBF 2.0 and at the 123 m-long PANTER facility of MPE near Munich. Both facilities cannot be used to routinely test 600 MMs for acceptance and integration into the Mirror Assembly Module (MAM). The BEaTriX facility was designed as a compact (18 m x 9 m) screening facility with fast vacuum pump-down, and a broad beam to fully illuminate the aperture of the MM under test. This makes BEaTriX different from other light sources, such as synchrotrons, where the X-ray beam is very narrow and needs to be scanned throughout the MM aperture.

Banner image (by Stefano Basso): BEaTriX facility - INAF Brera Astronomical Observatory site of Merate (LC) - Web site credit: B. Salmaso, D. Spiga, and the BEaTriX team - M.R. Panzera