Manufacturing and testing of the X-ray collimating mirror for the BEaTriX facility

The BEaTriX (Beam Expander Testing X-ray) facility under construction at INAF-Brera Astronomical Observatory aims at performing the acceptance tests of the Silicon Pore Optics mirror modules of the ATHENA (Advanced Telescope for High-ENergy Astrophysics) X-ray observatory. The facility implements a grazing-incidence collimating mirror that, together with a monochromator and a beam expander stages based on crystals, enables the full X-ray illumination of the mirror modules under test. We present the development and test of the collimating mirror, a paraboloid sector having an optical surface of 400 mm ´ 60 mm and sagittal radii of about 155 mm. The ground and lapped optics made of HOQ 310 fused quartz was corrected by bonnet polishing. In this paper, we report on the smoothing of mid-to-high spatial frequency error by pitch-tool polishing process, and on the correction of residual surface shape errors by ion-beam figuring process, both performed at INAF-Brera Astronomical Observatory. We present the X-ray test campaigns carried out on the mirror at PANTER facility, before and after coating it with a Pt layer via magnetron sputtering at DTU Space. The results provide an overview of the mirror performance in terms of angular resolution pre- and post-coating deposition.