Calibration of the XRT-SOLARB flat mirror samples at the XACT Facility of INAF-OAPA

The X-Ray Telescope (XRT) experiment on-board the Japanese satellite SOLAR-B (launch in 2006) is equipped with a modified Wolter I grazing incidence X-ray telescope (focal length 2700 mm) to image the full Sun at ~ 1.5" angular resolution onto a 2048 x 2048 back illuminated CCD focal plane detector. The X-ray telescope consisting of one single reflecting shell is coated with ion beam sputtered Iridium over a binding layer of Chromium to provide nearly 5 square centimetres effective area at 60 Å. We present preliminary results of X-ray calibrations of the XRT flat mirror samples performed at the X-ray Astronomy Calibration and Testing (XACT) facility of INAF-OAPA. We describe the instrumental set-up, the adopted measurement technique, and present the measured reflectivity vs. angle of incidence at few energies.