X-ray scatter measurements from thermally slumped thin glass substrates for the HEFT hard X-ray telescopes

We have performed X-ray specular reflectivity and scattering measurements of thermally slumped glass substrates on X-ray diffractometers utilizing a rotating anode X-ray source (8.048 keV) at the Danish Space Research Institute (DSRI) and synchrotron radiation (18 & 28 keV) at the European Synchrotron Radiation Facility (ESRF) optics Bending Magnet (BM05) beamline. In addition, we tested depth graded W/Si multilayer-coated slumped glass using fl-ray specular reflectivity measurements at 8.048 keV (DSRI) & 28 keV (ESRF) and energy-dispersive measurements in the 20-50 keV range at a double-axis diffractometer at the Orsted Laboratory, University of Copenhagen. The thermally slumped glass substrates will be used to fabricate the hard X-ray grazing incidence optics for the High-Energy Focusing Telescope (HEFT).(7,8) We compared the measurements to the SODART-mirrors(1) from the SRG(17) telescope mission program. The surface scatter measurements of the thermally slumped glass substrates yields Half Power Diameters (HPD's) of single-bounce mirrors of full-illuminated lengths of similar to 40 arcseconds for typical substrates and as low as similar to 10 arcseconds for the best substrates, whereas the SODART mirrors yields HPD's of similar to 80 arcseconds with very little variation. Both free-standing glass substrates and prototype mounted and multilayer-coated optics were tested. The results demonstrate that the surface scatter contribution, plus any contribution from the mounting procedure, to the Half Power Diameter (HPD) from a telescope using the slumped glass optics will be in the subarcminute range. In addition we measured low surface microroughness, yielding high reflectivity, from the glass substrates (similar to 2 Angstrom), as well as from the depth graded W/Si multilayer-coated glass (interfacial width similar to 4.2 Angstrom).