X-ray evidence for particulate contamination on the AXAF VETA-1 mirrors

X-ray testing of the AXAF outer mirror pairs, in the VETA-1 configuration, reveals a point spread function (PSF) with unexpectedly large wings at low energies. Although the angular dependence in the wings of the PSF is close to that expected for diffractive scattering from surface roughness, the energy dependence differs substantially. Analyses of the observed x-ray PSF, images near ring focus, and single-quadrant images at conjugate focus suggest that the excess scattering observed at low x-ray energies results from diffractive scattering by relatively small grains (as small as a few tenths micrometer in radius). Subsequent visual inspection of the mirror (under grazing incidence illumination) and scanning electron microscopy of tape lifts from the mirrors provides qualitative support for this conclusion. We develop a simple model for the contribution of scattering by particulates to the PSF. Merging this model with that for scattering by surface roughness, we fit the combined model to the observed energy-dependent PSF, in order to estimate parameters and associated uncertainties characterizing the grain-size distribution and the surface-roughness power spectral density. In particular, we find that the fractional coverage of the mirrors by particulates is approximately 1x104 (for grain radii between 0. 1 jim and 10 j.tm), and that the rms surface-roughness is approximately 0.7 nm (for spatial frequencies between 1 mm1 and 1000 mm1).