  The International X-ray Observatory (IXO) is a very ambitious mission, aimed
at the X-ray observation of the early Universe. This makes IXO extremely
demanding in terms of effective area and angular resolution. In particular, the
HEW requirement below 10 keV is 5 arcsec Half-Energy Width (HEW). At higher
photon energies, the HEW is expected to increase, and the angular resolution to
be correspondingly degraded, due to the increasing relevance of the X-ray
scattering off the reflecting surfaces. Therefore, the HEW up to 40 keV is
required to be better than 30 arcsec, even though the IXO goal is to achieve an
angular resolution as close as possible to 5 arcsec also at this energy. To
this end, the roughness of the reflecting surfaces has to not exceed a
tolerance, expressed in terms of a surface roughness PSD
(Power-Spectral-Density). In this work we provide such tolerances by simulating
the HEW scattering term for IXO, assuming a specific configuration for the
optical module and different hypotheses on the PSD of mirrors.
