  The imaging performance of X-ray optics (expressed in terms of HEW,
Half-Energy-Width) can be severely affected by X-ray scattering caused by the
surface roughness of the mirrors. The impact of X-ray scattering has an
increasing relevance for increasing photon energy, and can be the dominant
problem in a hard X-ray telescope like SIMBOL-X. In this work we show how, by
means of a novel formalism, we can derive a surface roughness tolerance - in
terms of its power spectrum - from a specific HEW requirement for the SIMBOL-X
optical module.
