  Future X-ray telescopes like SIMBOL-X will operate in a wide band of the
X-ray spectrum (from 0.1 to 80 keV); these telescopes will extend the optical
performances of the existing soft X-ray telescopes to the hard X-ray band, and
in particular they will be characterized by a angular resolution (conveniently
expressed in terms of HEW, Half-Energy- Width) less than 20 arcsec. However, it
is well known that the microroughness of the reflecting surfaces of the optics
causes the scattering of X-rays. As a consequence, the imaging quality can be
severely degraded. Moreover, the X-ray scattering can be the dominant problem
in hard X-rays because its relevance is an increasing function of the photon
energy. In this work we consistently apply a numerical method and an analytical
one to evaluate the X-ray scattering impact on the HEW of an X-ray optic, as a
function of the photon energy: both methods can also include the effects of
figure errors in determining the final HEW. A comparison of the results
obtained with the two methods for the particular case of the SIMBOL-X X-ray
telescope will be presented.
