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.
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