It is not foreseen to perform absolute in-orbit effective area measurements. However the vignetting function of the telescope has been verified on constant X-ray sources such as cluster of galaxies and supernovae remnants. Also, contamination effect if any during launch and telescope tube outgazing phase could induce CK and OK absorption edges as well as a modification of the mirror reflectivity close to the Au M absorption edges. Such effect were not detected in the spectra of lineless BL LAC objects. Bright BL LAC object with hard spectra have been used for that purpose in order to obtain a high enough ratio signal to noise in the 2.2 to 3.4 keV range of the Au M edges.