Calibration of the sky background count rate between XMM-Newton and ROSAT-PSPC. Left panel: Relation between the sky background count rate as measured using the XMM-Newton mean SX profiles and ROSAT-PSPC in the R5, [0.56-1.21] keV, energy band. The black points represent the clusters for which is less than 12 arcmin and that have been used to fit the relations. The grey points are the clusters filling the field of view, their being greater than 12 arcmin. The solid line represents the cross correlation obtained via the linear regression. The dashed lines represent the intrinsic scatter of the relation. Right panel: Same as the left panel except for the fact that the XMM-Newton count rate is measured using the median SX profiles.