Demonstration of the quality of the modelling of the MOS1/pn cross-calibration bias for A1795 using Method 1 (left panel) and Method 2 (right panel). When applying Method 1, we evaluate Eq. (3), i.e. we approximate the cross-calibration bias after the convolution (blue crosses in the upper-left panel). The best-fit model (Eq. (4)) to such data is shown as a red line. When applying Method 2, we fit the MOS1 spectra (blue symbols in the upper-right panel; the statistical uncertainty is smaller than the plot symbol) with a model consisting of a fixed component for accurate modelling of the pn data (see the main text for details), while the cross-calibration bias is modelled as a fourth-order polynomial (the total model is shown as a red line in the upper-right panel; see Eq. (5)). In both cases, the lower panel shows the ratio of the best-fit model to the data.