Comparing the SAS-produced 2D PSFs and default PSFs with true source data. Left to right: (1) the stacked counts in the data; (2) the stacked counts in the model; (3) a χ²-like statistic; (4) r, a measure of whether the model consistently (white) under- or (black) overestimates the data (see text for all details), for (top) the 2D PSF and (bottom) the default PSF. The colourbar corresponds to the χ² plot. The 30 radial bins are each 4″ wide, and the 32 azimuthal bins are each 11.25° wide, and arranged to be alternately fully on-spoke, then fully off-spoke. The example shown is for MOS2, on-axis, 0.5−1 keV.