Illustration of the I_E magnitude offset with respect to pre-launch estimates of the ZP, per detector, using one exposure on the Perseus galaxy cluster. The horizontal line indicates the average offset, while the green symbols represent stars with a Gaia colour of (G_BP – G_RP) < 1.8. These stars were specifically used to calculate the average offset, highlighting the variations across different detectors and emphasising the impact of stellar colours on the observed magnitudes.