Mapping the PSF-ZP shift as a function of time and position on the ACS detector, with two different diagnostics (see text). The first (left panel) is the difference between the median magnitudes  ⟨ m ⟩ _beg and  ⟨ m ⟩ _end measured in frames taken respectively at the beginning and the end of the orbit, while the second (right panel) is the difference between the median magnitude of the star  ⟨ m ⟩ ^7th measured during the seventh “visit” of the program (2 453 451 < JD < 2 453 52), and the median magnitude  ⟨ m ⟩  of its full light curve. Top panel: all the high-S/N light curves (σ_m < 0.02) have been registered to their average magnitude  ⟨ m ⟩ . In all panels, the color scale spans the range  −0.02−0.02 mag from black to red.