a) Measured photon transfer curve of amplifiers #1 to #8 of the CCD E2V-250 expressed in ADU. b) PTCs are converted in e⁻ using the readout gain measured as the inverse of the slope at the origin (see Sect. 4.3). Because departure from linearity of the PTCs is similar for different amplifiers this indicates that the cause of the effect is to be found in the CCD itself rather than in the electronic readout.