Photon transfer curve of channel 0 fitted with three different models: the full model, a quadratic fit, and the “simple model” with b = 0. The bottom plots display the fit residuals. The full model and the b = 0 model describe well the data, although the χ² is 10% higher for the latter. We believe that part of the fit residuals are artifacts caused by the non-linearity and deferred charge corrections.