a) Temporal evolution of corrected dark frame histograms. Dark frames with a weekly regularity and T′ = 7.4 s integration time have been selected. Their histogram is coded with a logarithmic gray scale. Most pixels display a corrected signal near zero, as expected. b) Average histogram of all above dark frames. Both axes have log scales. The number of occurrences corresponding to negative signals are represented by the dashed line. The fitted Gaussian function is overplotted with a blue dotted line. Its center (the model bias), its standard deviation (model error), and its full width at half maximum (FWHM) is worth ~3.1 ADU ~14.7 ADU, and ~34.6 ADU, respectively.