Normalized histogram of the Jul. 2010–Nov. 2012 maps reconstructed by the dark signal model for the image zone. The histogram mode represents the cool pixels. It can be fitted by a lognormal function (blue dotted line), which gives 2.39 ADU  pxl^-1 s^-1 ≈ 4.1 e⁻ pxl^-1 s^-1. The first small shoulder around 6−20 ADU  pxl^-1 s^-1 comprises the pixels that are polluted by some spurious signal in the bottom of the image. The second shoulder describes the hot pixel dark current distribution. It appears to be flat from 10–20 ADU  pxl^-1 s^-1 to 200 ADU  pxl^-1 s^-1. The hottest pixels deliver 2000 ADU  pxl^-1 s^-1 ≈ 3.5 ke⁻ pxl^-1 s^-1.