Background subtraction in the camera images. Top: instrumental flux in 5.65 mm apertures measured in dark images as a function of the background light proxy build from the never exposed region of the sensor. The best-fit linear relation (solid black line) gives an estimate of background contribution in exposed images. Bottom: residuals to the best-fit relation. The measured RMS gives the level of the background subtraction noise which is the dominant noise contribution in our QE measurement.