Imperfections in the digitization of the CCD output voltage are causing some ADU values to appear more frequently than others, as can be seen in figure 11. The additional noise may be detectable on exposures with very little signal, where the noise normally would be read-out-noise limited. With a few hundred counts or more, the photon noise will dominate the noise from the ADC.
Figure 11:
Upper: Histogram of a flat-field exposure.
Lower: Frequency spectrum of the histogram.
The ADU levels are seen to be modulated, in particular every fourth
and eighth value
seems to appear with increased frequency. This is caused by errors in
the digitization process.