Left: features of a CCD quadrant. This quadrant from a raw bias image shows (i) a noticeable jump in the readout pedestal during readout (with more subtle variations observable throughout the readout), (ii) a top-down intensity gradient indicative of light injection during readout, and (iii) an electronic ringing effect on the left at the start of each line readout. Right: high-S/N master bias frame (median of tens of raw images) used to process all ERO data. The brightness variation here does not exceed 0.6 ADU, while the typical raw ERO signal – zodiacal light background – is around 40 ADUs, necessitating this correction.