Sketch of a frame-transfer CCD and scenario for the dark signal build-up. A frame-transfer CCD, such as SODISM’s, has two zones, IZ and MZ, which have (quasi) the same format. During the CCD integration time, the signal is generated and stored in the IZ on the right side of the figure. This is when the shutter can be opened and the device exposed to observe the Sun. This is also when the IZ component of the dark signal accumulates at a rate of . The image frame is then transferred to the MZ. At this occasion, the signal charges that were generated at coordinates (i,j) of the IZ are moved to (i,j) of the MZ. During the next and last phase, the readout phase, the CCD lines (represented vertically here) are transferred one by one to the serial register. The readout phase lasts 22 s, which is sufficient to accumulate a significant amount of extra dark signal while sojourning in the i pixels (k,j) of the MZ.