The DN value passed out of the camera when there is no light incident on the CCD and the dark current generation is zero will not be 0 DN. The minimum level is set electronically to be slightly higher, plus the spurious charge during readout causes another offset. The following are the approximate DN levels for no light incident on the cooled CCD.
This means that if you have one raw image (which has not had the dark current image removed) with an average signal of 15 DN and another with 20 DN, that is not a 25% difference, it is over 300% different. This offset must be watched carefully when working with low signal levels, even after the dark image has been removed.
The above values are appropriate for data acquired prior to 13-Nov-92. After the entrance filter failure, all values increased 1-2 DN due to a change in the spurious charge. NOTE: A separate effect is that the dark current has been increasing gradually during the mission, and will continue to do so, which is caused by X-ray damage.
DARK_SUB (which is what SXT_PREP calls), will remove a single dark frame which is closest in exposure level to the input image. When removing that single dark image, it takes off the digital offset, plus the dark current (exposure dependent) portion. An even better method is to use the /DC_INTERPOLATE option, which will interpolate two dark images to correct for the exposure dependent dark current, and will also remove the digital offset.