The Automatic Exposure Control of the SXT is described in the ``Red Book'' and briefly in the Reference Guide. One aspect of the AEC algorithm is that if the image is overexposed, and the AEC software has reduced the exposure to the lowest exposure level possible, then the logic could insert a ``thicker'' filter to avoid saturation. This option is enabled or disabled by the sequence table. There has been confusion on occasion when the filter has changed for a given sequence ``slot,'' even when there has been no table upload, or change in DP mode or DP rate. In most cases, the change in filter was due to AEC Filter Alternation.
The bottom line is that if exposure and ND filter adjustment do not suffice to maintain the desired exposure a thicker or thinner filter could be selected. Beware of this when analyzing flare data and an unexpected filter suddenly appears in the sequence!