SXT has automatic exposure control (AEC) available for partial frame images. The algorithm works on a per observing region basis and runs separately for each ``slot'' shown in the sequence above (image 3-3 exposure control is independent of image 5-3, even if they are for the identical filter and location). An upper and lower intensity, or Data Number (DN), threshold is set and the number of pixels above and below that threshold is set. An allowable number of pixels above and below is also set, and if the image exceeds this set level, then the exposure duration is shortened or lengthed, depending on whether it is over exposed or under exposed. See the ``Red Book'' article for more details on the exposure control.