Creating a dimming detection mask. Percentage running difference images (PRD) and normal RD images are binarized using both high and low thresholds. The resulting low threshold and high threshold masks are combined by keeping only the larger clusters from the low threshold mask that intersect with the high threshold mask. These RD and PRD masks are combined into a potential dimming mask (PDM) by only keeping those clusters from the RD mask that intersect with clusters in the PRD mask. Finally, to reduce the number of false detections this PDM is refined by discarding clusters that do not have an overlap with clusters in the previous PDM. This means that the dimming characterization process will only be triggered by events that last longer than one image. As an example, this procedure is illustrated here for October 23, 2012, 07:42 UTC.