Prior to the execution of the background determination algorithm, the fluxes and flags of each Pass 1 swath are evaluated to ensure that enough valid pixels exist in a swath to produce a reliable fit to the data. Any pixels containing flags with excessively negative ITF extrapolations or cosmic ray hits are added to a "pixels-to-avoid" working template for each Pass 1 swath. A few checks are then made to ensure that the background-determining algorithm will run properly. For example, if too few valid pixels can be found in a given Pass 1 swath to permit a reliable background extraction, the solution is set to a zero-flux array. If this is not repeated for the next swath, then the solution for the zeroed-out swath is reset to the mean of the two adjacent swaths.