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.