code: def find_npolfile(flist,detector,filters):
      
      npolfile = None
      for f in flist:
          fdet = fits.getval(f, 'detector', memmap=False)
          if fdet == detector:
              filt1 = fits.getval(f, 'filter1', memmap=False)
              filt2 = fits.getval(f, 'filter2', memmap=False)
              fdate = fits.getval(f, 'date', memmap=False)
              if filt1 == 'ANY' or \
               (filt1 == filters[0] and filt2 == filters[1]):
                  npolfile = f
      return npolfile
docs: Search a list of files for one that matches the configuration
          of detector and filters used.