code: def _open_fits_files(filenames):
      
      kw = { 'mode' : 'update', 'memmap' : False }
  
      def _fh(fn):
          """ Returns a filehandle or None if file does not exist """
          return fits.open(fn, **kw) if os.path.exists(fn) else None
  
      return collections.OrderedDict(
              (corr, tuple(_fh(fn) for fn in files))
          for corr, files in filenames.iteritems() )
docs: Given a {correlation: filename} mapping for filenames
      returns a {correlation: file handle} mapping