docs: def _hdu_on_disk(self, hdulist_index):
          """
          IRAF routines such as daophot need input on disk.
  
          Returns:
            filename: str
              The name of the file containing the FITS data.
          """
          if self._tempfile is None:
              self._tempfile = tempfile.NamedTemporaryFile(
                  mode="r+b", suffix=".fits")
              self.hdulist[hdulist_index].writeto(self._tempfile.name)
          return self._tempfile.name