def find_DQ_extension(self):
        """ Return the suffix for the data quality extension and the name of
        the file which that DQ extension should be read from.

        """
        dqfile = None
        # Look for additional file with DQ array, primarily for WFPC2 data

        indx = self._filename.find('.fits')

        if indx > 3:
            suffix = self._filename[indx-4:indx]
            dqfile = self._filename.replace(suffix[:3],'_c1')

        elif indx < 0 and len(self._filename) > 3 and \
             self._filename[-4] == os.extsep and \
             self._filename[-1].lower() == 'h':

            # assume we've got a GEIS file
            dqfile  = self._filename[:-2]+'1'+self._filename[-1]
            hdulist = readgeis.readgeis(dqfile)
            prih    = hdulist[0].header
            if 'FILETYPE' in prih:
                dq_suffix = prih['FILETYPE'].strip().upper()
            else:
                # assume extension name is 'SDQ' for WFPC2 GEIS files
                dq_suffix = 'SDQ'
            hdulist.close()
            return dqfile,dq_suffix

        else:
            raise ValueError("Input file {} does not appear to be neither " \
                        "a FITS file nor a GEIS file.".format(self._filename))

        if os.path.exists(dqfile):
            dq_suffix = fits.getval(dqfile, "EXTNAME", ext=1, memmap=False)
        else:
            dq_suffix = "SCI"

        return dqfile, dq_suffix