code: def _to_fits(self, uniq, optional_kw_dict=None):
          
          depth = self.max_order
          if depth <= 13:
              fits_format = '1J'
          else:
              fits_format = '1K'
  
          tbhdu = fits.BinTableHDU.from_columns(
              fits.ColDefs([
                  fits.Column(name='UNIQ', format=fits_format, array=uniq)
              ]))
          tbhdu.header['PIXTYPE'] = 'HEALPIX'
          tbhdu.header['ORDERING'] = 'NUNIQ'
          tbhdu.header.update(self._fits_header_keywords)
          tbhdu.header['MOCORDER'] = depth
          tbhdu.header['MOCTOOL'] = 'MOCPy'
          if optional_kw_dict:
              for key in optional_kw_dict:
                  tbhdu.header[key] = optional_kw_dict[key]
  
          thdulist = fits.HDUList([fits.PrimaryHDU(), tbhdu])
          return thdulist
docs: Serializes a MOC to the FITS format.
  
          Parameters
          ----------
          uniq : `numpy.ndarray`
              The array of HEALPix cells representing the MOC to serialize.
          optional_kw_dict : dict
              Optional keywords arguments added to the FITS header.
  
          Returns
          -------
          thdulist : `astropy.io.fits.HDUList`
              The list of HDU tables.