code: def to_fits(self, filename, wavelengths=None, **kwargs):
          
          w, y = self._get_arrays(wavelengths)
  
          kwargs['flux_col'] = 'Av/E(B-V)'
          kwargs['flux_unit'] = self._internal_flux_unit
  
          # No need to trim/pad zeroes, unless user chooses to do so.
          if 'pad_zero_ends' not in kwargs:
              kwargs['pad_zero_ends'] = False
          if 'trim_zero' not in kwargs:
              kwargs['trim_zero'] = False
  
          # There are some standard keywords that should be added
          # to the extension header.
          bkeys = {'tdisp1': 'G15.7', 'tdisp2': 'G15.7'}
  
          if 'expr' in self.meta:
              bkeys['expr'] = (self.meta['expr'], 'synphot expression')
  
          if 'ext_header' in kwargs:
              kwargs['ext_header'].update(bkeys)
          else:
              kwargs['ext_header'] = bkeys
  
          specio.write_fits_spec(filename, w, y, **kwargs)
docs: Write the reddening law to a FITS file.
  
          :math:`R(V)` column is automatically named 'Av/E(B-V)'.
  
          Parameters
          ----------
          filename : str
              Output filename.
  
          wavelengths : array-like, `~astropy.units.quantity.Quantity`, or `None`
              Wavelength values for sampling.
              If not a Quantity, assumed to be in Angstrom.
              If `None`, ``self.waveset`` is used.
  
          kwargs : dict
              Keywords accepted by :func:`~synphot.specio.write_fits_spec`.