Write a catalog file of the likelihood region including
        membership properties.

        Parameters:
        -----------
        filename : output filename
        
        Returns:
        --------
        None

def write_membership(self,filename):
        """
        Write a catalog file of the likelihood region including
        membership properties.

        Parameters:
        -----------
        filename : output filename
        
        Returns:
        --------
        None
        """
        # Column names
        name_objid = self.config['catalog']['objid_field']
        name_mag_1 = self.config['catalog']['mag_1_field']
        name_mag_2 = self.config['catalog']['mag_2_field']
        name_mag_err_1 = self.config['catalog']['mag_err_1_field']
        name_mag_err_2 = self.config['catalog']['mag_err_2_field']

        # Coordinate conversion
        #ra,dec = gal2cel(self.catalog.lon,self.catalog.lat)
        glon,glat = self.catalog.glon_glat
        ra,dec    = self.catalog.ra_dec

        # Angular and isochrone separations
        sep = angsep(self.source.lon,self.source.lat,
                     self.catalog.lon,self.catalog.lat)
        isosep = self.isochrone.separation(self.catalog.mag_1,self.catalog.mag_2)

        # If size becomes an issue we can make everything float32
        data = odict()
        data[name_objid]     = self.catalog.objid
        data['GLON']         = glon
        data['GLAT']         = glat
        data['RA']           = ra
        data['DEC']          = dec
        data[name_mag_1]     = self.catalog.mag_1
        data[name_mag_err_1] = self.catalog.mag_err_1
        data[name_mag_2]     = self.catalog.mag_2
        data[name_mag_err_2] = self.catalog.mag_err_2
        data['COLOR']        = self.catalog.color
        data['ANGSEP']       = sep.astype(np.float32)
        data['ISOSEP']       = isosep.astype(np.float32)
        data['PROB']         = self.p.astype(np.float32)
     
        # HIERARCH allows header keywords longer than 8 characters
        header = []
        for param,value in self.source.params.items():
            card = dict(name='HIERARCH %s'%param.upper(),
                        value=value.value,
                        comment=param)
            header.append(card)
        card = dict(name='HIERARCH %s'%'TS',value=self.ts(),
                    comment='test statistic')
        header.append(card)
        card = dict(name='HIERARCH %s'%'TIMESTAMP',value=time.asctime(),
                    comment='creation time')
        header.append(card)
        fitsio.write(filename,data,header=header,clobber=True)