def transformToRef(self,ref_wcs,force=False):
        """ Transform sky coords from ALL chips into X,Y coords in reference WCS.
        """
        if not isinstance(ref_wcs, pywcs.WCS):
            print(textutil.textbox('Reference WCS not a valid HSTWCS object'),
                  file=sys.stderr)
            raise ValueError
        # Need to concatenate catalogs from each input
        if self.outxy is None or force:
            outxy = ref_wcs.wcs_world2pix(self.all_radec[0],self.all_radec[1],self.origin)
            # convert outxy list to a Nx2 array
            self.outxy = np.column_stack([outxy[0][:,np.newaxis],outxy[1][:,np.newaxis]])
            if self.pars['writecat']:
                catname = self.rootname+"_refxy_catalog.coo"
                self.write_outxy(catname)
                self.catalog_names['ref_xy'] = catname
Transform sky coords from ALL chips into X,Y coords in reference WCS.