code: def transformToRef(self,ref_wcs,force=False):
          
          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
docs: Transform sky coords from ALL chips into X,Y coords in reference WCS.