code: def _build_src_index(self):
          
  
          self._srcs = sorted(self._srcs, key=lambda t: t['offset'])
          nsrc = len(self._srcs)
          radec = np.zeros((2, nsrc))
  
          for i, src in enumerate(self._srcs):
              radec[:, i] = src.radec
  
          self._src_skydir = SkyCoord(ra=radec[0], dec=radec[1], unit=u.deg)
          self._src_radius = self._src_skydir.separation(self.skydir)
docs: Build an indices for fast lookup of a source given its name
          or coordinates.