def _build_src_index(self):
        """Build an indices for fast lookup of a source given its name
        or coordinates."""

        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)