def boundaries_lonlat(self, healpix_index, step):
        """
        Return the longitude and latitude of the edges of HEALPix pixels

        This returns the longitude and latitude of points along the edge of each
        HEALPIX pixel. The number of points returned for each pixel is ``4 * step``,
        so setting ``step`` to 1 returns just the corners.

        Parameters
        ----------
        healpix_index : `~numpy.ndarray`
            1-D array of HEALPix pixels
        step : int
            The number of steps to take along each edge.

        Returns
        -------
        lon, lat : :class:`~astropy.units.Quantity`
            The longitude and latitude, as 2-D arrays where the first dimension is
            the same as the ``healpix_index`` input, and the second dimension has
            size ``4 * step``.
        """
        return boundaries_lonlat(healpix_index, step, self.nside, order=self.order)