def cone_search_lonlat(self, lon, lat, radius):
        """
        Find all the HEALPix pixels within a given radius of a longitude/latitude.

        Note that this returns all pixels that overlap, including partially, with
        the search cone. This function can only be used for a single lon/lat pair at
        a time, since different calls to the function may result in a different
        number of matches.

        Parameters
        ----------
        lon, lat : :class:`~astropy.units.Quantity`
            The longitude and latitude to search around
        radius : :class:`~astropy.units.Quantity`
            The search radius

        Returns
        -------
        healpix_index : `~numpy.ndarray`
            1-D array with all the matching HEALPix pixel indices.
        """
        if not lon.isscalar or not lat.isscalar or not radius.isscalar:
            raise ValueError('The longitude, latitude and radius must be '
                             'scalar Quantity objects')
        return healpix_cone_search(lon, lat, radius, self.nside, order=self.order)