Converts from sky coordinates to pixel indices.

        Args:
            coords (:obj:`astropy.coordinates.SkyCoord`): Sky coordinates.

        Returns:
            Pixel indices of the coordinates, with the same shape as the input
            coordinates. Pixels which are outside the map are given an index
            equal to the number of pixels in the map.

def _coords2idx(self, coords):
        """
        Converts from sky coordinates to pixel indices.

        Args:
            coords (:obj:`astropy.coordinates.SkyCoord`): Sky coordinates.

        Returns:
            Pixel indices of the coordinates, with the same shape as the input
            coordinates. Pixels which are outside the map are given an index
            equal to the number of pixels in the map.
        """

        x = self._coords2vec(coords)
        idx = self._kd.query(x, p=self._metric_p,
                             distance_upper_bound=self._max_pix_scale)
        return idx[1]