Converts from sky coordinates to unit vectors. Before conversion to unit
        vectors, the coordiantes are transformed to the coordinate system used
        internally by the :obj:`UnstructuredDustMap`, which can be set during
        initialization of the class.

        Args:
            coords (:obj:`astropy.coordinates.SkyCoord`): Input coordinates to
                convert to unit vectors.

        Returns:
            Cartesian unit vectors corresponding to the input coordinates, after
            transforming to the coordinate system used internally by the
            :obj:`UnstructuredDustMap`.

def _coords2vec(self, coords):
        """
        Converts from sky coordinates to unit vectors. Before conversion to unit
        vectors, the coordiantes are transformed to the coordinate system used
        internally by the :obj:`UnstructuredDustMap`, which can be set during
        initialization of the class.

        Args:
            coords (:obj:`astropy.coordinates.SkyCoord`): Input coordinates to
                convert to unit vectors.

        Returns:
            Cartesian unit vectors corresponding to the input coordinates, after
            transforming to the coordinate system used internally by the
            :obj:`UnstructuredDustMap`.
        """

        # c = coords.transform_to(self._frame)
        # vec = np.empty((c.shape[0], 2), dtype='f8')
        # vec[:,0] = coordinates.Longitude(coords.l, wrap_angle=360.*units.deg).deg[:]
        # vec[:,1] = coords.b.deg[:]
        # return np.radians(vec)

        c = coords.transform_to(self._frame).represent_as('cartesian')
        vec_norm = np.sqrt(c.x**2 + c.y**2 + c.z**2)

        vec = np.empty((c.shape[0], 3), dtype=c.x.dtype)
        vec[:,0] = (c.x / vec_norm).value[:]
        vec[:,1] = (c.y / vec_norm).value[:]
        vec[:,2] = (c.z / vec_norm).value[:]

        return vec