docs: 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