def get_galactic_coordnates(metadata_list):
    """
    Read galactic coordinates of all given Metadata objects, and check that
    they are all the same (otherwise, raise RuntimeError).
    Returns a float tuple (gl_deg, gb_deg).
    """
    def galc(md):
        coord = md['skycoord'].galactic
        return coord.l.deg, coord.b.deg

    ref = galc(metadata_list[0])
    if not all([galc(md) == ref for md in metadata_list]):
        raise RuntimeError("Coordinates are NOT identical across all dedispersed time series")
    return ref