def add_relative_src_offset(metadata):
    """Get the source offset relative to the lens center

    Parameters
    ----------
    metadata : pd.DataFrame
        the metadata generated by Baobab

    Returns
    -------
    pd.DataFrame
        metadata augmented with relative source offset columns added

    """
    metadata['src_light_pos_offset_x'] = metadata['src_light_center_x'] - metadata['lens_mass_center_x']
    metadata['src_light_pos_offset_y'] = metadata['src_light_center_y'] - metadata['lens_mass_center_y']
    return metadata