def _parse_cal_product(cal_product):
    """Split `cal_product` into `cal_stream` and `product_type` parts."""
    fields = cal_product.rsplit('.', 1)
    if len(fields) != 2:
        raise ValueError(f'Calibration product {cal_product} is not in the format '
                         '<cal_stream>.<product_type>')
    return fields[0], fields[1]