def check_FS_true(output_hdul):
    """
    This function checks if the fits file is a Fixed Slit.
    Args:
        output_hdul: the HDU list of the output header keywords

    Returns:
        result: boolean, if true, the file is assumed to be Fixed Slit
    """
    result = False
    if "EXP_TYPE" in output_hdul:
        if output_hdul["EXP_TYPE"] == "NRS_FIXEDSLIT":
            result = True
    return result