def read_header_key(fits_file, key, hdu=1):
    """Read the header key key from HDU hdu of the file fits_file.

    Parameters
    ----------
    fits_file: str
    key: str
        The keyword to be read

    Other Parameters
    ----------------
    hdu : int
    """
    from astropy.io import fits as pf
    hdulist = pf.open(fits_file)
    try:
        value = hdulist[hdu].header[key]
    except:
        value = ''
    hdulist.close()
    return value
