def apStar_url(telescope, location_id, field, file_,
               url_header=None):
    """ apStar url generator
    which in principle is able to generate file path

    Parameters
    ----------
    telescope: string
        TELESCOPE, {"apo1m', 'apo25m'}
    location_id: int
        for 'apo1m', it's 1
        for 'apo25m', it's like PLATE
    field: string
        for 'apo1m', it's 'hip'|'calibration'|...
        for 'apo25m', it's non-sense
    file_: string
        FILE
    url_header: string
        if None|'sas', it's set to be
        "https://data.sdss.org/sas/dr13/apogee/spectro/redux/%s/stars"%version

    Returns
    -------
    url: string
        the url of apStar file

    Note
    ----
    version: string
        currently it's 'r6' @20161031
    """

    if url_header is None or url_header is "sas":
        url_header = ("https://data.sdss.org/sas/dr13/apogee"
                      "/spectro/redux/r6/stars")

    url_header = url_header.strip()
    telescope = telescope.strip()
    field = field.strip()
    file_ = file_.strip()

    if telescope == "apo1m":
        # apo1m
        url = "%s/%s/%s/%s" % (url_header, telescope, field, file_)
    elif telescope == "apo25m":
        # apo25m
        url = "%s/%s/%s/%s" % (url_header, telescope, location_id, file_)
    else:
        raise(ValueError("@Cham: This is not an option!"))
    return url