| Description: The name of the filter(s), grating(s), polarizer(s), or other optical elements used for the observation. If two or more are combined, then both names are given, separated by a plus sign "+". For WFPC2 products, this is the name of either FITS keyword FILTNAM1 or FILTNAM2, whichever is not blank. If neither is blank, the two values are combined using "+", with the shorter-wavelength filter (the filter name containing the smaller three-digit number) appearing first. For ACS products: - if FILTER1 is CLEAR1S or CLEAR1L and FILTER2 is CLEAR2S or CLEAR2L, the value is "CLEAR"; - otherwise, if FILTER1 is CLEAR1S or CLEAR1L, the value of FILTER2 is used; - otherwise, if FILTER2 is CLEAR2S or CLEAR2L, the value of FILTER1 is used; - otherwise, the values of the two filters are combined with "+". If a polarizer is used (FILTER1 or FILTER2 starts with "POL"), the name of the polarizer appears second; otherwise, the filter with the shorter center wavelength (the three-digit number inside the filter name) appears first. For STIS products, wavelength response is defined by a grating (FITS keyword OPT_ELEM) and the filter (FITS keyword FILTER). If the FILTER name is "Clear", the grating name appears alone; otherwise, the two values are appended, separated by a "+". For FOC products, this is constructed from the values of the FITS keywords FILTNAM1, FILTNAM2, FILTNAM3, and FILTNAM4. After omitting "CLEAR" values, the remaining values are sorted and concatenated with "+". For FOS products, this is the value of FITS keyword SPEC_1, which is found in the _shf.fits file. For GHRS products, this is the value of FITS keyword GRATING. For other HST instruments, this is the value of FITS keyword FILTER. For instruments that do not use filters, the value is "Not applicable". |