Description
A decrease in sensitivity has been detected below 1200Å in G140L sensitivity monitoring data using BD+28D4211. Each time the monitor is run, the current data is compared to the first BD+28D4211 data at the same wavelength. A ratio and errors are calculated every 10Å from ~1100Å to ~1350Å. The change with time is evident in the plot showing Ratio vs. Wavelength (PostScript, 168 Kb). The apparent increase in sensitivity above 1200Å is not understood but has remained fairly constant with time.
The change with time (PostScript, 184 Kb) for the wavelengths up to 1215Å has also been plotted.
A similar monitor is done on Side 2 at 5 standard wavelengths. The plot of Ratio vs. Time (PostScript, 160 Kb) is beginning to show a slight decrease at all 5 wavelengths. No correction for longer wavelength (Side 2) data is planned at this time.
Solution
New G140L sensitivity files are available in CDBS. There is one net wavelength (NETHFILE) file, h4s1130qz.r4h, for many absolute flux header (ABSHFILE) files. The new baseline post-COSTAR sensitivity is h4s1130pz.r3h; there are also time variable files for specific USEAFTER dates. (Their names range from h5m10094z.r3h to h5m1009fz.r3h; names like h4s1135?z.r3h are the same but are limited to wavelengths between 1140 and ~1630Å). Please also see the updated Reference File Additions page. GHRS-ISR-085 describes the creation of these files.
Starview can tell you which file is appropriate to your data. Look under the Searches Menu for GHRS Reference Files. Qualify that screen with a dataset name or proposal ID and begin the search. Look at the lines for the Absolute Flux Sensitivities and Wavelength Net. The recommended column tells you which file is deemed best for your data and the Level of Change column indicates if the recommended file is different from the one used and if the change affects the results of the calibration (SEVERE), or just the header of the calibration file (TRIVIAL).
If you want to re-flux-calibrate your G140L data using one of the new
sensitivity files closest in time to your observation, mark the
recommended files and retrieve them. Then go back to STSDAS, strfits
the retrieved files. Make sure you have a complete set of "de-fits-ed"
calibration files. Then use
stsdas.hst_calib.ctools.chcalpar on your *.d0h files to
change the ABSHFILE to the appropriate *.r3h file and the NETHFILE to
the one *.r4h file listed above. Make sure FLX_CORR is set to PERFORM
and then run CALHRS.
List of Figures
Side 1, ratio vs. wavelength for many epochs (PostScript, 168 Kb)
Side 1 <1200Å w/time (PostScript, 184 Kb)
Side 2 Five wavelengths w/time (PostScript, 160 Kb)