The synphot throughput files have now been updated for all the gratings. These synphot files are primarily used for the ETC calculations; the actual reference files used by the pipeline have been up-to-date with the data on which these updates are based since about February 1999.

Some of the earlier throughput files in synphot were based on pre-flight data. The updates are done with on-orbit, calibration data and a correction for an infinite aperture. For the Echelle modes, the earlier throughput files were based on 'gross' counts, which included the scattered light from other orders. The throughput files have been corrected so that the throughputs now reflect the 'net' counts (='gross' - the scattered component). The corrected Echelle throughputs are accordingly lower as enumerated below. The ETC takes the scattered contribution into account in the signal-to-noise calculations.

Note that no significant evolution in sensitivity larger than about 1% has been seen for any of the modes since launch; all of the changes reflect improvements in calibration rather than physical changes in STIS.

Throughput changes in the synphot files, as a result of the on-orbit calibrations, are as follows:

 
L modes:
   changes on the 2-3% level

M modes:  
   g140m, g430m - change on the 2-3% level, 
                  decrease in throughput
   g230mb- significant change; throughput up 
           ~15% longward of 2200 A, INCREASE 
           in throughput
   g750m- change on the 5% level, decrease
          in throughput
   g230m- corrects for mistake- wavelength 
          regions was cut off prematurely at 
          ~2775 in former version; significant 
          change in throughput at red end since 
          now goes to ~3150 A, decrease in 
          throughput at a given wavelength, but 
          increase in total throughput
        
echelles: maximum changes in throughput caused by the 
          scattering correction are as follows:

          e140h - 15% decrease 
          e140m - 20% decrease 
          e230h - 7% decrease 
          e230m - 7% decrease