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Space Telescope Imaging Spectrograph
Doppler correction errors

An error was found in the phasing of the Doppler correction applied by the OTFR pipeline to MAMA TIME-TAG data retrieved from the archive before 2004-Apr-21. This error was fixed by the release of OPUS 15.0b. Users for whom accurate velocities are critical or who wish to obtain the maximum resolution from the data should re-request their TIME-TAG data from the archive if the data were retrieved prior to that date. The error occurred in an early stage of the OTFR processing, and cannot be fixed without re-obtaining the data. Doppler corrections for ACCUM mode data are applied on-orbit and were not affected by this software problem.

The second error affects all E140M data (TIME-TAG and ACCUM) taken prior to 2000-Mar-01. The on-orbit Doppler correction was systematically under-estimated because the wrong dispersion was used for the E140M grating (120000 instead of 187000). Since the erroneous correction was applied on-orbit, affected ACCUM data cannot be fully corrected. We are currently working on a fix for this problem that would correct E140M TIME-TAG data during OTFR processing. In the mean time, an IRAF script is available on our website http://www.stsci.edu/hst/stis/software/analyzing/scripts/axis1_descrip.html which allows users to correct affected E140M TIME-TAG data files after they are retrieved from the archive. In order to avoid the phase error problem discussed above, be sure to only use data obtained from the archive after 2004-Apr-21.