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Space Telescope Imaging Spectrograph
Doppler correction for STIS E140M Time-tag data

CL script to correct early E140M Time-tag data for an error in the magnitude of the HST Orbital Doppler Correction

Download the script

STIS E140M data taken prior to 2000-Mar-01 systematically under-estimated the Doppler correction for HST's orbital motion because the wrong dispersion (120000 instead of the correct 187000) was used in the flight software for the E140M grating. Since the erroneous correction was applied on-orbit, affected ACCUM data cannot be fully fixed as there is no way of knowing when in the orbit each photon was detected. However, for TIME-TAG observations the Doppler correction is applied on the ground, and therefore can be corrected. A fix that will allow the OTFR pipeline to do this automatically is in progress. In the interim, the script axis1.cl can be used to fix affected STIS tag files. In order to avoid errors from an unrelated TIME-TAG doppler correction software problem, be sure to use only STIS TIME-TAG data obtained from the OTFR pipeline after 2004-Apr-21 as input to axis1.cl.

STIS *_tag.fits files are FITS tables with four columns. The AXIS1 column contains the Doppler corrected position of each photon event along the dispersion direction, while the DETAXIS1 column contains the original, uncorrected position. The script axis1.cl, when given a corrected value for the DOPPMAG keyword, will recalculate corrected values for the AXIS1 column. The DOPPMAG keyword gives the half-amplitude of the Doppler correction in units of hi-res MAMA pixels. The INTTAG task in the STIS package can then be used to create a new raw file (or files) from the the corrected tag file, and CALSTIS can be used to create new calibrated data files from the raw file.

There are only 70 E140M TIME-TAG datasets that are affected by this error. Below we list all of these datasets along with the original and the corrected values for the DOPPMAG keyword. Note that only integral pixel shifts are applied for the Doppler correction.

The STIS task DOPPINFO can also be used to calculate the proper DOPPMAG value for any MAMA spectroscopic dataset. The DOPPINFO task should also be used if it is necessary to estimate the errors found in pre-2000-March E140M ACCUM data.

STIS E140M TIME-TAG observations taken before 2000-Mar-01:

--------------------------
STIS       DOPPMAG value
dataset    old   corrected
--------------------------
o49wa1010   1        1
o49wa2010   0        1
o49wa3010   0        1
o49wa4010   0        1
o49wa5010   0        0
o4gt02010   2        3
o4gt02020   2        3
o4gt02030   2        3
o4gt02040   2        3
o4mu01010   3        5
o4mu01020   3        5
o4mu02010   2        4
o4mua1010   3        5
o4mua2010   2        4
o4mua2020   2        4
o4v402010   3        4
o4v402020   3        4
o4v402030   3        4
o4v402040   3        4
o4v402050   3        4
o4v403010   3        4
o4v403020   2        4
o4v403030   2        4
o4v403040   2        4
o4v403050   2        4
o4v404010   2        4
o4v404020   2        4
o4v404030   2        4
o4v404040   2        4
o4v404050   2        4
o4v405010   3        4
o4v405020   3        4
o4v405030   3        4
o4v405040   3        4
o4v405050   3        4
o4v406010   3        4
o4v406020   3        4
o4v406030   3        4
o4v406040   3        4
o4v406050   3        4
o4v407010   3        4
o4v407020   3        4
o4v407030   3        4
o4v407040   3        4
o4v407050   3        4
o4v408010   3        4
o4v408020   3        4
o4v408030   3        4
o4v408040   3        4
o4z101020   2        3
o4z102020   2        3
o4z301010   3        5
o4z301020   3        5
o4z301030   3        5
o4z301040   3        5
o50101010   3        4
o50101020   3        4
o50101030   3        4
o50101040   3        4
o50102010   3        4
o50102020   3        4
o50102030   3        4
o50102040   3        4
o5b612010   1        1
o5b616010   2        3
o5bi07010   2        3
o5e306010   3        5
o5e306020   3        5
o5e306040   3        5
o5e306050   3        5
o5kc01030   2        3
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