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Hubble Space Telescope
January 2016 STAN

A new version of CalCOS, version 3.1, has been installed in the operational pipeline and the available cache of COS data has been reprocessed with the latest version. Detailed release notes for this, and previous, versions of CalCOS can be found on the “CalCOS Release Notices” web page. This new version includes minor bug fixes from CalCOS 3.0, the addition of a new reference file, and code changes to handle localized transient events in FUV data. These changes went into effect on December 1, 2015. The changes produced by CalCOS 3.1 are relatively minor for data obtained at Lifetime Position 3 (i.e. all data obtained after Feb 9 2015 except with the G130M/1055/1096 cenwaves) and are insignificant for data obtained prior to the LP3 move. However, users who prefer to get the most up-to-date calibrations can re-retrieve their data from https://archive.stsci.edu

CalCOS v3.1 fixes some minor bugs discovered in CalCOS v3.0; the most significant of which was a slight mis-alignment in the cross_dispersion direction between the two-dimensional COUNTS image and data quality (DQ) arrays in the _FLT file. This bug could result in any DQ region to be up to +/-3 pixels offset from the correct location, which in turn could cause bad regions to be included in spectral extraction, or conversely, good regions to be excluded from the extracted spectrum. While CalCOS v3.0 was in operations, the BPIXTAB reference file (which maps the bad regions to be flagged) was padded by +/-3 pixels in the cross-dispersion direction to compensate for this bug and provide a robust calibration. As this bug has now been addressed in v3.1, the BPIXTAB has been reverted to it’s original form.

In 2015, monitoring activities began to notice transient, isolated regions of increased count rates on the COS FUV detectors. Though these so-called hot spots have so far had minimal impact on science spectra, due to their location (below Lifetime Position 3), but future spots could potentially impinge on the spectral extraction region. To mitigate the effects of such hot spots, CalCOS v3.1 introduces code changes and a new reference file to exclude identified transients. The new reference file, SPOTTAB, contains the physical location and duration of known transient regions. CalCOS uses this reference file to flag affected detector regions with a DQ value of 2 if the region’s temporal duration overlaps with the time of observation. Because the magnitude of the detected spots is occasionally comparable to observed spectra, any columns flagged with 2 will be completely discarded from the extracted spectrum. This applies even when the region occurs in the outer zone of COS two-zone extraction. See the COS Data Handbook for more information on the COS extraction methods.