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August 2017 STAN

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STScI Analysis Newsletter (STAN)

Space Telescope Imaging Spectrograph (STIS)

August 2017

This STAN describes various updates to STIS calibration, including the geometric distortion, time dependent sensitivity, echelle blaze function, and the cosmic ray rejection algorithm.


Different OCRREJECT Versions Yield Different Results

The STIS Team has discovered that different versions of OCRREJECT yield different results. This article discusses this discrepancy and recommends a solution until a software update is found and implemented.

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TDS Reference File Updates

New time dependent sensitivity reference files (TDSTAB) were delivered on March 10, 2017, replacing the reference files that have been in place since 2009. These files are used during the flux calibration step of CALSTIS to correct for both the time-varying changes in the sensitivity of the individual STIS spectral elements as well as the temperature-varying sensitivities of each detector. All non-PRISM spectroscopic observations taken after May 01, 2009 have been reprocessed with these new reference files. The improvements of these new files on the final flux calibration range from modest (<2-3%) to significant (~8%) depending on the wavelength and time of observation. Users working at the shortest wavelengths who are interested in absolute flux calibrations should ensure they are working with data processed with these new files, especially if their observations were taken within the last few years.

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FUV-MAMA Geometric Distortion Correction

On July 7, 2017, the STIS team delivered an updated IDCTAB reference file that corrects geometric distortions for images obtained with FUV-MAMA. This new IDCTAB supersedes older versions, and is currently being applied to all images taken on or after August 2009 with the STIS FUV-MAMA in ACCUM mode. This article provides details on how to manually correct for geometric distortions on images obtained before August 2009. Please note that this IDCTAB update only applies to STIS FUV-MAMA data obtained in imaging mode.

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Echelle Blaze Function/FUV PHOTTAB Update

An updated STIS FUV spectroscopic photometric conversion reference file (PHOTTAB) was delivered to the HST data pipeline on May 24, 2017. The new PHOTTAB contains updates to the time component coefficients of the echelle blaze function shifts for the E140H grating modes, for observations obtained after STIS’s repair in Servicing Mission 4 (SM4) in 2009. This file improves the relative flux accuracy of recent E140H grating observations in overlapping spectral regions of adjacent spectral orders by up to ~5-10%.

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