JUNE 18, 2019

June 2019 STAN

 In this STAN we provide an update on new reference file deliveries and general updates of interest to COS users.

New COS/FUV Gain Sag Reference File Delivered

The COS FUV detector experiences a phenomenon known as gain sag over the course of its lifetime due to continuous charge extraction that causes a decline in registered signal. This is mitigated long term by raising the high voltage of the detector and moving lifetime positions (LPs) to pristine areas of the detector. In the interim, the gain sag reference file (GSAGTAB) tracks the location of regions of gain sag and the date of gain sag to be used in the calibration pipeline. Since the last GSAGTAB delivery in March of 2018, gain sag regions have developed on the FUVB detector in the LP4 region. A new version of the GSAGTAB is now available that accounts for sagged regions in the Lifetime Position 4 (LP4) region of FUVB for high voltage levels of 163 and 167 up to March of 2019. The nominal operating high voltage level for all FUV modes at LP4 is 163 with the exception of G130M/1222, which operates at 167. Users who have obtained data taken after March 2018, particularly for datasets affected by these regions, are encouraged to re-retrieve their data from the HST archive so that these newly sagged regions can be properly taken into account by the calibration pipeline. Furthermore, regions of the detector corresponding to Lifetime Position 3 (LP3) are beginning to sag as well due to use of this lifetime position by a few programs with specific science use cases. We expect to release an updated version of the gain sag reference file that addresses gain sag at LP3 within the next few months.

A 2019 gain map at high voltage of 163 for COS FUVB
The figure shows the gain of the FUVB detector as of Feb 14 2019, at HV level = 163. The approximate locations of the different lifetime positions are marked as well. A region is flagged as “gain sagged” and therefore recorded in the GSAGTAB if the modal gain is less than 3 (corresponding to white contours, and highlighted by a circle and arrow at the LP4 location). Since the move to LP4, regions in that part of the detector have become sagged primarily due to use of G130M/1291 with  FP-POS  3 and 4 (two white contours inside circle). These regions have been flagged and added to the GSAGTAB just released. A similar exercise was performed for a high voltage level of 167, and the regions flagged were also included in the updated gain sag reference file.


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New COS/FUV Bad Pixel Reference File Delivered

The bad pixel reference file (BPIXTAB) identifies rectangular regions on the detectors that are known to perform sub-optimally. It contains data quality (DQ) flags for gridwire shadows, poorly calibrated regions, low response regions (<50% sensitivity), very low response regions (<20% sensitivity), background features, and low pulse height amplitude (PHA) features. Of these spatial DQs, we classify the poorly calibrated regions and very low response regions as serious, or SDQs, and events flagged in these portions of the detector will be removed from the final calibrated products (*x1dsum*.fits) if they fall within the core of the extraction profile.

A new COS/FUV BPIXTAB (36d1836ml_bpix.fits) has been delivered for use in the HST pipeline on June 13, 2019. This file was created by using new deuterium lamp data and Cycle 26 dark data to update the boundaries of the existing low response regions, very low response regions, background features, and low PHA features. A majority of the previously flagged regions were expanded, and seven new low response regions, two new background features, and two new low PHA features were added. We also added a new DQ flag for enhanced background counts regions, which enclose the edges of the FUVA detector where enhanced dark counts have been observed over time (see the March 2019 STAN for more information). Therefore, COS/FUV users should re-retrieve their data from the HST archive to ensure that any changed SDQ regions have been properly removed from their data. This applies to COS/FUV data taken at all lifetime positions.

The updates to the bad pixel reference file affect COS FUV data taken at all lifetime positions. Users concerned about the effect of bad pixel regions on their data are encouraged to re-retrieve their data from the HST archive.

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Job Opportunity: Senior Research & Instrument Analyst

The Space Telescope Science Institute (STScI) has openings for Senior Research and Instrument Analysts (RIAs) in our Instruments Division. At least one position to work in the COS team is available. Opportunities on other HST instruments and on James Webb Space Telescope (JWST) instrument teams also exist. Senior RIAs provide support in all phases of instrument calibration, including the collection, reduction, organization, analysis, and interpretation of data to optimize the scientific return of our flagship missions.

This is a great opportunity to join a dynamic and collaborative team operating a cutting-edge science facility. A PhD in relevant fields is preferred but all academic levels will be considered. Experience with spectroscopy, either from the ground or space telescopes, is a plus. Salary will be commensurate with your education and experience. STScI provides a comprehensive benefit package, flexible work schedules and generous retirement.

The application deadline is July 12, 2019. Full details and a link to the online application are available here.


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