April 2021 STAN
In this STAN we introduce a webpage with instructions for responding to reports of observing problems, discuss the possibility that NUV fluxes of extended objects may be underestimated, and announce the delivery of a new gain sag table that affects data obtained starting in May 2020.
What to Do When You Get an Exception Report
If an anomaly occurred on the spacecraft during your COS observations, you receive an automated email containing an HST exception report. This email asks you to download your data from MAST, determine whether the S/N and/or flux of your data reached the expected levels, and determine from a quick-look inspection whether your data appear anomalous in any other way. We have added a checklist to the COS website with more detailed steps for how to inspect COS data for potential problems that might lead to filing a Hubble Observation Problem Report (HOPR) within 90 days. Investigators who decide that their spectra are suitable for their own needs in spite of an observing problem are asked to still report the nature of the problem so the affected datasets can be flagged for future users of archival data. A link has been added to the automated email to direct users to this checklist.
Underestimated NUV Fluxes of Extended Objects
The default regions used by CalCOS to estimate the background are read from the XTRACTAB reference file. In the NUV, these background regions are currently adjacent to the regions from which the spectra are extracted. For point sources, the background regions lie sufficiently far from the spectrum, but for extended objects, the background regions may contain counts from the target. In this case, the subtracted background will be too large, and the calibrated target fluxes will be underestimated. Users whose NUV targets have angular diameters greater than 1.3 arcseconds should investigate whether their data may be affected and contact the help desk for any assistance needed in performing custom background subtraction. An NUV XTRACTAB with background regions suitable for both extended and non-extended targets will be delivered in the future.
New Gain Sag Table Delivered
The COS FUV detector experiences gain sag, a loss in efficiency as it is exposed to incoming photons. Regions on the detector that are exposed to more photons experience gain sag more rapidly, particularly around where the Lyman Alpha (Ly α) airglow falls. For a given high voltage (HV) and segment (FUVA or FUVB), the gain sag table (GSAGTAB) reference file flags locations at a given time on the FUV detector where the peak of the pulse height distribution of the gain has fallen below a threshold of 3.0.
A new GSAGTAB (54c1542dl_gsag.fits ) was delivered on 2021 April 12 with updated flagging of several regions. The gain sag holes caused by Ly α airglow on segment FUVB were updated at an HV of 169, which became the operating HV value at Lifetime Position 4 (LP4) starting on 2020 October 5. COS users with FUV spectra obtained after 2020 October 5 are encouraged to re-retrieve their data from MAST so that regions of their spectra affected by the gain sag will be properly flagged and not included in the final x1dsum spectra. Additionally, several regions on FUVA were flagged at an HV of 163, which was the LP4 operating HV value from 2017 October 2 through 2020 October 5. These new regions affect data obtained between 2020 May 16 and 2020 October 5. COS users with FUV spectra obtained during this date range are also encouraged to re-retrieve their data from MAST. The blue modes (G130M cenwaves 1055 and 1096) are observed at LP2 and are not affected by this change.