WFC3 STAN Issue 27, July 2018
In the last few months, HST has experienced increased jitter in its pointing stability as one of its gyros has begun to demonstrate performance issues and another gyro has failed. We have put together an ISR to describe the current level of jitter and document the impact of jitter on PSFs and on astrometry (which is much more sensitive to jitter than photometry).
The increase in jitter comes at a time when we are starting to get an excellent handle on the PSF. The HST PSF has long been known to vary both spatially (with location on the detector due to geometric optics and charge-diffusion effects) and temporally (with breathing-related variations in focus). Many ISRs over the years have detailed the spatial variations of the PSF at nominal focus (see for example ISR 2013-11), but we have also begun to understand the temporal variations (see ISR 2015-08).
Anderson & Bedin (2017 MNRAS 470 948) found in their GO-12911 study of stars in M4 that the F467M PSF varies in a regular way along a single-parameter curve, where the single parameter can be mapped to telescope focus. The WFC3 team has been using the same approach to construct focus-diverse PSF models for the more commonly used filters (F275W, F336W, F435W, F606W, and F814W) using archival images that have an abundance of stars. These focus-diverse PSFs can be fitted to stars in exposures in order to determine an empirical measure of the focus for each exposure, which in turn allows us to fine-tune a PSF for each exposure.
As we were in the process of rolling out these PSFs to the community, along with the tools needed to make use of them, the increase in jitter has forced us to consider this additional mode of PSF variation. ISR-2018-07 examines how an increase in jitter affects the WFC3/UVIS PSF. We find that when the jitter is less than 11 mas, its effect on the PSF will be much less than the unavoidable spatial and temporal variations. At about 11 mas, the jitter will begin to cause an increase in astrometric residuals. At about 16 mas, the jitter will double astrometric errors and will cause an in-focus PSF to be as broad as an out-of-focus PSF.
From early 2017 to early April 2018 one of the gyros exhibited increasing signs of instability, and the jitter increased from nominal (3 mas along each axis) to about 8 mas along one axis. In April, one of the good gyros failed and was replaced, and the new combination of gyros changed the jitter direction and amplitude. The current level of jitter is larger than before the failure, but we find by comparing recent images with images taken in late 2016 when the jitter was nominal that there is still very little impact on astrometry, which is the most demanding measurement a PSF can make.
The HST mission office and the WFC3 instrument team will continue to monitor the jitter and its impact on the PSF in images. As we make the new focus-diverse PSFs available to the community in the coming months, we will explore ways of including jitter in the PSF models.
For more details about the WFC3 PSF, including a link to the MAST PSF database, please refer to this WFC3 page.
For more details about the recent gyro issues, please see the STScI Newsletter article.
M. Gennaro, E. Sabbi, S. Baggett, J. Mack.
The WFC3 team has completed a major update of their Data Handbook (DHB). The WFC3 DHB version 4.0 has been released and is available here.
The DHB is the starting point for all users who desire to better understand the processing steps for WFC3 data. Among its contents are: a description of the WFC3 data products, the calwf3 data pipeline workflow, several technical challenges, as well as sources of error and strategies to mitigate known WFC3 problems.
This version of the DHB has undergone several major updates following major changes to how the WFC3 team completes WFC3 data processing.
The DHB updates include, but are not limited to:
- An overview of the refactored calwf3 data reduction pipeline which was implemented in February 2016, including the recent UVIS chip-dependent photometric calibration.
- An updated description of the UVIS charge transfer efficiency correction applied by the pipeline to all UVIS data, as well as the flagging of sink pixels.
- Updates to the description of how data taken in SCAN mode are processed.
- Mitigation strategies for variable IR background.
- A description of the workflow necessary to obtain calibrated photometry with WFC3, with particular attention to the differences for UV and QUAD filter photometry.
- A list of software tools dedicated to WFC3 data processing.
- Examples of how to use the pipeline for manual data reprocessing; these have been modernized and are now written in Python.
One of the newest additions to the WFC3 Internal QuickLook application is the spatial scan monitor. This automated code uniformly reduces all WFC3/IR time-series spatial scan observations (typically of transiting exoplanets). Since 2011, there have been over 150 such observations, providing insight into the instrument's behavior over time. In particular, the slow degradation of one of HST's gyros has led to a heightened awareness of the telescope's measured pointing drift in recent years. Here we define drift as the measured time-dependent change in pointing within an observation. Nominally, well-behaved observations should exhibit rms drift values that do not exceed 15 mas (~0.124 pixels).
The figure below depicts the measured x-axis (dispersion direction) rms drift as a function of time, while limiting our sample to those observations that are well-behaved. Black circles represent individual observations. The red curve and shaded regions are the rolling median and standard deviation, respectively, for a window size of 10. The seven-year median x-axis rms drift is 5.1 +\- 2.3 mas; however, since mid-2016 the drift has steadily increased. The current 2018 median x-axis rms drift is 7.4 +\- 2.2 mas. The median y-axis rms drift (cross-dispersion direction, not shown) remains constant at 3.7 +\- 3.3 mas.
We continue to monitor the drift (and other key parameters) of newly-acquired spatial scan data and will advise PIs should any further increases in drift negatively impact the quality of the data.
C. Martlin, V. Kozhurina-Platais.
The geometric distortion solutions found in the Instrument Distortion Coefficients Table (IDCTAB) and the distortion reference files for individual filter distortions (NPOLFILEs) are used in the HST calibration pipeline to correct WFC3/UVIS images for distortions.
Based on observations of the globular cluster Omega Cen (calibration program CAL-14393), the IDCTAB reference file has been updated recently with newly obtained unique polynomial coefficients for the following narrow and unique band UVIS filters F280N, F343N, F373N, F395N, F469N, F487N, F502N, F631N, F645N, F656N, F658N, F665N, F680N, F475X, F475W, and F600LP.
The solutions for the geometric distortion coefficients were derived for each UVIS filter by comparing the X and Y positions from the images to the X and Y positions from the standard astrometric catalog OMCCOOLR_BR.RIGID.XYVI in the vicinity Omega Cen (Anderson, et al. 2010). The previous distortion corrections for all these UVIS filters were based on the reference F606W solution. The newly-derived solutions improve the astrometric correction to the level of 2 milli-arcseconds (0.05 pixels).
The NPOLFILE reference files (2-D look-up tables of filter-dependent low scale distortion) are also now newly available for the 16 UVIS filters listed above. Previously, there were no NPOLFILE solutions available for these UVIS filters thus the updates will help reduce the positional systematic errors due to the filter-dependent distortions.
Note that the updates discussed here only affect the UVIS filters listed above. Observers with UVIS data taken in any of these filters prior to July 4th 2018 and who have used packages that utilize the IDCTAB and/or the NPOLFILEs (such as the STSDAS package Drizzlepac/TweakReg/Astrodrizzle) should re-request their data from MAST (Mikulski Archive for Space Telescopes) to ensure they have the most up-to-date calibration files assigned by the HST pipeline.
For more specific information on the IDCTAB and NPOLFILE reference files and how they are used please visit the Calibration Database System (scroll to the bottom) or the WFC3 Data Handbook for more details.
There has been a re-delivery of all UVIS darks (*_drk.fits, *_dkc.fits ) used by WFC3/UVIS observations taken between the end of the anneal on August 10th 2017 at 08:50:19 and the start of the anneal on October 4th 2017 at 09:45:12. The previous dark reference files created were using an incorrect September 2017 anneal date and time. This allowed the dark reference file creation pipeline to produce dark reference files which will likely have incorrect hot pixel flagging. The updated reference files were created with the correct anneal date/time and have now been delivered to CRDS while the sub-optimal dark reference files have been removed from CRDS. All observations taken with WFC3/UVIS between those dates will here-on-out be processed with the correct dark reference files; if users previously downloaded their data taken between those dates they may need to re-request their data from MAST (Mikulski Archive for Space Telescopes) to ensure they have the most up-to-date dark calibration files assigned by the HST pipeline.
- ISR 2018-01 Accuracy of the HST Standard Astrometric Catalogs w.r.t. Gaia – V. Kozhurina-Platais, N. Grogin, E. Sabbi.
- ISR 2018-02 Comparing the ACS/WFC and WFC3/UVIS Calibration and Photometry – S.E. Deustua and J. Mack.
- ISR 2018-03 Persistence in the WFC3 IR Detector: Intrinsic Variability – Knox S. Long, & Sylvia M. Baggett.
- ISR 2018-04 Persistence in the WFC3 IR Detector: An Area Dependent Model – Knox S. Long, & Sylvia M. Baggett.
- ISR 2018-05 A Characterization of Persistence at Short Times in the WFC3/IR Detector – M. Gennaro, V. Bajaj, K. Long.
- ISR 2018-06 WFC3/IR Blob Monitoring – B. Sunnquist.
- ISR 2018-07 Impact of Increased Jitter on WFC3/UVIS PSFs – J. Anderson and E. Sabbi.
- ISR 2018-08 WFC3 Color Term Transformations for UV Filters – A. Calamida, J. Mack, S. Deustua, E. Sabbi
The archive of all WFC3 Instrument Science Reports (ISRs) is here.
The most recent available version of the WFC3 Data Handbook can be found here.