JANUARY 2, 2020
WFC3 NEWSLETTERS

WFC3 STAN Issue 30, January 2020


1. WFC3 in Action! Visit us at the 235th AAS Meeting

Several posters will be presented at the upcoming AAS related to WFC3 data reduction as well as recent WFC3 science results. We invite all WFC3 users at the AAS to stop by during scheduled poster sessions to review these projects and their available tools and results. Below we include condensed summaries of the various posters - full abstracts are available through the AAS; we link to individual poster abstracts below.   

109.18 Wide Field Camera 3 (WFC3) Python Data Analysis Tools and Jupyter Notebooks - Session 109 - Computation, Data Handling, Image Analysis - January 5th (Morning Session).
C. Shanahan;
New functionality to aid users in data analysis has been added to the `wfc3tools` Python package which is maintained by the WFC3 team at STScI. In addition, Jupyter notebooks outlining best practice workflows have been created to further guide users in using the tools available in this package.

109.07 A PSF Fitting Workflow for HST/WFC3 - Session 109 - Computation, Data Handling, Image Analysis - January 5th (Morning Session).
V. Bajaj, J. Anderson;
A new Python interface, along with a full workflow within a Jupyter notebook, have been created to aid users in fitting Point Spread Functions (PSF) on HST WFC3 images to generate high precision catalogs of positions and fluxes of stars. Other functionality of the software interface and Jupyter notebook workflow includes matching catalogs, zeropoint correction, as well as input checking. 

372.07 WFC3 DASH Reduction Pipeline Development and Launch - Session 372 - Instrumentation: Space Missions I - Poster Session - January 7th (Evening Session). 
R. O'Brien, C. Martlin, I. Momcheva, M. Gennaro;
A new Python package has been developed to aid users in reducing the initial data acquired via the DASH (drift-and-shift) observing technique where WFC3/IR images are taken under gyro control. The software creates the partial images and aligns them and includes options for creating mosaics, segmentation maps and association files. We provide documentation and Jupyter notebook tutorials to guide users through the reduction pipeline.

110.12 HST/WFC3/UVIS Survey of Stellar Clusters in the Magellanic Clouds: Search for multiple populations - Session 110 - Stellar Evolution, Stellar Populations - January 5th (Morning Session).
V. Kozhurina-Platais , N. Bastian, S. Saracino, S. Martocchia, I. Platais;
The magic combination of HST/WFC3 UVIS filters from ultraviolet-wide to red-wide along with the narrow F343N is sensitive to C-N-O abundance variations for stars on the Red Giant Branch. Not only for Galactic star clusters, but also for clusters in the Magellanic Clouds. The HST Magellanic Cloud cluster survey shows a strong correlation between the cluster age and the properties of multiple populations (MP) - tracing detailed abundances of N, O, and He as a function of age. The HST observations allowed us to explore in detail the chemical variations and its characterization for large numbers of stars in the cluster, providing fresh constraints on the origin of MPs in the stellar clusters.

2. WFC3 Instrument Handbook for Cycle 28

L. Dressel

The Cycle 28 edition of the WFC3 Instrument Handbook has been posted here. A PDF version of the entire Handbook is linked to the top of the page. The web version can be searched for information on specific topics by using the Search box below the Table of Contents. For example, "single guide star" produces a list of articles where this phrase occurs. The question mark icon that appears in the search box is linked to a menu that provides descriptions of how to refine this search using logical statements, wildcards, etc. A more limited search can be conducted by selecting one of the chapters in the Table of Contents and using the Search box provided under Chapter Contents.

hdox website screenshot

Updates to the previous edition of the Handbook are described in Section 1.1:

  • We discuss recent results from studies of count rate non-linearity on the WFC3/IR detector (Section 5.7.6).
  • We discuss the construction of focus-diverse PSF models for five commonly used UVIS broad-band filters and their relationship to HST focus (Section 6.6.4).
  • We have extensively updated the description of photometric calibration (Section 6.10).
  • We point out the existence of an offset between spatial scan images made in forward and reverse modes (Section 6.12.3).
  • We update advice to observers using the spatial scan mode and demonstrate the advantage of monitoring UVIS system throughput with spatial scan data (Section 6.12.3).
  • We state the increased drift rate associated with using a single guide star (Section 6.12.4, Section 7.10.4).
  • We summarize the results of recent studies of persistence on the WFC3/IR detector and advise the use of dither steps greater than 10 pixels for some photometric programs (Section 7.9.4).
  • We note that the solar exclusion angle for HST observations has increased from 50 degrees to 55 degrees (Section 7.9.5, Section 9.7.1).
  • We provide new references to studies of the "blobs" in WFC3/IR images and discuss the date-dependent bad pixel tables that track the appearance of blobs (Section 7.9.6).
  • We discuss the impending release of tools and a Jupyter notebook to assist users of the DASH mode to calibrate their observations (Section 7.10.6).
  • We provide a more detailed discussion of the background in WFC3/IR grism exposures and illustrate the three components of the background (zodiacal light, He I airglow emission, and scattered light) in G102 and G141 images (Section 8.5).
  • We confirm the stability of the geometric distortion solution for the WFC3/UVIS F606W filter and summarize the delivery of IDCTAB reference files for the WFC3/IR detector and the WFC3/UVIS detector (Appendix B).
  • We have added a summary of the WFC3 calibration plan for the upcoming cycle in Appendix E.

3. New Astrometric Solutions in WFC3 Imagess

V. Bajaj

As of December 3, 2019, WFC3 FLT/FLC images have new absolute astrometric solutions.  These improvements mainly reduce the pointing errors (generally a few tenths of an arcsecond) present in WFC3 data, but do NOT affect the distortion solution.  The new solutions are derived in a number of ways, but fall into two different categories.  The first is an "a Priori" solution, which is derived using Gaia Data Release 1 (DR1) coordinates for the guide stars used in the observation, as the Gaia positions for these stars are much more accurate than the previous Guide Star Catalog positions.  The other type of solution is an "a Posteriori" solution derived via matching sources detected in an image to an external catalog and correcting the image for the offsets between matches.  If one of these solutions exists, the external catalog will either be the Hubble Source Catalog Version 3, Gaia DR1, or Gaia DR2 (the final choice depends on the quality of solution each catalog was able to generate for any given dataset); other catalogs may be added in the future.

If a new solution is available for a dataset, products retrieved from the MAST archive will have the best solution applied by default.  In this case, the application of the solution changes the World Coordinate System (WCS) in the headers of the FLT/FLC files, but does NOT affect the pixel values for these data (though the values in DRZ/DRC images will likely be affected).  This is because the WCS defines transformations from pixel to sky coordinates for an image.  When a new WCS is applied, this is reflected in the "WCSNAME" header keyword in the science extensions of the FLT/FLC files.  Several solutions are available for a given dataset, and are contained in extra "headerlet" extensions appended to the end of FLC/FLC files (see here for more details ).

However, because the drizzle software uses the WCS's of the input FLT/FLC images to project those images onto an undistorted frame, changes in the WCS may affect the pixel values in the drizzled products (DRZ/DRC).  In general, these effects should be insignificant, though in a small number of cases the relative astrometry between exposures may be changed, altering the final drizzled result.  As always, drizzled images directly from the archive should be treated as quick look/preview images only and should not be assumed to be science-ready.

In general, the absolute astrometry of the data products should be significantly improved when new solutions are present (especially with the a Posteriori solutions), i.e. the WCS of the updated images should be very close, if not matching the Gaia frame.  However, as with the drizzle products from the archive, a broad set of parameters were used in the derivation of the a Posteriori solutions.  Rarely, this may cause a degradation in both the absolute and relative astrometry, especially in sparse/small fields with very few Gaia sources.  If the relative astrometry between input images of a drizzled image is compromised, then the resulting drizzled image may have spurious rejection of sources as cosmic rays.  Furthermore, since the derivation of the a Posteriori solutions is done on an individual dataset basis, it is possible that images taken in the same visit, but part of different associations (such as those taken in different filters) may not be aligned to each other.  Thus, we strongly recommend assessing datasets visually to verify alignment, e.g. by opening up images in SAOImage DS9, matching the WCS's of the images and blinking them.

If the original (pre June 2019) or other available solution is desired as the "active" solution/WCS, it can be activated using the tools in the Python package "stwcs".  A further description of the solutions with small code examples for working with/switching solutions is available here.  To propagate a solution into DRZ/DRC images will first require applying the solution to the FLT/FLC images followed by re-drizzling the images.  A brief example notebook is available at github.

4. WFC3 IR PSF Database Available on MAST Portal 

C. Shanahan. 

The WFC3 PSF database on the MAST archive, which hosts over 23 million PSFs extracted from UVIS observations from 2009-2018, has recently been extended to include WFC3/IR PSFs. Nearly 4.6 million high signal-to-noise PSFs have been identified in full-frame IR exposures and parameters including flux, PSF fit quality, (x, y) detector position, (RA, Dec) sky position, and observation metadata parameters such as exposure time and filter are stored. Among other use cases, these PSFs are intended to assist users in building PSFs that are more representative of their observations (filter/detector/position/focus-value etc.). 

To access this database, select 'WFC3 PSF' from the drop down menu on the MAST portal . From there, selecting either 'IR' or 'UVIS' and clicking 'Advanced Search' will lead to the database interface where the full set of PSFs can be accessed. The database can be filtered to select a subset of PSFs based on any of a number of observation-specific and PSF-specific parameters. Once the desired PSFs have been selected, users can download a CSV file with PSF and observational parameters, as well as 11x11 pixel FITS cutout images of the individual PSFs. Updates will be made periodically to this database to include PSFs from data that have recently passed the proprietary date period. 

mast portal screenshot

5. Comparison of WFC3/UVIS Geometric Distortions Solutions to Gaia Data Release 2  

C. Martlin, V. Kozhurina-Platais.

The globular cluster Omega Centauri (ω Cen) has been monitored over the last 10 years using the WFC3 UVIS and IR instruments for changes in the geometric distortion calibrations. The HST software DrizzlePac/TweakReg was used to investigate the linear part of the current WFC3/UVIS geometric distortion solutions over time with respect to the Gaia DR2 in the vicinity of ω Cen, as seen in figure 1. The linear terms (offsets, scale and rotation) were calculated by TweakReg from the transformations between each individual UVIS frame and the Gaia DR2 coordinate system. We are able to determine the stability of the WFC3/UVIS geometric distortions over time is < 0.01 pixels and find no discernible temporal variation (C. Martlin et al. 2019). We conclude that the WFC3/UVIS geometric distortion linear terms are stable with insignificant changes over the last 10 years. The geometric distortions of WFC3/UVIS will continue to be monitored in the future. Further information is available in ISR 2019-09.

footprints of WFC3 F606W images on Gaia DR2 sources

Figure 1: Overlay of the Gaia DR2 sources (blue dots) of the final 125 WFC3/UVIS F606W images (unfilled footprints) that were used with TweakReg to compare to the catalog of Gaia DR2 sources. Color scale is applied to footprints to aid in distinguishing them visually.

6. Vacancy Announcement 

Share in the thrill of space astronomy and join the Space Telescope Science Institute (STScI) team! STScI has openings for Science Support Analysts in our Instruments Division. We have openings at several levels, from entry-level to highly experienced, so we welcome you to apply whether you have a Bachelor’s, Master’s, or PhD in relevant fields. As a science support analyst, you will typically work on an instrument team to support the development and operations of one of our flagship missions: the Hubble Space Telescope (HST), the James Webb Space Telescope (JWST), and the Wide-Field InfraRed Survey Telescope (WFIRST). You will be expected to provide support in all phases of instrument calibration, including the collection, reduction, organization, analysis, and interpretation of data (from e.g. spectrographs, cameras, coronagraphs) to optimize the scientific return of our flagship missions. Additional experience with spectroscopy, either from the ground or space telescopes, is a plus. For full details, please see the science support analyst job posting.

7. New Documentation

  • ISR 2019-08: Periodicity in the WFC3/UVIS Bias Pre-Scan. - H. G. Khandrika, T. D. Desjardins.
  • ISR 2019-09: Comparison of WFC3/UVIS Geometric Distortions Solutions to Gaia Data Release 2. - C.Martlin, V. Bajaj, V. Kozhurina-Platais.
  • ISR 2019-10: WFC3/UVIS CTE Monitor: Efficacy of Post-Flash in the UVIS Darks - J. V. Medina, M. Bourque, S. Baggett. 
  • ISR 2019-11: WFC3/UVIS: 2018 Superbias Reference File -  B. Kuhn, H. Khandrika. 
  • ISR 2019 12: Analyzing Eight Years of Transiting Exoplanet Observations Using WFC3's Spatial Scan Monitor -  K. B. Stevenson & J. Fowler. 
  • ISR 2019-13: Pre-Flashing WFC3/IR Time-Series, Spatial Scan Observations - K. B. Stevenson & W. Eck.

The complete WFC3 ISR archive is at: /hst/wfc3/documents/ISRs/

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