JUNE 1, 2020

WFC3 STAN Issue 31, June 2020

1. WFC3 in Action! Visit us (online) at the 236th AAS Meeting

Several iPosters and an oral presentation 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 review these projects and their available tools and results. Below we include summaries of the various posters and talks and links to their full abstracts.   


6/2/2020 5:30 - 6:30 PM

242.04:  Filter-Dependent Sky Flats for WFC3/IR - Heather Olszewski, J. Mack, & N. Pirzkal
We present filter-dependent sky flats for Hubble’s WFC3/IR detector, computed by stacking observations of sparse fields (with sources masked) over the instrument lifetime. The new flats replace the current set of P-flat reference files, which were based on ground test data with a single ‘grey’ correction for all filters derived from early inflight data. The sky flats correct for wavelength-dependent residuals ~1% at the center of the detector and ~2% at the detector edges with Poisson errors of 0.2% or better. We also present ‘delta’ flat corrections in six filters for all known ‘blobs’ by stacking observations before and after their individual appearance dates, and these correct for sensitivity residuals of ~5-10%.

6/3/2020 5:30 - 6:30 PM
341.01:  Hubble's WFC3 in 2020- Joel Green and WFC3 Team. 
Hubble's Wide Field Camera 3 (WFC3) is the workhorse instrument for HST, providing numerous modes in imaging, slitless spectroscopy, covering from the near-IR to near-UV. Installed during the most recent HST servicing mission in 2009, WFC3 has logged numerous observations and exciting scientific discoveries over the past 11 years. We review the status of the instrument, including recent adjustments to its technical capabilities and performance.


6/3/2020 11:20 - 11:30 AM
305.03: A new time-dependent photometric calibration of the WFC3/UVIS detector on HST - A. Calamida, J. Mack, C. Shanahan, J. Medina, V. Bajaj
In this talk I will describe a new time-dependent photometric calibration of the Wide Field Camera 3 (WFC3) UVIS detector on HST. The calibration is based on ten years of imaging data collected for four CALSPEC (HST flux spectrophotometric standard) white dwarfs across more than 40 filters. A sensitivity decline is observed in all filters, with a typical loss of ~0.15-0.2% per year. New inverse sensitivities are derived and will be provided to WFC3 users as a function of observing time. Scanned photometry for two CALSPEC white dwarfs is also available and demonstrates that photometric repeatability is improved by up to a factor of 5 compared to the staring mode photometry. I will describe plans to leverage the scanning observing mode for further refining the WFC3 photometric calibration in the future.

2. Cycle 28 Tips and Advice

J. Green, J. Anderson, S. Baggett, L. Dressel 

The deadline for cycle 28 GO program Phase II's is June 30th, 2020. We encourage PIs using the WFC3 to read our Phase II Proposal Roadmap for a step by step guide in preparing Phase II proposals. 

We are introducing additional UVIS dither patterns. The WFC3 Instrument Handbook (Appendix C) displays the definitions of a 2 point line dither, a 3 point line dither, and a 4 point box dither, which have been implemented in APT. In some cases there is also a need for more points in a pattern (e.g., 6 full frame exposures fit in an orbit). An upcoming ISR will provide supplemental dither patterns for sets of more exposures - unique n > 4 dither line patterns. These new patterns will be available in the form of prescribed POS TARGs for Phase II Cycle 28 preparations.

We remind the user that IR dither patterns should be at least 10 pixels to avoid self-persistence for sources that are larger than 5 pixels in size. The standard box pattern is significantly smaller than this. For more information, see the ISR by V. Bajaj (2019).  J. Anderson (2016) provides supplemental dither patterns for sub-pixel dithers for the3 (undersampled) IR case.

We are updating the advice for how best to mitigate charge transfer efficiency (CTE) losses. We will be releasing this guidance in time for Phase II preparations. When beginning your Phase II's, please check the updated recommendations at that time. This guidance will also be located under the Current Status section on WFC3's main page.

3. New Absolute Astrometry in MAST Data Products

J. Mack, V. Bajaj, R. Avila

Improved Absolute Astrometry for WFC3 and ACS imaging data was released in MAST in December 2019. Calibrated data now include an updated WCS (world coordinate system) in the image header which is used to produce the combined visit-level drizzled data products. In the majority of cases, alignment is performed by matching stars in each HST frame directly to the Gaia DR2 reference catalog reducing astrometric errors to ~10 mas (see ‘Improvements in HST Astrometry’ for more detail). For fields with insufficient overlap with Gaia, the WCS is defined by the Hubble Guide Star Catalog (GSC 2.4) or the Hubble Source catalog (HSC 3.0), which correct the coordinates of HST’s guide stars with their positions from Gaia DR1 with typical ~200 mas accuracy.

In January 2020 the WFC3 team published a Newsletter article describing ‘New Astrometric Solutions in WFC3 Images’ and highlighted a Jupyter notebook for 'Using updated astrometry solutions' and evaluating the quality of the alignment. For some datasets, the new WCS solutions may not be optimal, and users may wish to reset the WCS to the original solution, as described in Section 4 of the notebook.  If desired, users may also manually align observations to the Gaia frame, following the workflow described in the notebook ‘Aligning HST images to an absolute reference catalog'. 

4. WFC3/IR Sensitivity Over Time 

V. Kozhurina-Platais, S. Baggett

The multi-cycle astrometric calibration programs of WFC3/IR observations of the globular cluster Omega Centari have been used to examine the long-term trend of the IR throughput and its variation over time (ISR 2020-05).

Analysis of accurate ePSF fitting photometry obtained from several thousand stars is used to calculate the photometric offsets over ~11 yrs. Meticulous analysis of the calculated photometric offsets in the different magnitude ranges and the images taken at different time on orbit and different epochs shows consistent results that the sensitivity of WFC3/IR has declined by ~0.2+/-0.04%/yr. However, we also were able to show that the loss of WFC3/IR sensitivity may not be a gradual change with time; there is a possibility of abrupt changes in the sensitivity at 2011.5 and that before and after this event the sensitivity appears to be stable.

In addition to the long term changes of WFC3/IR sensitivity over time, there is also short term variation at the level of ~2% in the timescale of a few HST orbits that is due to the well-known breathing effect, thermal changes on the HST orbital timescale.

In summary, we conclude that the changes in WFC3/IR sensitivity, are of the order of ~0.2%/year, consistent with found by Bohlin & Deustua (2019) in the WFC3/IR grism data.

It is also important to note here that the changes are relatively small - on the order of 0.002 magnitude per year and may not significantly affect the relative and/or absolute WFC3/IR photometry. However, the general observer must be aware that there is evidence for a a sensitivity loss in WFC3/IR detector and that should be taken into consideration when WFC3/IR high precision photometry is required. 

5. EPER Update  

H. Khandrika

The behavior of the Charge Transfer Efficiency was measured via the Extended Pixel Edge Response method. The analysis has been updated from the 2016 results (Khandrika et al 2016) to include all on-orbit data from Cycles 22 to Cycle 26 (present). As expected, the CTE has 1) continued to decline with time and 2) is steeper for the fainter signal levels. The CTE has declined by as much as 0.0011 over the last 10 years for the lowest illumination level of 160 electrons, corresponding to a decline rate of ~0.0001 per year. We re-confirm the findings from 2016 that found that the rate of decline is no longer well-matched by a linear fit but instead, is better fit by a quadratic or cubic function.

Figure 1 shows the three fits (linear, quadratic, and cubic) along with the residuals of each fit to the data for all illumination levels. The trend for the lowest level linear fit residuals appear to have a cycle of at least 10 years, the range of data available in this study. The decadal pattern of the residuals to the linear fit was suggestive of the solar cycle. To explore this further, sunspot activity data were obtained from the Sunspot Index and Long-term Solar Observations (Royal Observatory of Belgium, Brussels).

Figure 2 shows the last 10 years of sunspot activity as compared to the linear fit residuals of the 160 electron level illumination data. Both of the datasets have been normalized to their respective maxima in order to be placed on the same relative scale: the CTE residuals are anti-correlated with the sunspot counts i.e. solar activity. These EPER findings form a consistent picture with what was found in external data after the installation of WFC3. At that time (2009), the CTE losses in WFC3 were developing at a faster-than-expected pace. The ACS instrument had not experienced such strong degradation when it was installed ~7 years earlier. The authors of a CTE white paper (Baggett et al. 2011) suggested that the solar minimum played a role: the strength of the South Atlantic Anomaly, a region particularly damaging to instruments, is inversely correlated to the solar cycle. Based on data from the ESA ERS and ENVISAT satellites and the Casadio SAA index (Casadio, Arino, and Serpe 2010) as a measure of the SAA radiation environment, the exposure levels were significantly higher at the start of the WFC3 mission (2009) than they were at the start of the ACS mission (2002), in agreement with the external CTE measurements early in the lifetimes of those instruments. This is the first time this effect has been observed in practice in WFC3.


Figure 1: Top left: Decline of EPER CTE over time as a function of illumination level (electrons) with overplotted linear fits to the data. Top right: Quadratic fit to the CTE measurements versus time. Bottom Left: Cubic fit. Bottom Right: Residuals of each fit to the data points. The equivalent illumination level remains the same as in the other figures.


Figure 2: Sunspot activity versus time as compared to the residuals of the 160 electron illumination level. Both the sunspot data and the residuals are normalized to their respective maximum values, in order to fit on the same scale.

6.WFC3 Quicklook Anomalies Database

J. Medina

The WFC3 Quicklook Anomalies database, previously available only to internal (STScI) users, is now available to the public in the form of a table in an ascii document. This database is a record of all WFC3 datasets (since 2009) whose science images contain at least 1 anomaly (e.g. satellite trails, persistence, etc). Users can parse through this database to determine if their datasets contain any anomalies and which kind. Two examples on how to utilize the database are described in ISR 2020-02, as well as an explanation of the caveats involved in detecting and flagging anomalies. The database will be updated periodically, with the file name denoting when it was last updated (download current database file). 

7. The Dispersed infrared background in WFC3 G102 and G141 observations

N. Pirzkal, R. Ryan

Background removal is one of the, if not the most, important step during the extraction and calibration of slitless observations. Any error in the amount of dispersed background light has an immediate effect on the quality of the extracted spectra. We derived three new independent background components (Zodi background, HeI emission, and ''Scatter'' Earth light) which have different spectral signature and therefore contribute differently to the overall background of the final observation. We also introduced a generalized implementation of the variable background subtraction described in ISR 2017-01. This approach allows us to account for a temporal variation of both the HeI and ``Scatter'' light and retain the use of on-the-ramp fitting in CALWF3. We also investigate, using all of the available G102 and G141 archival data, the conditions under which we measure an increased levels of HeI and ''scatter'' light emission. Details are available in ISR 2020-04.

8. HST 30th Anniversary Image Information

 J. Green 

The aquatic-inspired "Cosmic Reef"  image released for the 30th Anniversary of the launch of Hubble on April 24, 2020 is a 3 x 4 tiled mosaic of NGC 2020 and 2014, part of a vast star-forming region in the Large Magellanic Cloud, several hundred parsecs across. The image was taken using WFC3's F475W, F502N, F657N, and F814W filters to highlight the starlight/continuum, as well as gaseous oxygen, hydrogen, and nitrogen. The data are also available as a High-level Science Product (HLSP) in MAST.

Downloadable versions:
News Story/Image

Video: Flight into the Cosmic Reef

Video: Zoom into the Cosmic Reef

9. New Documentation

ISR 2020-01: Recommendations for Optimizing Rapid Ultraviolet HST Observations of Gravitational Wave Optical Counterparts. L. Strolger, A. Rest, O. Fox, A. Calamida, R. Ryan, N. Reid.
General guidelines for Target of Opportunity observations.

ISR 2020-02: WFC3 Quicklook Anomalies Database. J. V. Medina, S. Baggett, & The Quicklook Team.
Release and description of the public WFC3 Quicklook Anomalies database.

ISR 2020-03: IR `Snowball' Occurrences in WFC3/IR: 2009-2019 Snowballs Database. J. D. Green, H. Olszewski.
IR "snowball" artifacts have not changed in occurrence rate over 11 years of WFC3, constraining their likely cause.

ISR 2020-04: The Dispersed Infrared Background in WFC3 G102 and G141 Observations. N. Pirzkal, R. Ryan.
The development of background subtraction routines using a generalized multi-component algorithm including Earth-based scattered light, on G102 and G141 observations.

ISR 2020-05: WFC3 IR Sensitivity Over Time. V. Kozhurina-Platais, S. Baggett.
The sensitivity of WFC3/IR appears to be changing at the level of 0.2% (0.002 mag) per year.

ISR 2020-06: WFC3/UVIS EPER CTE 2009 - 2020. H. Khandrika. CTE loss has been levelling off since 2009, and is anti-correlated with solar activity.

The complete WFC3 ISR archive is available here.

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