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WFC3 STAN Issue 49, June 2025

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June 5, 2025
WFC3 NEWSLETTERS

About This Article

1. WFC3 at the 246th AAS Meeting

A. O'Connor

WFC3 will be at the 246th AAS Meeting in Anchorage, AK! We encourage AAS attendees to stop by our iPoster to learn about the many incredible WFC3 software tools and Jupyter notebooks available for observers. We welcome your questions and feedback on proposing, analysis, documentation, or any other aspects of your experience with WFC3. 

Below, we provide session information and link to the listing in the AAS meeting itinerary, where the abstracts can be found. Times are given in Alaska Daylight Time. We look forward to meeting you in Anchorage, Alaska! 

HST/WFC3: Jupyter Notebooks and Software Tools for Data Analysis- A. O'Connor (presenting), M. K. Alam, M. Revalski, F. Dauphin, B. Kuhn, A. Pidgeon, S. Baggett, J. Green,  and the WFC3 Team

 Wednesday, June 11, 2025, 9:00 AM - 10:00 AM

 

2. STScI Help Desk Updates

The WFC3 Help Desk Team

We would like to inform users that the STScI Help Desk email address (help@stsci.edu) for all observatories will be deactivated by the end of this year. Moving forward, any help desk inquiries that you may have are to be relayed through the existing ServiceNow web portals for the relevant mission/division:

STScI has used the ServiceNow platform for the help desk since early 2017, but we have kept the email address active as a courtesy to the user base. These portals offer announcements, knowledge base articles that address frequently asked questions, options to submit your questions directly to the appropriate experts, and access to your support history. A free MyST account is required to use these services. For any issues accessing ServiceNow, please contact helpsn@stsci.edu.

 

3. Masking Diffraction Spikes using HST1PASS

J. Anderson

HST1PASS is a FORTRAN-based software package to perform HST photometry (ISR 2022-05). The software has recently been enhanced to allow observers to mask diffraction spikes in their data. The correction is controlled via a new set of parameters, as shown in Figure 1 below (updates to the documentation and a tutorial ISR are in progress).  By means of these parameters, observers can set the level (in DN or DN/s) above which spikes should be masked, specify which flags to set in the data-quality (DQ) file for pixels in spikes, and specify the radial range over which spike flagging should be performed.  In addition, the software can generate both an image showing the expected spike impact as well as new version of the flt/flc with the spike-pixels flagged in the DQ array.  Figure 2 below presents an F160W WFC3/IR image stack before and after application of the diffraction-spike flagging.  HST1PASS can be obtained via the WFC3 Software Tools page. Please refer to ISR 2022-05 for the HST1PASS user's guide.

new_HST1PASS_params.png
Figure 1: New parameters within HST1PASS to control masking of diffraction spikes.
F160W-spike-masking.png
Figure 2:  A stack of WFC3/IR F160W images before (left) and after (right) HST1PASS correction of diffraction spikes.

 

4. New Calibration Initiatives for Cycle 32

J. Mack, J. Anderson, V. Bajaj, A. Calamida, G. Anand

Several new calibration initiatives were approved in Cycle 32 and are currently executing. The ID's for each program are linked in Table 1 and summarized here. For more detail, see the PDF slides posted on the WFC3 Calibration Plan webpage.  

The first program will test the pointing stability of HST under single-star guiding, which is the only guiding option for some targets in Reduced Gyro Mode (RGM).  In each of five independent visits, this program will take five 350s exposures with no commanded POS TARG between them.  Any shift between the exposures will thus represent pointing drifts.  We can measure the pointing drift with a precision of better than 0.01 pixel (0.4 mas) in WFC3/UVIS---much better than the nominal 7-mas pointing jitter.  This dataset will allow us to evaluate trailing in exposures from 350s to over 1750s (a full orbit) in length.  In addition to fitting for shifts, PSF fitting will also be used to quantify the apparent blurring in the PSF for trailed images of various exposure times.  Image blurring can be quantified by a decline in the fraction of light contained in stars' central pixels. 

The next program will revisit the primary astrometric field in the LMC which lies in JWST's continuous viewing zone. In 2006 and 2017, an extensive set of ACS observations were acquired and used to create a standard astrometric catalog for calibrating the distortion and scale of the JWST instruments (Anderson et al. 2021).  Limited WFC3 observations were taken in 2009 with UVIS and IR and cover only a small portion of the field. A new epoch of observations will help constrain proper motions for improved astrometric cross-calibration.  

Two new cross-mission flux calibration programs will observe twenty CALSPEC spectrophotometric standards in WFC3 and ACS filters that have wavelength overlap with some JWST and Roman filters.  For WFC3, IR grism observations will be used to improve the Spectral Energy Distribution (SED) of these standards in CALSPEC, and will be useful for cross-calibration with the JWST (NIRSPEC) grisms. WFC3 imaging in both UVIS and IR channels will be useful for cross-calibration with JWST (NIRCAM, NIRISS) and Roman (WFI) bandpasses.  A coordinated ACS program will observe the same flux standards in four filters for cross-mission overlap (F606W, F625W, F775W, F814W), plus a fifth filter (F435W) to sample the entire WFC detector wavelength range.

ID PI Orbits Title
17893 Anderson 5 Impact of Single-Star Guiding on HST Pointing Stability with WFC3/UVIS
17905 Bajaj 3 Astrometric Cross Calibration for JWST and HST (ACS and WFC3)
17906 Calamida 28 HST/WFC3 Observations for Cross-Mission Flux Calibration
17895 Anand 14 HST/ACS Observations for Cross-Mission Flux Calibration


Table 1: Program IDs for new calibration programs approved midway through Cycle 32 and are currently underway.

 

5. New Analysis of the UVIS Time-Dependent Sensitivity

A. Pidgeon, J. Mack

New filter-dependent sensitivity losses for UVIS1 and UVIS2 have been computed using 16 years of in-flight monitoring data (Pidgeon and Mack, 2025, in prep.). While the slopes for UVIS1 are broadly consistent with the values in the current image photometry (IMPHTTAB) reference file (see Calamida et al.,  WFC3 ISR 2021-04), the slopes for five filters—F390M, F390W, F410M, F475W, and F555W—are significantly steeper in UVIS2 and agree more closely with the UVIS1 slopes. For these filters, the total sensitivity loss from the WFC3 reference epoch (June 2009) to the current epoch (June 2025) differs by more than 1% from the value predicted by the current (2021) IMPHTTAB. Here, a method for correcting the UVIS2 observed count rates with the new slope values is described, and an ISR with the results for additional filters is in preparation. 

Figure 3 shows the F555W observed-to-synthetic (obs/syn) flux ratio for five CALSPEC standards observed from 2009-2025 on UVIS1 (CCD amp A) and UVIS2 (CCD amp C). The slope (percent loss per year) and the total loss over 16 years are indicated at the top of each panel. To account for global offsets in the obs/syn ratios, the inverse-variance-weighted mean slope is computed based on a linear fit to each individual star. While the UVIS1 slope is consistent with the current IMPHTTAB (delivered in 2021), the UVIS2 slope is steeper in the new fit. 

F555W_mean_slope.png
Figure 3: Observed-to-synthetic photometry of five CALSPEC stars observed from 2009-2025 in the F555W filter on UVIS1 (amp A) and UVIS2 (amp C). The slope (percent loss per year) and the total loss over 16 years are indicated at the top of each panel for the current (2021) IMPHTTAB and for the new analysis.

For these filters, UVIS2 photometry may be corrected without rerunning calwf3. First, divide the observed count rate (in electrons sec-1) by the value of the keyword PHTRATIO. This 'backs out' the FLUXCORR step of calwf3, which multiplies the UVIS2 science array by the time-dependent chip inverse sensitivity ratio (PHTRATIO). For more details on calwf3 processing and photometric keywords, see the WFC3 Data Handbook Section 3.2.12 and Section 9.1.4.

\(\text{Countrate}_{UVIS2,corr} = \text{Countrate}_{UVIS2,obs} / \text{PHTRATIO}\)

Next, compute a new inverse sensitivity keyword (PHTFLAM2MJD , in units of erg sec-1 cm-2 A-1 per electron sec-1) for UVIS2 using the EXPSTART value from the image header and the data from Table 1, where Slope is the percent loss per year and PHTFLAM2ref is the inverse sensitivity of UVIS2 at the reference epoch (MJD=55008) from Calamida et al., 2021 (WFC3 ISR 2021-04).

\(\text{PHTFLAM2}_{MJD} = \text{PHTFLAM2}_{ref} \times (1 - \frac{\text{Slope}}{100} \times \frac{\text{EXPSTART} - 55008}{365.25})\)

Finally, multiply the corrected UVIS2 count rate by the new inverse sensitivity to convert to absolute flux units (erg sec-1 cm-2 A-1 ).

\(\text{FLUX}_{UVIS2,corr} = \text{Countrate}_{UVIS2,corr} \times \text{PHTFLAM2}_{MJD}\)

Filter

Slope (% yr-1)

 IMPHTTAB Slope ( % yr-1)

PHTFLAM2ref

F390M

 -0.162 ± 0.005  -0.067 ± 0.287 2.5166e-18
F390W -0.133 ± 0.004  -0.025 ± 0.017 4.9674e-19
F410M -0.130 ± 0.007  -0.034 ± 0.357 2.3529e-18
F475W -0.162 ± 0.004  -0.061 ± 0.099 2.5139e-19
F555W -0.154 ± 0.005 -0.054 ± 0.207 1.8482e-19


Table 2: UVIS2 slopes for five filters with the largest differences with the current IMPHTTAB reference file (WFC3 ISR 2021-04).  

6. Astroquery Now Supports Downloads Through HST and JWST Search Interfaces

S. Bianco

The astroquery.mast.MastMissions class now includes enhanced capabilities for retrieving and downloading data products! As a Python wrapper for the modern mission-specific search forms, MastMissions allows users to search mission-specific metadata and access data products more efficiently. The workflow for querying datasets and retrieving products is outlined in our “Searching for Mission-Specific Data with Astroquery” notebook. Other new features include authentication for proprietary data access and the ability to switch between missions to quickly change the collection being searched.

To check out the latest release (0.4.9.post1) of Astroquery and take advantage of these improvements, run the following command in a terminal:

    $ python -m pip install -U –-pre astroquery

You can verify your version by running these lines of code in a Python script or interactive session:

    import astroquery

    print(astroquery.__version__)

If you have any questions or feedback, feel free to open a GitHub issue or email us at archive@stsci.edu. Thanks for using astroquery.mast!

astroquery_notebook.png
Figure 4: A screenshot of the “Searching for Mission-Specific Data with Astroquery” notebook tutorial. The main notebook content in the middle of the page outlines learning goals, including using the astroquery.mast module to access mission-specific data from MAST, run metadata queries, filter and download datasets, and search multiple missions. Below the learning goals, a table of contents lists sections such as querying for datasets, getting and downloading data products, and practice exercises.

7. New Documentation

ISR 2025-01: Analyzing Exoplanet Transits Observed with the WFC3/UVIS G280 Grism - M. K. Alam, F. Dauphin, A. Pagul

ISR 2025-02: WFC3/UVIS EPER CTE 2009-2025 - A. O'Connor, H. Khandrika

 

The complete WFC3 ISR archive is available here. Additional information about WFC3 calibration, performance, data analysis, software tools, and more can be found online.

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