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WFC3 STAN Issue 48, January 2025

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January 10, 2025
WFC3 NEWSLETTERS

About This Article

1. Meet the WFC3 Team at the 245th AAS Meeting

M. Revalski, M, Marinelli

Are you attending the 245th Meeting of the American Astronomical Society? So are we! We encourage AAS attendees to stop by our two iPosters to learn about the latest WFC3 tools and news. 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 Eastern Standard Time (EST). A full list of STScI-related events at the AAS meeting is available in the STScI Newsletter. We look forward to meeting you at National Harbor! 

203.07: New Tools for Modeling the Point Spread Function of Wide Field Camera 3 - Mitchell Revalski (presenting), Frederick Dauphin, Varun Bajaj, Jay Anderson, and the WFC3 Team

Tuesday January 14th, 9:00 AM - 10:00 AM

361.04: HST/WFC3 Photometric Calibration Updates – Mariarosa Marinelli (presenting), Varun Bajaj, Ky Huynh, Annalisa Calamida, Jennifer Mack

Wednesday January 15th, 5:30 PM - 6:30 PM

2. Notes on WFC3 in the Call for Proposals for Cycle 33

A. Pagul, J. Green

The Call for Proposals for Cycle 33 has come out: https://hst-docs.stsci.edu/hsp/hubble-space-telescope-call-for-proposals-for-cycle-33

Please note that the Call continues to include opportunities to propose for WFC3/IR and ACS/WFC, but as shared risk observations. We anticipate minimal levels of support for any selected WFC3/IR programs.

Proposers can refer to the updated Cycle 33 Instrument Handbook for additional detail: https://hst-docs.stsci.edu/wfc3ihb

For example, note the new section comparing the bandpasses of WFC3/UVIS and IR filters with overlapping filter wavelengths on JWST.

Finally, when using the HST ETC (https://etc.stsci.edu/) to plan observations and estimate orbits needed to reach certain signal-to-noise values, users should carefully consider the "number of frames" keyword (i.e. the number of dithers) needed per target. Dithering is strongly recommended, which for faint sources can require additional time for overheads.

 

3. WFC3 Instrument Handbook for Cycle 33

M. Marinelli, J. Green

The Cycle 33 edition of the WFC3 Instrument Handbook has been published! Updates to the previous edition of the Handbook are described in Section 1.1, and include: 

  • New comparisons between select WFC3/UVIS and WFC/IR bandpasses with JWST/NIRCAM filters (Section 3.3.4). 
  • Discussion of recent analysis regarding the impact of x-CTE (serial CTE) in Section 6.9.
  • Revisions to Section 6.11.5 and 7.10.6 to reflect that the Drift-and-Shift (DASH) observing method is unavailable in reduced gyro mode (RGM).
  • Updates to Sections 7.8 and 9.3 to reflect the new (2024) WFC3/IR time-dependent photometric calibration.
  • Inclusion of Table 7.11 to share WFC3/IR sensitivity loss rates for F098M and all five wide-band filters.
  • Addition of further context regarding the 2020 calibration of the G280 grism to Section 8.2.
  • Edits to Section 8.6 to reflect new limitations to spatial scan rates (as of June 2024) due to RGM. 
  • Summary of the WFC3 calibration plan for Cycle 32 in Appendix E (Section E.19)

A PDF version of the entire Handbook is linked at the top webpage (orange box in Figure 1). The web version is searchable using the Search box above the Table of Contents (blue box in Figure 1). For example, searching "single guide star" will produce a list of sections in which this phrase occurs. On the result page (Figure 2), the question mark icon in the search box provides descriptions of how to refine this search using logical statements, wildcards, etc.

wfc3_ihb_home.png
Figure 1: Web version of the Cycle 33 WFC3 Instrument Handbook. A PDF version is downloadable using the link in the top left corner, while the web version is searchable using the search box.
wfc3_ihb_search.png
Figure 2: Results page from searching the online version of the WFC3 IHB for “single guide star”; overlain is the Search Help window, which appears when a user clicks the question mark icon in the search box.

4. New Time-Dependent WFC3/IR Inverse Sensitivities

A. Calamida, M. Marinelli, V. Bajaj, A. Pidgeon, J. Mack

The automated calibration pipeline has been updated to provide IR time-dependent zeropoints, based on new time-dependent inverse sensitivities for the WFC3/IR detector that have been delivered (ISR 2024-13). These were calculated using the sensitivity change slopes measured by Marinelli et al. (ISR 2024-06) and photometry of five CALSPEC standards (the white dwarfs GRW+705824, GD153, GD71, G191B2B, and the G-type star P330E) collected from 2009 to 2023. The new inverse sensitivities account for losses of 1-2% over 15 years, depending on wavelength, and provide an internal photometric precision better than 0.5% for all wide-, medium-, and narrow-band filters (see Figure 3). 

An updated version of calwf3 (v3.7.2) has been developed for use with a new time-dependent image photometry table (IMPHTTAB, 8bq13281i_imp.fits).  New values of the photometric keywords PHOTFLAM (erg/cm2/A/e-) and PHOTFNU (Jy/s/e-) will be populated in the FLT image headers for WFC3/IR images downloaded after December 2024.  Previously-retrieved data should either be re-downloaded to obtain the new header keywords or re-processed with calwf3 (v3.7.2 or higher) and the new IMPHTTAB reference file. Alternately, the new inverse sensitivities may be manually computed for a specific observation date by running stsynphot.

It is important to note that the new inverse sensitivities do not impact the science data (pixel values) of WFC3/IR images. The only change appears in the FLT image header, wherein the PHOTFLAM and PHOTFNU keywords now reflect the inverse sensitivity for a specific date recorded in the EXPSTART keyword.  Therefore, in order to calibrate prior observations, it is only necessary to apply the new time-dependent inverse sensitivities to the flux measurements. For more details, see the IR Photometric Calibration webpage.

wfc3_IR_final_ratio_july18_2024.png
Figure 3: Ratio of the observed to synthetic count rates in five wide-band and four medium-band WFC3/IR filters as a function of pivot wavelength for the five CALSPEC standards used in the calibration. The dashed black line indicates a unity ratio. Error bars are displayed. 

 

5. Temporal Stability of the WFC3/UVIS Geometric Distortion Solution

A. O'Connor, J. Mack, V. Bajaj, A. Calamida

A new WFC3 report (ISR 2024-15) presents analysis showing a small time evolution in the linear terms of the UVIS geometric distortion (shift, rotation, scale, and skew) with respect to the Gaia DR3 catalog.  Scale offsets between UVIS filters are found at all epochs (Figure 4), and a large residual rotation is observed between 2018-2022 (Figure 5).

MAST pipeline processing includes an alignment step which corrects WFC3 images for any residual linear terms with respect to Gaia DR3.  In addition to pointing errors, this step corrects for any residual offsets in the linear terms of the distortion solution, but the accuracy of the fit depends on the number of matched sources.  For observers requiring high-precision astrometry, we recommend using TweakReg to realign images, and we provide links to DrizzlePac tutorials for improving both relative and absolute astrometry in WFC3 observations (see the next article).

 

wfc3_uvis_scale_evolution.png
Figure 4: Scale ratio: WFC3/UVIS to Gaia DR3. Each color and shape combination represents a separate filter. A line of best fit is plotted in black.
wfc3_uvis_rotation_evolution.png
Figure 5: Rotation term offsets: WFC3/UVIS to Gaia DR3. Each color and shape combination represents a separate filter. A line of best fit is plotted in black.

 

6. New DrizzlePac Tutorials for WFC3, ACS, & WFPC2

J. Mack, M. Revalski, V. Bajaj, R. Avila

MAST data products include improved astrometry for WFC3, ACS, and WFPC2 images by aligning to Gaia DR3 (see ISR 2022-06). We highlight new python tutorials for checking the quality of the alignment in the FITS data and then selecting either the active world coordinate system (WCS), an alternate WCS, or re-aligning the images manually. The recommended workflow will depend on the science goals and the number of point sources in the frame with well-measured positions.

In particular, we highlight several specific use cases: 

 

7. New Documentation

ISR 2024-11: Sensitivity Evolution of the HST WFC3/UVIS G280 Grism - M. Alam, A. Pagul, A. Calamida, B. Kuhn, D. Som, S. Baggett

ISR 2024-12: WFC3/UVIS: Pixel-to-Pixel Quantum Efficiency 2013-2023 - F.Dauphin & B. Kuhn

ISR 2024-13: New Time-Dependent WFC3/IR Inverse Sensitivities - A. Calamida, M. Marinelli, V. Bajaj, A. Pidgeon, J. Mack

ISR 2024-14: Using machine learning for object classification and filtering - V. Bajaj & F. Dauphin

ISR 2024-15: WFC3/UVIS Geometric Distortion - Time Evolution of Linear Terms w.r.t. Gaia - A. O’Connor, V. Bajaj, J. Mack, A. Calamida

ISR 2024-16: New WFC3 Superdarks for HST Cycles 26-30 - A. Pidgeon, 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.

Need help? stsci.service-now.com/hst

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