About This Article
1. Cycle 34 Call for Proposals News and Updates
J. Green, S. Baggett
The Phase I Cycle 34 deadline is April 16, 2026!
Please keep the following points about WFC3 in mind when preparing your proposal submission:
- Phase 1 proposals should be as explicit as possible about requirements and restrictions. For example, Special Requirements such as timing and orientation constraints MUST be specified in Phase 1 (HST New and Important Features)
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Full support for WFC3/IR will continue for the remainder of Cycle 33 (through September 30, 2026). As noted in the New and Important Features linked above, WFC3/IR may be offered as shared risk in Cycle 34, depending on support NASA budgets for FY27 and beyond. In the case that WFC3/IR calibration support decreases in future cycles, we expect WFC3/IR data quality to remain relatively stable. The calibration changes only very slowly over time; any impact to science data is anticipated to be minor over several years.
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Starting in Cycle 34, single guide star (1GS) guiding will become the default for WFC3 observations with exposure times < 1001 sec in visits with length of 1 or 2 orbits (this does NOT include moving targets or spatial scans). There will be an opportunity to opt-out of 1GS with a strong scientific justification after Phase II notifications are sent out. Further information about 1GS is available in WFC3 ISR 2025-07.
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Observations requiring more than 6 consecutive orbits must be justified in the Special Requirements section of the Phase I. In addition, the visit will be executed at shared risk (i.e., not be eligible for repeat if impacted by observatory problems). Observers should consider alternate strategies, as there is significant risk for any observation lasting more than a few orbits.
Other tips for completing the Phase I submission:
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List all filters required, even those for the pre-imaging (direct) images necessary for grism observations.
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Search for and address all duplications with previous sources.
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Include any postflash needed for UVIS exposures in your S/N calculations e.g. using ETC. To mitigate charge transfer efficiency losses, image background levels should be a minimum of 20 e-/pixel total (natural sky + dark + postflash if needed). See the WFC3 Instrument Handbook for CTE details.
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Add a note in the Duplication Justification section if your science targets involve variability.
Late-breaking news items are announced on the Cycle 34 Call for Proposals coverpage.
2. New Pixel-based Non-linearity Correction for WFC3/IR
K. Huynh, S. Shenoy, V. Bajaj, J. Mack, M. Marinelli, N. Grogin
The WFC3/IR detector has an intrinsic non-linear response to incident photons, which is corrected in the calwf3 pipeline with the non-linearity reference file (NLINFILE). Pixel values below the full well limit are modeled by a third order polynomial fit which relates the measured and the idealized signal for each pixel. The past solution used a set of quadrant-averaged polynomials derived from heavily saturated Tungsten lamp flats, making it difficult to constrain the non-linearity at high fluence levels.
A new pixel-based non-linearity correction has been computed using a large set of on-orbit flats and is described in Shenoy et al. (2025). Extensive testing on a variety of targets and observing modes, along with an additional modification to the coefficient derivation to further improve the calibration is described in Huynh et al. (2025). The testing was performed on a variety of images processed with the new linearity correction (internal flat fields, star clusters, and HST flux standards — both imaging and grisms) acquired over a range of sample sequences.
The new correction improves the linearity for sources with fluence levels above ~50,000 e-, making sources brighter in the calibrated FLT data. Figure 1 shows improvements up to ~6% for stars with peak pixel values approaching the full well limit of ~80,000 e-. The new correction also significantly decreases the number of cosmic-rays flagged during calwf3's up-the-ramp fit, especially in the upper-left quadrant, as shown in Figure 2. As a result, the new NLINFILE improves not only the photometric accuracy of FLT calibrated data but also increases the signal-to-noise by rejecting fewer reads. The new calibration has negligible impact (∼0.1–0.2%) on the published IR Zeropoints by Calamida et al. (2024), which have an intrinsic RMS error of 0.5%.
The new reference file (NLINFILE=a2412448i_lin.fits) was delivered to CRDS on 04 Feb 2026 and is currently used for pipeline processing of new WFC3/IR observations. MAST reprocessing for all prior observations is complete as of March 2026, reflected in the NLINFILE name in the science data headers.
3. New Video Tutorials Now Available for WFC3
M. Revalski
Figure 3: Previews of the new Hands-on Hubble tutorials page, which includes beginner-friendly video guides and details for upcoming HST Office Hours.
The WFC3 team is excited to share a new video tutorial series that guides users through the fundamentals of using WFC3 data and resources for their astronomical research. This first release includes guides that cover everything from the basics of the instrument to a closer look at several practical data analysis tools. These guides can be found on the new HST Video Tutorials webpage, which includes high-resolution video players, and external links to all of the key resources found in each tutorial. This first set of video tutorials cover the following core topics:
- Tutorial #1: Instrument Essentials and Online Documentation
- Tutorial #2: The HST Jupyter Notebooks Tutorial Repository
- Tutorial #3: Using DrizzlePac to Align and Combine Exposures
- Tutorial #4: Creating PSF Models for Astrometry & Photometry
Please share this new resource with your colleagues, collaborators, and students. The first release provides foundational guides for a variety of users, and future tutorials may cover additional advanced content based on community suggestions. If you have topics that you would like to see covered in a tutorial video, then please submit your suggestions using the HST Help Desk Portal.
Stay tuned! These videos will be paired with monthly HST office hours where you can join a WFC3 expert on Webex to ask your questions on any aspect of WFC3 observations and software. The first session is scheduled for Thursday, April 2nd, 2026 at 11:00am EDT. Connection details will be posted on the tutorials webpage. No pre-registration is required, and users of all experience levels are encouraged to ask questions.
4. New Documentation
ISR 2025-08: Pixel-Based Non-Linearity Correction for the WFC3 IR Detector - S. Shenoy, K. Huynh, V. Bajaj, J. Mack
ISR 2025-09: Testing and Validation of the Updated Pixel-Based Non-Linearity Calibration File for WFC3/IR - K. Huynh, V. Bajaj, M. Marinelli, J. Mack, S. Shenoy, N. Grogin
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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