About This Article
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Contents:
1. Notes for HST Cycle 34 Phase 2 Submissions
2. Have Cycle 34 Phase II Questions? Join our August 6th Office Hours!
3. New HST Notebook for WFC3/UVIS: Equalizing Amplifier Offsets in FLC/FLT Images
4. Updates to WFC3/UVIS Encircled Energy Values
5. Updates to the MAST PSF Cutout Database
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1. Notes for HST Cycle 34 Phase II Submissions
A. Pagul
HST Cycle 34 proposal results came out last week and observers will have until August 18th, 2026 to prepare their Phase II submissions. Here are a few notes and tips on Phase II submissions for Cycle 34:
- Remember to choose the appropriate postflash level to mitigate charge transfer inefficiency (dark + sky + postflash should total 20e-/pix; see WFC3 ISR 2021-09).
- Remember that anything not explicitly included in the accepted Phase I proposal must be approved. For any major change (ie. one that affects schedulability), a TTRB request is required.
- The default mode for WFC3 exposures <1001 sec is single star guiding (1GS), which has been shown to provide high quality imaging; see WFC3 ISR 2025-07. Longer exposures as well as moving targets and spatial scans will continue to be done under 2GS. PI’s can request an exception to the single guide star default by submitting written justification by August 3rd. Those should be emailed to your assigned PI/CS (not to the TTRB) with cc to the proposal history folder. More information and resources are available in this Primer and the STScI Newsletter on 1GS.
2. Have Cycle 34 Phase II Questions? Join our August 6th Office Hours!
M. Revalski
On July 21st, astronomers around the world received decisions letters for their Cycle 34 Hubble Space Telescope proposals. A complete list of accepted proposals will soon be provided on the Approved Programs page, and Guest Observers have until August 18th to submit their Phase II program using the Astronomer’s Proposal Tool (APT). As in earlier cycles, the Primary Investigator for each WFC3 program will be assigned a Contact Scientist (CS) who can answer any technical questions that may arise while crafting their Phase II.
We encourage all users with Phase II questions to join a WFC3 expert live via Webex on Thursday, August 6th at 11:00am eastern, as part of our monthly WFC3 Office Hour series. Do you have questions about selecting the best aperture for your target? Optimizing dither patterns, post-flash values, or orbit packing? Join us online for practical advice on how to optimize your observations and squeeze every second of exposure time out of your science program.
Users that are unable to join WFC3 Office Hours are encouraged to submit their questions via the HST Help Desk. In addition, the team now maintains the Hands-on Hubble video tutorial series that covers a wide range of WFC3 resources that may be useful for crafting new programs. The WFC3 team is constantly searching for ways to better support the community, and we welcome your questions and feedback during Office Hours. Please visit the HST Video Tutorials webpage for a list of upcoming dates and the Webex link.
We look forward to seeing you on August 6th, 2026 at 11:00am eastern time.
3. New HST Notebook for WFC3/UVIS: Equalizing Amplifier Offsets in FLC/FLT Images
A. O'Connor
A new WFC3 notebook is available on the HST Notebooks GitHub page to help observers identify and correct small amp-dependent bias offsets in WFC3/UVIS data.
The WFC3/UVIS detector has four amps (A, B, C & D), and small bias level differences can sometimes remain between them even after standard calwf3 processing. These residual offsets can appear as visible amp-to-amp background variations. This notebook provides a step-by-step guide for measuring these offsets and equalizing the bias levels across all four amps to produce a more uniform image background. We encourage WFC3 observers to use this notebook whenever residual amp-dependent offsets are evident in their UVIS images.
You can access this new notebook directly at:
https://spacetelescope.github.io/hst_notebooks/notebooks/WFC3/uvis_amp_equalization/uvis_amp_equalization.html
This notebook is part of the HST Notebooks repository on GitHub, which provides practical, hands-on tutorials for HST data analysis and calibration for all instruments, as well as DrizzlePac, HASP, and HSLA tutorials. Explore the full repository at:
https://spacetelescope.github.io/hst_notebooks/
4. Updates to WFC3/UVIS Encircled Energy Values
A. O'Connor, J. Mack, and V. Bajaj
Updated WFC3/UVIS encircled energy (EE) table are available for a subset of filters, using improved analysis techniques and new measurements of the PSF wings. The revised EE values are more accurate at small radii (r ≤ 10 pixels) and bring the calibration into closer agreement with a recent large archival PSF study. The impact on photometry can be significant for small apertures, e.g. the EE fraction between 5 and 10 pixels, EE(5)/EE(10), changes by up to 3% for some filters.
Because UVIS zeropoints are derived from photometry measured within a 0.4″ (10-pixel) aperture and corrected to a 6″ aperture using EE tables, the revised EE solutions lead to modest changes in the UVIS photometric calibration. For several filters (F218W, F225W, F275W, F775W, F814W, and F845M), the EE at 0.4″ is now larger by ≳0.5% (≈0.005 mag), particularly for UVIS2. In contrast, the EE values for the long-pass filters F200LP, F350LP, and F850LP are now smaller, with F850LP showing the largest difference at ∼2% (≈0.02 mag).
Updated EE solutions will be delivered with revised UVIS inverse sensitivity tables and zeropoints later in 2026. In the meantime, Appendix A of the report provides EE tables for commonly used filters, while Appendix B includes a worked example demonstrating how observers can correct measured fluxes to account for the revised EE values.
For additional details, see WFC3 ISR 2026-02.
5. Updates to the MAST PSF Cutout Database
I. Rivera, M. Revalski
The WFC3 team has significantly expanded the archive of stellar cutouts that provide examples of the instrumental point-spread function (PSF) available through the Mikulski Archive for Space Telescopes (MAST). Over the last two and a half years (2024 through April 2026), approximately 3.7 million new stellar cutouts have been added for WFC3/UVIS and 1.6 million for WFC3/IR. These additions bring the total archive to more than 37 million PSFs for WFC3/UVIS and 27 million PSFs for WFC3/IR.
The database provides access to stellar cutouts extracted from both raw and calibrated observations with WFC3. Sources are measured using the HST1PASS software package, which employs empirical HST PSF models and one-pass photometry to identify and characterize isolated stellar sources across all calibrated WFC3 FLT data. The database is updated annually as new public observations become available. A similar database is available for WFPC2, containing 25 million sources from external observations during the instrument’s lifetime. See WFC3 ISR 2021-12, "The WFPC2 and WFC3 PSF Database" for an overview of the PSF pipeline used to extract stellar sources and update the databases. For information on the WFC3 PSF archive, access tools, and documentation, visit the WFC3 PSF Search Page.
Users can access the data through the MAST Portal or programmatically through the MAST API . The HST notebook “Downloading WFC3 and WFPC2 PSF Cutouts from MAST” provides examples for querying, filtering, and downloading subsets of PSFs tailored to specific scientific applications. A wide range of search parameters is available, enabling users to retrieve stellar cutouts based on characteristics such as instrument and detector (WFC3/UVIS, WFC3/IR, WFPC2), filter, exposure time, detector position, telescope focus value, observation date, and stellar flux and quality metrics. All searchable fields are described on the WFC3 PSF MAST API Documentation page.
These observed PSFs enable a variety of science and calibration applications. For example, Section 2.3 of the "HST WFC3 Point Spread Function Modeling" notebook demonstrates how users can generate custom stacked PSFs matched to their observations, which may help in more accurate photometry and astrometry.
Example Calibration Science Application: Mapping the WFC3/UVIS Saturation Limit
A recent example of the power of this database is described in WFC3 ISR 2025-06, “Updates to the WFC3/UVIS Saturation Map.” In that study, approximately one million stellar PSFs retrieved from MAST were used to determine the flux level at which the central pixel of a stellar image begins to deviate from linearity due to saturation. The large number of observed stellar cutouts enabled saturation measurements across 1,024 regions of the UVIS detector, resulting in a new spatially varying saturation map that is now implemented in the calwf3 calibration pipeline. This work demonstrates how large samples of archived stellar PSFs can be used not only for photometric and astrometric studies, but also for improving instrument calibrations that directly benefit the entire HST user community.
As the PSF cutout database continues to grow, it will provide increasingly powerful opportunities for both scientific investigations and calibration efforts, enabling users to exploit the full precision of HST imaging data.
6. New Documentation
ISR 2026-02: Updates to WFC3/UVIS Encircled Energy Values in Select Filters
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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