About This Article
Callout
Table of Contents
- Full support for ACS in Cycle 33 and prospects for future Cycles
- CR-SPLITs disallowed in Cycle 34 Phase II programs
- Empirical SBC backgrounds available in ETC 34.1.1
-
Recommendations for observations of extended sources in light of CTE degradation
End callout
1. Full support for ACS in Cycle 33 and prospects for future Cycles
The ACS Team
In light of positive budget news, full support for ACS/WFC is confirmed for the remainder of Cycle 33. ACS/WFC may be offered as shared risk in Cycle 34. The level of operational support in future cycles is contingent on NASA budgets for FY27 and beyond. In the “shared risk” scenario:
- ACS/WFC would still be operational for science. Only basic calibration activities such as anneals, and dark and bias reference file deliveries would be performed. Calibrated and drizzled data products would be available via MAST, as they are now.
- While very limited helpdesk services may be available, there would be no contact scientists assigned to programs using ACS/WFC, no ACS/WFC representation on the telescope time review board (TTRB), and no documentation updates.
- Hubble Observation Problem Reports (HOPRs) would not be allowed, as there would be no staff to assess data quality in case of suspected observation failure.
Despite these limitations, it is expected that the calibration accuracy of ACS/WFC data would remain very good in a “shared risk” scenario. A recent internal study indicated the calibration could drift up to a few tenths of a milli-mag per year if only basic calibration activities were carried out (darks and biases).
ACS/SBC remains fully supported in current and future Cycles.
2. CR-SPLITs disallowed in Cycle 34 Phase II programs
The ACS Team
Starting in Cycle 34, single guide star (1GS) guiding will become the default for ACS/WFC observations with exposure times <1001s in visits with length of 1 or 2 orbits (not including moving targets). There will be an opportunity to opt-out of 1GS with a strong scientific justification after Phase II notifications are sent out.
In order to mitigate any potential effects on data quality from telescope roll during 1GS observations, CR-SPLITs will be disallowed in APT for Cycle 34 Phase II programs. If your program requires images at the same detector location, alternative methods can achieve this (i.e., zero-spacing dither, non-interruptible sequence special requirement, 2GS). However, proposers are encouraged to minimize observing constraints as much as possible to improve schedulability. Special requirements must be thoroughly justified in the Phase I proposal. Please contact the HST Help Desk for assistance.
3. Empirical SBC backgrounds available in ETC 34.1.1
C. J. R. Clark
Analysis of historical ACS/SBC data has shown that airglow is usually significantly fainter than what was previously predicted by the ETC in four filters: by a factor of 2.51 for F115LP, 2.64 for F122M, 105 for F125LP, and 3.64 for F140LP. As a result, far shorter integration times are necessary for background-limited observations in these filters. ETC version 34.1.1 provides the option to use an empirical background model that uses the backgrounds measured from historical data; see ACS ISR 2025-04 and the ETC users guide backgrounds page for details.
4. Recommendations for observations of extended sources in light of CTE degradation
D. V. Stark
The ACS team recently conducted an analysis of how degraded Charge Transfer Efficiency (CTE) impacts photometry of extended sources (spanning size scales of tens to a few hundred pixels) in ACS/WFC imaging data. This analysis used observations of the same galaxy cluster field carried out in 2004, 2013, and 2021 and spanning a range of backgrounds from 15-40e-. Full details can be found in ACS ISR 2025-02.
Our analysis demonstrated that degraded CTE can impact extended source photometry. Based on our findings, we advise users to use FLC/DRC images for scientific analysis and continue to follow the standard practice of ensuring backgrounds stay above 30 e-/pixel. Doing so will yield reliable global brightnesses for targets that are brighter than 300e- per exposure. However, even when following this guidance, measurements on smaller scales may still be systematically underestimated. If users are conducting science that requires spatially resolved analysis, we recommend targets be placed within 512 pixels, or as close as possible, to the CCD serial registers to minimize the impact of degraded CTE. The ACS team is continuing to assess the impact of degraded CTE on extended source measurements.
5. Documentation for Cycle 34 released
The ACS Team
Documentation relevant to the preparation of HST Cycle 34 ACS proposals has been released. This includes:
6. Recent ACS notebooks
A. Guzman and The ACS Team
The ACS team has created a number of new Jupyter notebooks to guide users in ACS data exploration and analysis. For a full list of ACS and other HST notebooks, please visit the HST notebooks homepage.
7. Recent Instrument Science Reports
For a complete list with abstracts, please visit the ACS ISR webpage.
- ACS ISR 2026-01: “Using 23 Years of ACS/SBC Data to Understand Backgrounds: Explaining & Predicting Background Variations” (Clark et al.)
- ACS ISR 2025-04: “Using 23 Years of ACS/SBC Data to Understand Backgrounds: Significant Reductions in Expected Background Levels” (Clark et al.)
- ACS ISR 2025-03: “Principal Component Analysis for ACS/WFC Superbias Temporal Variation” (Guzman and Grogin)
- ACS ISR 2025-02: “The Impact of Degraded Charge Transfer Efficiency on Extended Sources in ACS/WFC” (Stark et al.)
- ACS ISR 2025-01: “The Effect of Roll-Drift in ACS/WFC Images” (Cohen)