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arxiv: 2604.06108 · v1 · submitted 2026-04-07 · 🌌 astro-ph.IM

Investigating ACS/WFC Amp-to-Amp Sensitivities

Gagandeep S. Anand , Norman A. Grogin This is my paper

Pith reviewed 2026-05-10 18:10 UTC · model grok-4.3

classification 🌌 astro-ph.IM
keywords ACS/WFCamp-to-amp sensitivityHST calibrationwhite dwarf standardsCTE mitigationUbercalphotometry
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The pith

White dwarf observations confirm ACS/WFC chips agree to 0.4% in F606W and F814W

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper performs a direct test of a reported 5% sensitivity offset between the two chips of the ACS/WFC detector. Using three white dwarf standards observed at four positions on different amplifiers with matched pixel transfers, they measure the fluxes in three filters. For F606W and F814W, the chip-to-chip agreement is 0.4% on average and better than 1% in all cases, contradicting the Ubercal result. This is important for ensuring accurate photometry across the entire detector in HST observations. Variations in F435W suggest the need for spectral-type aware flat fields.

Core claim

Observing three white dwarf standards in F435W, F606W, and F814W at four dither positions with equal x and y transfers shows that the F606W and F814W measurements agree to 0.4% on average and within 1% individually across all stars and positions. This provides very strong evidence against the ~5% global sensitivity offset between WFC1 and WFC2 reported by the Ubercal framework. Larger variations in F435W are attributed to sensitivity of the flat field to spectral type.

What carries the argument

Matched-CTE dither observations of white dwarf flux standards to compare amplifier sensitivities

If this is right

  • The two WFC chips have consistent sensitivity for F606W and F814W photometry at the 0.4% level.
  • The reported Ubercal offset is likely an artifact of model degeneracies.
  • No chip-specific sensitivity correction is needed for these filters.
  • F435W flat fields require updates to handle spectral type dependence.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Re-examination of other Ubercal results may be warranted if this offset was spurious.
  • Targeted tests like this can validate or refute complex calibration models.
  • Improved F435W flats could enhance accuracy for blue observations.

Load-bearing premise

The white dwarf standards deliver position-independent fluxes and the dither strategy fully mitigates differential CTE effects.

What would settle it

Repeating the observations and finding a persistent 5% difference between the chips would support the original Ubercal offset.

Figures

Figures reproduced from arXiv: 2604.06108 by Gagandeep S. Anand, Norman A. Grogin.

Figure 1
Figure 1. Figure 1: Schematic of the observations used in this work. Three WD standards were observed on each of the four amplifiers, with their positions (stars) such that the number of x and y pixel transfers are identical, as shown by the 512 pixel scale bars at the top right. The stars were observed in each of the four positions using the F435W, F606W, and F814W filters. F606W was the filter used in the Ubercal work, and … view at source ↗
Figure 2
Figure 2. Figure 2: Summary plot of the photometric results from this program. Data points are shown for each combination of individual stars (symbols), filters (colors), and amplifiers (columns). The green banding in the center shows a region of ±1% as a visual aid. serial CTE correction, the delta-flat provides a potential avenue towards reducing the issues repeatedly seen for blue sources in F435W (while acknowledging that… view at source ↗
Figure 3
Figure 3. Figure 3: Delta flat-field for blue sources observed in F435W (Bohlin et al. 2017). The values in this delta flat-field cover a range of 4%, from 0.975 (black) to 1.015 (white). Summary and Future Outlook Recent work by the ACS team using an Ubercal framework (Ryan et al., 2024) uncovered a potential concern regarding a global sensitivity difference across the two chips (WFC1 vs. WFC2) at the level of 0.05 mag, or 5… view at source ↗
Figure 4
Figure 4. Figure 4: Same as [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗
read the original abstract

Recently, the ACS team applied an Ubercal framework to assess the photometric repeatability of stars observed across the WFC detector using 15 years of post-SM4 calibration data in the globular cluster 47 Tuc (Ryan et al., 2024). A surprising finding was an apparent 0.05 mag global difference in sensitivity between the WFC1 and WFC2 chips, which had not been seen in prior tests of sensitivity variations around the field-of-view. Given the many degenerate variables within the Ubercal framework such as CTE losses, time-dependent sensitivity, and flat-field corrections, we obtained new calibration data to perform a straightforward test of the reported $\sim$5$\%$ flux offset between detectors. We observed three white dwarf standards with three filters at four positions on the detector (each on a different amplifier), but with the same number of x and y pixel transfers to mitigate differential CTE-related effects. For the F606W and F814W filters, the agreements are good to 0.4$\%$ on average, and always 1$\%$ or better in individual cases. The consistency of these two filters over all three stars and the four dither positions provides very strong evidence against the large global sensitivity offset between WFC1 and WFC2 as seen in the Ubercal work. Larger variations seen in the bluer F435W filter are likely a result of a sensitivity of the flat field in that filter to underlying spectral type, warranting a future solution.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The manuscript presents new ACS/WFC observations of three white dwarf standards observed in F435W, F606W, and F814W at four dither positions, each landing on a different amplifier but with matched x- and y-pixel transfer counts. The authors report that F606W and F814W fluxes agree to 0.4% on average (always ≤1%) between WFC1 and WFC2, providing evidence against the ~5% global sensitivity offset found in the Ubercal analysis of 47 Tuc data; larger F435W variations are attributed to flat-field spectral-type dependence.

Significance. If the result holds after addressing CTE concerns, the work supplies valuable independent calibration data that challenges a key Ubercal finding and supports the use of standard-star observations to isolate detector properties. The matched-transfer design is a sound attempt to control one major systematic, and the consistency across three stars and two filters is a positive feature of the experiment.

major comments (2)
  1. [§2 and §4] §2 (Observing Strategy) and §4 (Results): The central claim that matched total x- and y-pixel transfers fully mitigate differential CTE rests on an assumption that may not hold. CTE losses are nonlinear with signal level, vary by amplifier readout chain, and include parallel/serial differences; the white-dwarf count rates also differ from the 47 Tuc stars used in Ubercal. Without a quantitative residual-CTE estimate or simulation, the observed <1% agreement does not yet constitute independent evidence against a true 5% offset.
  2. [§4] §4 (F435W results): The attribution of larger F435W variations to flat-field sensitivity to spectral type is plausible but lacks a quantitative test (e.g., comparison of flat-field residuals for the white-dwarf spectra versus the 47 Tuc population or a reference to the wavelength-dependent flat-field uncertainty). This weakens the interpretation that the redder filters are free of similar systematics.
minor comments (3)
  1. [Abstract and §4] The abstract and §4 should report the actual measured flux ratios or magnitude differences with uncertainties for each star, filter, and position rather than summary statistics alone.
  2. [§4] Add a short table or figure showing the per-position count rates (with errors) to allow readers to assess the 0.4% average agreement directly.
  3. [§2] Clarify the exact dither coordinates and amplifier assignments in §2 so the matched-transfer claim can be verified independently.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their positive assessment of the work's significance and for the detailed, constructive comments on the observing strategy and interpretation of results. We address each major comment below and indicate where revisions will be made to the manuscript.

read point-by-point responses

  1. Referee: [§2 and §4] §2 (Observing Strategy) and §4 (Results): The central claim that matched total x- and y-pixel transfers fully mitigate differential CTE rests on an assumption that may not hold. CTE losses are nonlinear with signal level, vary by amplifier readout chain, and include parallel/serial differences; the white-dwarf count rates also differ from the 47 Tuc stars used in Ubercal. Without a quantitative residual-CTE estimate or simulation, the observed <1% agreement does not yet constitute independent evidence against a true 5% offset.

    Authors: We acknowledge that CTE losses involve nonlinear signal dependence, amplifier-specific variations, and parallel/serial distinctions, and that matching total transfers does not remove every possible residual effect. Our design controls the dominant variable—the number of transfers—while the white-dwarf standards are sufficiently bright that standard ACS CTE models predict losses well below 1% for the relevant count rates. The observed agreement to ≤1% across three independent stars and four dither positions is therefore unlikely to be explained by a 5% differential CTE artifact. We will add a quantitative discussion in the revised §2 and §4 that references the ACS CTE model (ACS Data Handbook) to estimate residual losses for these sources and contrast them with the 47 Tuc data, thereby providing the requested context. revision: yes

  2. Referee: [§4] §4 (F435W results): The attribution of larger F435W variations to flat-field sensitivity to spectral type is plausible but lacks a quantitative test (e.g., comparison of flat-field residuals for the white-dwarf spectra versus the 47 Tuc population or a reference to the wavelength-dependent flat-field uncertainty). This weakens the interpretation that the redder filters are free of similar systematics.

    Authors: We agree that a direct quantitative comparison of flat-field residuals between the white-dwarf spectra and the 47 Tuc population would strengthen the argument. The larger F435W scatter is consistent with the known wavelength dependence of ACS/WFC flat-field uncertainties, which are substantially higher in the blue. In the revised manuscript we will add an explicit reference to the ACS Instrument Handbook documentation on filter-specific flat-field performance and note that the uncertainties drop markedly for F606W and F814W. While a full spectral-type residual analysis lies beyond the scope of the present data set, the tight agreement in the two redder filters across all stars and positions supports that the ~5% global offset reported by Ubercal is not reproduced in these bands. revision: partial

Circularity Check

0 steps flagged

New independent observations test prior Ubercal claim without reduction to fitted inputs or self-citations

full rationale

The paper's central claim rests on fresh ACS/WFC observations of three white dwarf standards at four dither positions chosen to match x/y pixel transfers. Measured count-rate ratios in F606W and F814W (0.4% average agreement, ≤1% individual) are reported directly from these data and compared to the Ryan et al. (2024) Ubercal result. No equations derive a 'prediction' from prior parameters, no ansatz is smuggled via self-citation, and the cited Ubercal work is by different authors. The derivation chain is therefore self-contained against external benchmarks and exhibits no circularity.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The test relies on standard domain assumptions in HST photometry that white dwarf standards have stable, position-independent fluxes and that matched pixel transfers control CTE variations. No free parameters or new entities are introduced.

axioms (2)
  • domain assumption White dwarf standards provide stable reference fluxes independent of detector position.
    Core premise for using them to measure sensitivity differences.
  • domain assumption Dither positions with equal x and y transfers isolate sensitivity from CTE losses.
    Key design choice stated in the abstract to control for charge transfer effects.

pith-pipeline@v0.9.0 · 5567 in / 1298 out tokens · 69203 ms · 2026-05-10T18:10:46.366558+00:00 · methodology

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Reference graph

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