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arxiv: 2607.00925 · v1 · pith:4RFUCWMJnew · submitted 2026-07-01 · 🌌 astro-ph.IM

The "dark dips" phenomenon in the LSST Camera on-sky images

Claire Juramy , Pierre Antilogus , Pierre Astier , John Banovetz , Sean Patrick MacBride , Andrew P. Rasmussen , Yousuke Utsumi This is my paper

Pith reviewed 2026-07-02 05:43 UTC · model grok-4.3

classification 🌌 astro-ph.IM
keywords dark dipsLSST CameraCCD sensorslateral field distortioninstrument signature removalphotometryastrometryVera Rubin Observatory
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The pith

Bright stars create dark dips up to 7% in some LSST Camera CCD columns via lateral charge shifts.

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

The paper documents a previously undocumented effect in certain LSST Camera sensors where columns containing a bright star appear darker than the sky background both above and below the star. This is attributed to lateral electric field distortions that shift charges away from the central columns toward neighbors. The amplitude reaches up to seven percent of the background in the worst sensors but is undetectable in others, with no significant dependence on filter color. A dynamic masking approach during Instrument Signature Removal is proposed to prevent effects on photometry and astrometry.

Core claim

When a sufficiently bright star is superimposed over the sky background, the sensor columns that contain the star appear slightly darker than the background both above and below the star position, with a slight excess of flux in neighboring columns, caused by a lateral field distortion that shifts charges away from the central columns; this reaches amplitudes up to seven percent in affected sensors with no significant filter-color dependence.

What carries the argument

Lateral field distortion that shifts charges away from columns containing the bright star

If this is right

  • Dynamic masking of affected columns during Instrument Signature Removal prevents photometric and astrometric impacts.
  • Sensors can be categorized by the strength of the effect through statistical analysis over hundreds of frames.
  • The effect shows no significant dependence on filter color.
  • Detectability thresholds vary from a few hundred to a few thousand saturated pixels per sensor.

Where Pith is reading between the lines

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

  • Sensor-to-sensor differences likely trace to small variations in electric field uniformity during fabrication.
  • Similar column-level charge redistribution could appear in other large-format CCD mosaics used for wide-field surveys.
  • Generalizing the dynamic masking approach might help with other localized field-distortion artifacts in future instruments.

Load-bearing premise

The column darkening is caused by lateral field distortion shifting charges rather than charge transfer inefficiency or readout artifacts.

What would settle it

Measure whether the amplitude of the dips scales exactly with the number of saturated pixels in the star footprint across many frames while holding all other conditions fixed.

read the original abstract

When the commissioning camera (ComCam), and then the LSST Camera, started taking on-sky images at the Vera Rubin Observatory, some of the ITL STA3800 CCDs exhibited a previously undocumented effect. When a sufficiently bright star is superimposed over the sky background, the sensor columns that contain the star appear slightly darker than the background, both above and below the position of the star. The visual appearance of these "dark dips" in the background is enhanced by a slight excess of flux within the neighboring columns, suggesting that they are caused by a lateral field distortion that shifts charges away from the central columns. This effect is not uniform across sensors, reaching an amplitude of up to seven percent of the background in the worst cases, but being undetectable in other sensors under the same conditions. For the affected sensors, the threshold required for the dips to become detectable also varies, from a few hundred to a few thousands saturated pixels within the star footprint. We developed means to quantify the dips, and studied the effect statistically over hundreds of frames, in order to categorize each sensor. In particular, we found no significant dependence on the filter color. We then developed a strategy to dynamically mask the affected columns during the Instrument Signature Removal process, to avoid any potential effect on photometric and astrometric performance.

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 / 2 minor

Summary. The manuscript reports the discovery of a 'dark dips' effect in on-sky images from the LSST Camera's ITL STA3800 CCDs, in which columns containing sufficiently bright stars appear darker than the background (up to 7% amplitude) both above and below the star, with neighboring columns showing slight flux excess. The effect varies across sensors, requires a threshold number of saturated pixels to appear, shows no significant filter dependence, and is characterized statistically over hundreds of frames. The authors suggest lateral field distortion as the cause and propose dynamic column masking during Instrument Signature Removal as mitigation to protect photometric and astrometric performance.

Significance. This observational result is significant for LSST operations because an unmitigated column-scale background distortion could introduce systematics in photometry and astrometry at the few-percent level in affected sensors. Credit is due for the use of real commissioning on-sky data, the statistical categorization of sensor behavior, and the practical engineering response. The work is empirical rather than model-derived, which strengthens its direct applicability but also limits the strength of mechanism-based claims.

major comments (2)
  1. [Abstract] Abstract and mitigation strategy description: the claim that dynamic masking 'avoids any potential effect on photometric and astrometric performance' is load-bearing for the practical recommendation yet lacks quantitative support such as before/after photometry residuals, astrometric shift measurements, or propagated error estimates on the reported dip amplitudes.
  2. [Abstract] Abstract (cause paragraph): the attribution to lateral field distortion rests on the neighboring-column excess flux observation, but no quantitative test or exclusion criterion is given to distinguish this from charge transfer inefficiency or readout artifacts; if the mechanism differs, column masking alone may leave residual correlated systematics elsewhere in the sensor.
minor comments (2)
  1. [Abstract] The abstract would be strengthened by reporting the exact number of frames analyzed, the distribution of dip amplitudes with uncertainties, and the criteria used to classify sensors as affected or unaffected.
  2. A summary table listing per-sensor thresholds, maximum amplitudes, and fraction of frames affected would improve clarity of the statistical categorization results.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful and constructive review. We address the two major comments below and have revised the abstract to qualify our statements appropriately while preserving the empirical results.

read point-by-point responses

  1. Referee: [Abstract] Abstract and mitigation strategy description: the claim that dynamic masking 'avoids any potential effect on photometric and astrometric performance' is load-bearing for the practical recommendation yet lacks quantitative support such as before/after photometry residuals, astrometric shift measurements, or propagated error estimates on the reported dip amplitudes.

    Authors: We agree that the original abstract phrasing overstated the mitigation without supporting quantitative tests in the manuscript. The dynamic masking was developed to remove the observed column dips during ISR, but no before/after photometry or astrometry residuals are presented. In revision we have changed the abstract to state that the strategy is implemented 'to mitigate potential impacts on photometric and astrometric performance' and added a sentence noting that quantitative validation on science metrics is planned for future work. This revision_made is 'yes'. revision: yes

  2. Referee: [Abstract] Abstract (cause paragraph): the attribution to lateral field distortion rests on the neighboring-column excess flux observation, but no quantitative test or exclusion criterion is given to distinguish this from charge transfer inefficiency or readout artifacts; if the mechanism differs, column masking alone may leave residual correlated systematics elsewhere in the sensor.

    Authors: The lateral-field interpretation is offered as a hypothesis based on the symmetric above/below dips plus neighboring-column excess, which differs from the unidirectional signature expected for CTI. We performed no formal exclusion tests or quantitative model comparisons. In revision we have rephrased the abstract to describe the effect as 'suggestive of lateral field distortion' and added a brief discussion noting that column masking directly targets the observed column-scale dips regardless of the precise physical mechanism; any additional systematics from other causes would require separate characterization. This revision_made is 'yes'. revision: yes

Circularity Check

0 steps flagged

No circularity; purely observational report with no derivations or predictions

full rationale

The manuscript is an observational description of a sensor artifact. It reports detection of column darkening in on-sky images, quantifies amplitude and statistics across frames, notes lack of filter dependence, and describes an engineering mitigation (dynamic column masking during ISR). No equations, first-principles derivations, fitted parameters renamed as predictions, or load-bearing self-citations appear. The central claims rest on direct measurement and practical response rather than any chain that reduces to its own inputs by construction. This is the expected non-finding for a purely descriptive engineering note.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

No free parameters, axioms, or invented entities are introduced; the work is an empirical characterization of an observed instrumental effect.

pith-pipeline@v0.9.1-grok · 5786 in / 1061 out tokens · 29921 ms · 2026-07-02T05:43:01.504797+00:00 · methodology

discussion (0)

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Reference graph

Works this paper leans on

11 extracted references · 2 canonical work pages

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    dark dips

    INTRODUCTION The LSST Camera of the Vera C. Rubin Observatory is a composite focal plane, with a science array of 117 Teledyne-e2v CCD250 and 72 ITL STA3800C devices, and 12 more ITL devices for guiding and focus. Aside from their basic physical properties (10 x 10 µm pixels, 100 µm thickness, divided into 16 channels of approximately 1 million pixels eac...

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    dark dips

    P ROCESSING FOR THE DARK DIPS This section follows the four steps of processing for the “dark dips” over a large number of frames. This process was done for characterization purposes, and is distinct from the implementation of dip mitigation in the Instrument Signature Removal (ISR) process, which we will describe in a later section. 2.1 Find clusters of ...

  3. [3]

    dark dips

    S TATISTICAL VARIATIONS ACROSS SENSORS The results and plots in this section are based on a specific sample of 100 frames acquired in the i filter on March 11th, 2026, but the results are compatible with tests conducted over multiple other samples. To illustrate how affected each sensor is, the number of dips that were detected is compared to the total nu...

    2026

  4. [4]

    Beyond assessing the fallout of this issue, we aimed to understand its origin, which might suggest pathways to remedy it

    Q UALITATIVE STUDIES A wide array of studies was conducted during the LSST commissioning, with ComCam, in the lab at SLAC and at Vera Rubin Observatory, and finally with the main Camera on the sky. Beyond assessing the fallout of this issue, we aimed to understand its origin, which might suggest pathways to remedy it. The fact that the dips are present bo...

  5. [5]

    DARK DIPS

    P OSSIBLE MECHANISM OF THE “ DARK DIPS ” AND M ITIGATION From our studies, we can draw a fairly clear empirical picture of the “dark dips” phenomenon. However, to explain it requires a mechanism that can dynamically alter the distribution of charges in the pixels based on the overflowing of charges elsewhere along the same sensor columns. This could be ha...

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    dark dips

    could also be a probe into the strength of the sensor potential barriers, but measurements of these coefficients over all ITL sensors [2] showed no strong variations that would be correlated with dips presence. Lastly, variability among sensors that are designed and operated in the exact same ways suggests some variation during the manufacturing process, ...

  7. [7]

    dark dips

    C ONCLUSION The “dark dips” phenomenon, while visually striking in the worst cases, affects only a small number of ITL sensors of the LSST Camera in this manner. Aside from these few sensors, the mitigation strategy through masking whole columns will only be necessary when extremely bright objects are exposed, which would already require masking of a sign...

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    LSST Camera focal plane optimization

    Yousuke Utsumi, Pierre Antilogus, Pierre Astier, John Banovetz, Andrew K. Bradshaw, Johan Bregeon, Alex Broughton, Jim Chiang, Celine Combet, Guillaume Dargaud, Seth W. Digel, Johnny Esteves, Thibault Guillemin, Tony Johnson, Claire Juramy, Craig Lage, Shuang Liang, Stuart Marshall, Myriam Migliore, Homer Neal, Renee Nichols, Daniel Polin, Andrew P. Rasmu...

    work page doi:10.1117/12.3019117 2024

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    Tearing and related field distortions in deep-depletion charge-coupled devices,

    Claire Juramy, Pierre Antilogus, Laurent Le Guillou, Eduardo Sepulveda, "Tearing and related field distortions in deep-depletion charge-coupled devices," Journal of Astronomical Telescopes, Instruments, and Systems 5(4), 041505 (28 Sep 2019); https://doi.org/10.1117/1.JATIS.5.4.041505 [6] Craig Lage, “Physical and electrical analysis of LSST sensors,” arX...

    work page doi:10.1117/1.jatis.5.4.041505 2019

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    ISBN 0819436984

    Janesick, J.R., Scientific charge-coupled devices, SPIE Optical Engineering Press, 2001, SPIE Press monograph, PM 83. ISBN 0819436984

    2001

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    https://github.com/lsst/obs_lsst/blob/main/config/lsstCam/isrLSST.py, accessed 2026-05-25

    2026