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arxiv: 2606.31939 · v1 · pith:QLQ6IS6Cnew · submitted 2026-06-30 · 🌌 astro-ph.IM · astro-ph.CO

Investigation and Mitigation of a Prominent Off-Axis Stray Light Path in Rubin Observatory Commissioning

Alex Drlica-Wagner , Alessio Taranto , Gabriele Rodeghiero , Joshua E. Meyers , John Andrew , Douglas R. Neill , Christian Aguilar , Brian Stalder
show 7 more authors
Robert H. Lupton Aashay Pai Lee S. Kelvin Aaron E. Watkins Luca Rosignoli Hannah M. M. Pollek Anastasia Alexov
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Pith reviewed 2026-07-01 02:47 UTC · model grok-4.3

classification 🌌 astro-ph.IM astro-ph.CO
keywords stray lightRubin ObservatoryLSST Cameralight bafflesoff-axis illuminationcommissioningprimary mirror reflection
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The pith

Light from large off-axis angles passes between light baffles, reflects off the primary mirror, and creates the scratched tape feature on the LSST Camera focal plane.

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

The paper traces the origin of the most prominent stray light artifact seen during Rubin Observatory commissioning, known as the scratched tape feature. Light at roughly 20 degrees off-axis enters the gap between the mid-level and center-section light baffles, reflects from the primary mirror, and reaches the camera focal plane. This path remained open because installation of the dome slit light-wind screen was delayed. The authors model the geometry, characterize the resulting pattern, and describe mitigation steps. Resolving the path improves image quality in early observations.

Core claim

The scratched tape stray light feature originates when light from large off-axis angles (~20 deg) passes between the mid-level and center-section light baffles, reflects off the primary mirror, and illuminates the LSST Camera focal plane. This scenario represented an unobstructed stray light path to the sky during Rubin commissioning due to delays in the integration of the dome slit light-wind screen.

What carries the argument

The gap between the mid-level and center-section light baffles that permits off-axis light to reach the primary mirror and then the focal plane.

Load-bearing premise

The delays in the integration of the dome slit light-wind screen left an unobstructed stray light path to the sky during commissioning.

What would settle it

An observation or ray-trace calculation showing that the scratched tape pattern on the focal plane does not match the illumination expected from 20-degree off-axis light reflected off the primary mirror, or that the feature persists after the wind screen is installed.

Figures

Figures reproduced from arXiv: 2606.31939 by Aaron E. Watkins, Aashay Pai, Alessio Taranto, Alex Drlica-Wagner, Anastasia Alexov, Brian Stalder, Christian Aguilar, Douglas R. Neill, Gabriele Rodeghiero, Hannah M. M. Pollek, John Andrew, Joshua E. Meyers, Lee S. Kelvin, Luca Rosignoli, Robert H. Lupton.

Figure 1
Figure 1. Figure 1: Examples of stray light features associated with the “scratched tape” stray light path in LSSTCam [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Images taken with the pinhole filter showing the gap between the mid-level and center-section [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Ray tracing models showing the scratched tape light path between the mid-level and center-section [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: (Left) Ray-tracing simulations with Zemax show the relative flux on the LSSTCam focal plane via the scratched tape light path for a unitary flux injected into the telescope pupil. The relative flux reaching the LSSTCam focal plane peaks at ∼ 10−3 and falls to ∼ 10−6 at off-axis angles ≳ 22 deg. (Right) Ray-tracing simulations run with batoid show the distribution of ∼ 2 × 107 photons fired in the direction… view at source ↗
Figure 5
Figure 5. Figure 5: Drawing of the 220 mm outward ex￾tension to the mid-level light baffle to block the scratched tape light path. The mid-level baffle ex￾tension is composed of 24 panels that were installed incrementally (colored green, pink, and red). Date Baffle Extension Component Completion Status 2026-02-19 −Y all panels (6/6) 2026-02-20 +X lower panels (4/6) 2026-02-24 −X lower panel (1/6) 2026-02-25 −X lower panel (2/… view at source ↗
Figure 6
Figure 6. Figure 6: Mitigation of the prominent scratched tape stray light feature by the mid-level light baffle extension. [PITH_FULL_IMAGE:figures/full_fig_p008_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Installation of the mid-level light baffle extension effectively removed the scratched tape stray light [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
read the original abstract

The "scratched tape" stray light feature is the most prominent and prevalent stray light artifact identified during the commissioning of the Vera C. Rubin Observatory. The scratched tape feature originates when light from large off-axis angles (~20 deg) passes between the mid-level and center-section light baffles, reflects off the primary mirror, and illuminates the LSST Camera focal plane. This scenario represented an unobstructed stray light path to the sky during Rubin commissioning due to delays in the integration of the dome slit light-wind screen. This document describes the identification, modeling, characterization, and mitigation of the scratched tape stray light artifact.

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

1 major / 0 minor

Summary. The manuscript identifies the origin of the prominent 'scratched tape' stray light artifact observed in LSST Camera data during Rubin Observatory commissioning. It attributes the feature to light incident at large off-axis angles (~20 deg) that passes between the mid-level and center-section light baffles, reflects from the primary mirror, and reaches the focal plane. This path existed because of delays in integrating the dome slit light-wind screen. The paper describes the identification process, ray-tracing modeling of the geometry, on-sky characterization, and the mitigation steps implemented.

Significance. The work addresses a concrete operational issue that affected commissioning data quality for a major survey facility. Successful diagnosis and mitigation of a specific stray-light path via physical modeling and hardware intervention provides a useful case study for stray-light control in large-aperture telescopes. The emphasis on commissioning timeline context and practical fixes is a strength.

major comments (1)
  1. [Abstract / Characterization] Abstract and characterization description: the claim that modeling and characterization were performed is not supported by any quantitative validation, error analysis, before-after intensity metrics, or statistical comparison of stray-light levels. This absence weakens the central assertion that the identified path was both the dominant source and successfully mitigated.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their constructive review and positive assessment of the work's significance. We address the single major comment below.

read point-by-point responses

  1. Referee: [Abstract / Characterization] Abstract and characterization description: the claim that modeling and characterization were performed is not supported by any quantitative validation, error analysis, before-after intensity metrics, or statistical comparison of stray-light levels. This absence weakens the central assertion that the identified path was both the dominant source and successfully mitigated.

    Authors: We agree that the manuscript would be strengthened by explicit quantitative validation. The current text describes the ray-tracing geometry, on-sky identification, and mitigation timeline but does not include error budgets, intensity metrics, or statistical before-after comparisons. In the revised manuscript we will add a dedicated subsection with (i) quantitative ray-tracing validation against measured artifact positions and intensities, (ii) before-and-after stray-light level measurements extracted from commissioning exposures, and (iii) a simple statistical comparison of the artifact amplitude pre- and post-installation of the dome slit light-wind screen. These additions will directly support the claims of dominance and successful mitigation. revision: yes

Circularity Check

0 steps flagged

No significant circularity detected

full rationale

The manuscript identifies a stray-light path via ray-tracing of the known Rubin telescope geometry, on-sky characterization, and mitigation steps. The central claim rests on physical description of baffle geometry and the factual commissioning timeline regarding the absent light-wind screen; neither is derived from fitted parameters, self-referential equations, nor load-bearing self-citations. No derivation chain reduces to its own inputs by construction, and the argument is self-contained against external geometric and observational benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Only the abstract is available; no free parameters, axioms, or invented entities are identifiable from the provided text.

pith-pipeline@v0.9.1-grok · 5698 in / 911 out tokens · 70366 ms · 2026-07-01T02:47:13.232801+00:00 · methodology

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Reference graph

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