🔭 astrophysics

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

This paper details the identification, modeling, and mitigation of the "scratched tape" stray light artifact, a prominent issue observed during the Rubin Observatory's commissioning caused by off-axis light reflecting off the primary mirror due to a delayed light-wind screen installation.

Original authors: Alex Drlica-Wagner, Alessio Taranto, Gabriele Rodeghiero, Joshua E. Meyers, John Andrew, Douglas R. Neill, Christian Aguilar, Brian Stalder, Robert H. Lupton, Aashay Pai, Lee S. Kelvin, Aaron E. Watki

Published 2026-07-01

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CC BY 4.0

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

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Vera C. Rubin Observatory as a giant, ultra-sensitive camera perched on a mountain, designed to take the clearest, darkest photos of the universe ever attempted. Its job is to see incredibly faint objects, like distant galaxies, against a pitch-black sky.

However, during its "training camp" (commissioning), the camera started taking photos with a strange, unwanted smudge on them. The scientists called this the "Scratched Tape" artifact.

Here is the story of how they found the problem, figured out what was causing it, and fixed it, explained in simple terms.

The Mystery Smudge

When the camera took pictures, a bright, straight line would sometimes appear across the image, looking a bit like a piece of tape that had been scratched or a sail unfurling. This wasn't a real star or galaxy; it was "stray light"—sunlight or starlight that took a wrong turn and hit the camera sensor directly.

This smudge was a big deal. It showed up in more than 5% of all the photos taken during the first year. If you were trying to study a faint galaxy, this bright smudge could ruin the picture, just like a flashlight shining into your eyes while you try to read a dim book.

The Detective Work: Finding the Culprit

The team of scientists acted like detectives. They noticed that the smudge only appeared when a very bright star system, Alpha Centauri (the third brightest star in our sky), was located about 20 degrees off to the side of where the telescope was pointing.

To solve the mystery, they used a special trick:

The Pinhole Test: They put a mask with tiny holes in front of the camera. This allowed them to see exactly where light was coming from inside the telescope's structure.
The Ray Tracing: They used computer simulations (like a video game physics engine) to trace the path of light.

The Discovery: They found a "secret tunnel."
Normally, the telescope has shields (called baffles) to block light coming from the sides. But, there was a small gap between two of these shields (the "mid-level" and "center-section" baffles).

The Path: Light from the bright star hit the telescope from the side, slipped through this gap, bounced off the main mirror, and zoomed straight into the camera sensor, bypassing the other mirrors entirely.
The Analogy: Imagine trying to take a photo of a candle in a dark room, but someone shines a flashlight through a crack in the door. The light hits the wall, bounces off a mirror, and blinds your camera. That's what was happening inside the telescope.

The Fix: Building a "Doorstop"

The scientists realized they couldn't just close the gap permanently because the telescope needed to move and breathe. Instead, they decided to build an extension to one of the shields to block that specific path.

The Solution: They designed and built a 22-centimeter (about 9-inch) extension to the mid-level light shield. Think of it like adding a little "doorstop" or an extra flap to a curtain to make sure no light leaks through the gap.
The Installation: They installed this extension in early 2026. It was made of 24 metal panels, painted with a special non-reflective black paint (like the inside of a black hole) to swallow any stray light that hit it. They installed it piece by piece over six weeks, working around the telescope's schedule.

The Result: A Clean Lens

After the extension was installed, they tested it again with the bright star Alpha Centauri.

Before: The "Scratched Tape" smudge was clearly visible.
After: The smudge disappeared completely.

There was a tiny, faint leftover spot called a "smudge" for a few weeks, but once they added a few small corner pieces to seal the very last gaps, even that vanished. The occurrence of these smudges dropped from affecting 5% of photos to effectively zero.

Why This Matters

The Rubin Observatory is built to see the faintest things in the universe. If stray light is 20% as bright as the dark night sky (which it was in the bluer colors), it creates "noise" that makes it hard to see the faint signals scientists are looking for.

By finding this gap and building a simple shield to block it, the team ensured that the telescope's "eyes" are now clear. This means the data the observatory will collect for the next decade will be much cleaner, allowing astronomers to study the universe without the distraction of "scratched tape" smudges.

In short: The telescope had a tiny crack letting in a beam of light that confused the camera. The team found the crack, built a patch, and now the camera sees the universe exactly as it should.

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

Problem Identification
During the commissioning of the Vera C. Rubin Observatory, a prominent and prevalent stray light artifact, termed the "scratched tape," was identified in images captured by the LSST Camera (LSSTCam). This feature manifests as a sharp, rectilinear, several-degree-long illumination on the focal plane, with a surface brightness reaching approximately 20% of the dark-sky background in bluer bands when sourced by bright stars. The artifact was visually apparent in over 5% of images collected during the first year of on-sky operations and science validation. The issue arose due to the delayed integration of the Rubin dome slit light–wind screen (LWS), which left an unobstructed optical path for astronomical sources located at large off-axis angles (approximately 20 degrees) to illuminate the primary mirror (M1) and reflect directly into the camera, bypassing the secondary (M2) and tertiary (M3) mirrors.

Methodology and Physical Characterization
The investigation employed a multi-faceted approach combining visual inspection, optical modeling, and targeted on-sky testing:

Visual Correlation and Pinhole Imaging: Initial identification correlated the artifact's appearance with the position of bright stars, specifically $\alpha$ Centauri ( $\alpha$ Cen), located $\sim$ 21 degrees off-axis. To trace the optical path, the team utilized a pinhole filter inserted into the LSSTCam filter changer to image the telescope pupil. These images revealed a direct connection between the stray light pattern and a physical gap between the mid-level and center-section light baffles on the Telescope Mount Assembly (TMA).
Ray-Tracing Modeling: Non-sequential ray-tracing simulations using batoid and Ansys Zemax OpticStudio were conducted to model the stray light path. These models confirmed that photons entering through the gap between the baffles could reflect off M1 and reach the detectors. The simulations determined that the effective off-axis angle range for this phenomenon is $19.8 \lesssim \theta \lesssim 26$ degrees, with peak illumination occurring between 20 and 22 degrees.
Flux Characterization: Surface brightness measurements were performed on 10 distinct instances of the $\alpha$ Cen-sourced artifact across multiple bands ( $u, g, r, i, z, y$ ). The analysis involved modeling and subtracting a smooth sky background using low-order polynomial fits on post-ISR (Instrument Signature Removal) data to avoid underestimating the feature's intensity.

Mitigation Strategy and Implementation
Based on the ray-tracing results, which indicated that extending the mid-level baffle outward would block the stray light path without impacting the telescope's clear aperture, a mechanical mitigation solution was designed and executed:

Design: A 220 mm outward extension of the mid-level light baffle was proposed. This width accounted for the 185 mm required to block simulated photons, plus a margin for manufacturing tolerances and uncertainties in the as-built TMA dimensions.
Construction: The extension consists of 24 modular panels (120 cm $\times$ 27 cm) made of 2-mm-thick low-carbon steel, painted with low-reflectivity Aeroglaze Z306. Small overlap panels were included to prevent gaps.
Installation: The installation was staged over approximately six weeks in early 2026 (February–April) to avoid interrupting the observing schedule. The process prioritized the $\pm$ Y quadrants, which allow a direct path to the sky, followed by the $\pm$ X quadrants, which are partially blocked by the dome structure.

Results
The effectiveness of the mitigation was validated through dedicated on-sky tests and opportunistic data taking:

Elimination of the Artifact: Dedicated observations of $\alpha$ Cen taken before and after the installation of the baffle extension demonstrated the complete removal of the "scratched tape" feature.
Residuals: A minor residual feature, termed the "smudge," was observed until March 19, 2026, attributed to remaining gaps between extension panels. This feature disappeared following the installation of final corner overlap panels.
Statistical Impact: Analysis of the occurrence rate showed a sharp decline in the fraction of exposures affected by the artifact immediately following the installation of the $\pm$ Y quadrants, with the feature effectively eliminated upon completion of the full installation.
Secondary Artifacts: Anecdotal evidence suggests that other morphologically similar artifacts (e.g., "muddy shoe" and "pillow") have also decreased in frequency, though they occur less frequently and require further characterization.

Significance and Claims
The paper asserts that the "scratched tape" artifact was the most prominent stray light issue identified during Rubin's commissioning. The successful identification, modeling, and physical mitigation of this path represent a critical step in ensuring the quality of LSSTCam data. While the impact on high-level scientific data products is partially mitigated through aggressive sky background modeling, the authors note that residual effects may persist in Rubin Data Preview 2. The primary significance of this work lies in the successful deployment of a hardware solution that blocks the stray light path prior to the start of the Legacy Survey of Space and Time (LSST), thereby preventing the contamination of future survey data by this specific off-axis illumination mechanism. The authors emphasize that the forthcoming LWS is expected to permanently block this path once fully operational, but the baffle extension was necessary to secure data quality during the commissioning and early operations phases.

The Mystery Smudge

The Detective Work: Finding the Culprit

The Fix: Building a "Doorstop"

The Result: A Clean Lens

Why This Matters

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