🔭 astrophysics

The VLT/ERIS grating vector Apodizing Phase Plate coronagraph

This paper presents the design, fabrication, and on-sky testing of the grating vector apodizing phase plate (gvAPP) coronagraph for the VLT/ERIS instrument, demonstrating that while it meets transmission and raw contrast specifications, electronic detector noise limits its post-processed contrast performance, though it remains a valuable tool for characterizing known substellar companions and serves as a pathfinder for future Extremely Large Telescope instruments.

Original authors: M. A. Kenworthy, F. A. Dannert, J. Hayoz, D. Doelman, B. J. Sutlieff, P. Liu, F. Snik, M. J. Bonse, S. P. Quanz, C. U. Keller, O. Absil, G. Orban de Xivry, R. J. De Rosa, C. Ginski, X. Chen, A. Zurlo

Published 2026-03-26

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: M. A. Kenworthy, F. A. Dannert, J. Hayoz, D. Doelman, B. J. Sutlieff, P. Liu, F. Snik, M. J. Bonse, S. P. Quanz, C. U. Keller, O. Absil, G. Orban de Xivry, R. J. De Rosa, C. Ginski, X. Chen, A. Zurlo, B. A. Biller, J. L. Birkby, A. Baruffolo, Y. Dalliliar, R. Davies, M. Dolci, H. Feuchtgruber, A. Glauser, P. Grani, K. Kravchenko, M. MacIntosh, A. Puglisi, C. Rau, A. Riccardi, E. Sturm, W. Taylor

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 trying to take a photograph of a tiny, glowing firefly sitting right next to a blindingly bright spotlight. If you point your camera directly at the firefly, the spotlight's glare washes out the entire image, making the firefly invisible. If you try to block the spotlight with your hand, you might accidentally block the firefly too, or the light might scatter around your hand and still ruin the photo.

This is the exact problem astronomers face when trying to see exoplanets (planets outside our solar system). These planets are incredibly faint and sit very close to their parent stars, which are billions of times brighter.

This paper is about a new, high-tech "sunglasses" and "light-bending trick" installed on the Very Large Telescope (VLT) in Chile, designed to solve this problem. Here is the story of how it works, what it achieved, and where it stumbled, explained simply.

1. The Magic Trick: The "Grating Vector" Glasses

The instrument is called ERIS, and the special tool inside it is the gvAPP coronagraph.

The Old Way: Traditional coronagraphs use a physical mask (like a tiny coin) to block the star's light. But this is like trying to block a spotlight with a coin; the light bends around the edges (diffraction) and still creates a messy halo.
The New Way (gvAPP): Instead of a physical blocker, this device uses a special piece of glass with a microscopic pattern etched into it. Think of it as a laser-cut stencil that doesn't block the light but twists it.
How it works: When the starlight hits this pattern, the device splits the light into two separate beams. It pushes the blinding glare of the star to the sides, leaving a "dark hole" in the middle of the image where the star's light is almost completely gone.
The Result: In this dark hole, the faint firefly (the planet) can finally be seen.

2. The Lab Test: A Success Story

Before putting this on the telescope, the team tested it in a laboratory.

Transparency: The glass is very clear. In the "K-band" (a specific infrared color), it lets 90% of the light through. In the "L-band," it lets through about 60%. This is excellent; it means the telescope isn't losing much of the precious light from the planets.
Contrast: They wanted to block the star's light so well that they could see a planet 100,000 times fainter than the star. In the lab, they hit this target perfectly.

3. The Real-World Test: The "Ghost" in the Machine

When they took the instrument to the VLT and pointed it at real stars in the sky, things got a bit messy.

The Good News: The "dark holes" worked! They successfully suppressed the star's glare, allowing them to see the area around the star clearly.
The Bad News (The Electronic Ghost): While the star's light was blocked, the detector (the camera's sensor) started acting weird. Because the star is so bright, even a tiny bit of its light hitting the sensor caused a "short circuit" effect between the electronic wires reading the image.
The Analogy: Imagine you are listening to a whisper in a quiet room, but the person next to you is shouting. Even if you cover your ears, the vibration from their shout travels through the floor and shakes your chair. In the telescope, the bright star's light "shook" the camera's electronics, creating a repeating pattern of noise (like a faint grid or stripes) across the image. This noise made it harder to see the faintest planets.

4. The Performance: Good, but not Perfect

Because of this electronic noise, the telescope couldn't see as faint as the designers hoped.

The Goal: They wanted to see planets that are 100,000 times fainter than the star.
The Reality: They could only see planets about 20,000 times fainter.
Why? The "shaking" of the camera electronics created a background noise floor that drowned out the faintest signals.

5. What Can We Do With It Now?

Even with this limitation, the instrument is a powerful tool, but it needs to be used smartly.

Best Use Case: It is perfect for studying known planets. If we already know exactly where a planet is (like a known firefly), we can aim the telescope so the planet lands right in the "dark hole." We don't need to hunt for it; we just need to look at it.
Time-Traveling Light: The instrument creates a small, unsaturated "leak" of the star's light in the center. Astronomers can use this as a reference point to measure how the planet's brightness changes over time (like watching clouds move across a planet's face).
The Future: This instrument is a "pathfinder." It taught engineers valuable lessons about how to build better electronics and fewer layers of glue for the next generation of telescopes, specifically the Extremely Large Telescope (ELT) coming in the 2030s. The next version will have even better "sunglasses" and cleaner electronics.

Summary

The team built a brilliant piece of technology that bends light to hide stars and reveal planets. It works beautifully in the lab and mostly works in the sky. However, the camera's electronics got a little "jittery" from the brightness of the stars, creating a static noise that hides the faintest planets.

Despite this, it is a major success. It proves the concept works, gives astronomers a new way to study known worlds, and paves the way for the massive telescopes of the future that will one day take pictures of Earth-like worlds around other stars.

1. Problem Statement

Direct imaging of exoplanets and circumstellar disks is hindered by the overwhelming diffraction halo of the central host star. While coronagraphs suppress this starlight, traditional designs often suffer from chromatic limitations (narrow bandwidths) or require complex optical setups. The Apodizing Phase Plate (APP) offers a pupil-plane solution that creates a "dark hole" (a region of suppressed starlight) without amplitude masks, but standard APPs are highly chromatic. The Vector APP (vAPP) uses geometric phase to achieve achromaticity, but requires additional optics (quarter-wave plates and Wollaston prisms) to separate polarization states. The Grating Vector APP (gvAPP) improves upon this by integrating a diffraction grating into the phase pattern, separating polarization states directly in the focal plane and eliminating the need for extra optics.

The specific challenge addressed in this paper is the design, manufacturing, and on-sky validation of the first gvAPP installed on the Enhanced Resolution Imager and Spectrograph (ERIS) at the Very Large Telescope (VLT). The authors aim to verify if the gvAPP meets its design specifications for transmission and contrast in the near-to-mid infrared (K to L bands) and to identify performance limitations in a real-world observing environment.

2. Methodology

The study employs a multi-stage approach combining optical design, laboratory characterization, and on-sky observations:

Optical Design: The gvAPP was designed using an algorithm capable of handling arbitrary pupil geometries. The design accounts for the VLT's primary mirror, secondary support structures, and a rectangular extension caused by the tertiary mirror. The optic is undersized ( $d=11.50$ mm) relative to the ERIS pupil ( $d=11.76$ mm) to ensure full illumination despite flexure.
Manufacturing: The device consists of four Calcium Fluoride ( $CaF_2$ ) substrates glued together, containing patterned liquid-crystal polymer (LCP) layers and an aluminum amplitude mask. The LCP layers utilize the geometric phase to create the required phase pattern. A sinusoidal phase ramp (25 cycles) is added to diffract the two circular polarizations into the $m=\pm1$ orders, separating the coronagraphic PSFs from the central leakage term.
Laboratory Testing: Transmission and phase patterns were measured using a Fourier spectrometer and a test bench to establish baseline performance.
On-Sky Observations: Observations were conducted in 2022 and 2023 using narrow-band filters (Br- $\gamma$ , Br- $\alpha$ , Br- $\alpha$ -cont, and K-peak) on targets including $\gamma$ Gru, HR 8799, and $\iota$ Cap.
Data Reduction: A custom pipeline using pynpoint and TRAP (Temporal Reference Analysis of Planets) was developed. Key steps included:
- AB nodding for background subtraction.
- Stitching the two coronagraphic PSFs (dark holes) into a single frame.
- Using unsaturated leakage PSFs for flux calibration.
- Applying Principal Component Analysis (PCA) and Angular Differential Imaging (ADI) for post-processing.

3. Key Contributions

First On-Sky Characterization of ERIS gvAPP: This paper provides the first comprehensive performance report of the gvAPP on the VLT/ERIS instrument, covering K and L bands.
Throughput Verification: The authors quantified the total system throughput, accounting for LCP absorption, glue layers, and pupil undersizing.
Contrast Performance Analysis: The study details the raw and post-processed contrast limits, comparing on-sky results against laboratory benchmarks and design specifications.
Identification of Detector Noise: A critical contribution is the identification of a specific electronic noise pattern (cross-talk) in the ERIS detector that degrades performance in the dark holes, a finding that impacts future instrument design.
Operational Recommendations: The paper provides specific observing strategies (nodding patterns, filter choices, and sub-array usage) to maximize the utility of the gvAPP for time-series photometry and companion characterization.

4. Key Results

Transmission: The gvAPP meets its design specifications for transmission:
- K-band: $\sim90\%$ (matches design).
- L-band: $\sim60\%$ (matches design).
- M-band: $\sim40\%$ (limited by absorption features in glue/LCP at 3.3 $\mu$ m).
Raw Contrast:
- The instrument achieves a raw contrast of $1 \times 10^{-5}$ at the Outer Working Angle (OWA), consistent with laboratory measurements.
- However, at the Inner Working Angle (IWA, $2.2 \lambda/D$ ), the contrast is limited to $\sim1.6 \times 10^{-3}$ , an order of magnitude worse than the design goal ( $10^{-4}$ ). This is attributed to scattering from polishing errors in the reimaging optics.
Post-Processed Contrast:
- The system failed to reach the target post-processed contrast of $5 \times 10^{-5}$ .
- Limiting Factor: A repeating horizontal noise pattern was detected in the dark holes, correlated with the high flux of the central leakage term and coronagraphic PSFs. This is attributed to electronic cross-talk between readout amplifiers in the detector.
- Sensitivity limits (scaled to 1 hour) range from 12.18 mag (Br- $\alpha$ ) to 15.98 mag (K-peak).
Throughput Efficiency: The total point-source throughput (including splitting between PSFs and encircled energy loss) is approximately 13.6% in the Br- $\alpha$ filter and 21.8% in the K-peak filter.
Flux Imbalance: A flux asymmetry was observed between the left and right coronagraphic PSFs, varying with the star's position on the detector, likely due to induced circular polarization in the telescope optics.

5. Significance and Conclusions

Scientific Utility: Despite the contrast limitations, the gvAPP is recommended as a powerful tool for characterizing substellar companions with known separations and position angles. By pre-calculating the orbit, observers can place the companion directly into the "dark hole," maximizing signal-to-noise.
Time-Series Photometry: The central leakage term serves as an internal photometric reference, allowing for variability studies of bright companions (e.g., brown dwarfs). However, the contrast performance and detector noise currently limit this application to the brightest targets.
Pathfinder for ELT: This work serves as a critical pathfinder for the Extremely Large Telescope (ELT) instruments, specifically METIS. The lessons learned regarding:
- Reducing the number of glue layers (from 4 to 2) to improve transmission.
- Using thinner optically active layers.
- Modeling and mitigating detector electronic cross-talk.
  ...will directly inform the design of next-generation gvAPP coronagraphs for the ELT, which will enable high-resolution spectroscopy of exoplanets at extremely small separations.

In summary, the VLT/ERIS gvAPP successfully demonstrates the feasibility of grating-based vector phase plates for high-contrast imaging in the infrared, achieving design transmission goals but revealing that detector electronic noise is a significant bottleneck for achieving the deepest contrast limits.