🔭 astrophysics

ANDES, the high-resolution spectrograph of the ELT: simulated performance of the CORO module and overview of the high-contrast capabilities for exoplanet observations

This paper details the design, features, and simulated performance of the ANDES coronagraph (CORO) module for the Extremely Large Telescope, demonstrating its high-contrast capabilities and predicted detection yields for characterizing exoplanet atmospheres using high-dispersion spectroscopy combined with adaptive optics.

Original authors: M. N'Diaye, A. Simonnin, A. Spang, P. Berio, A. Chiavassa, G. Agapito, C. Bailet, G. Carlà, Y. Caujolle, O. Carrión-Gonzalez, G. Chauvin, S. Cuevas, O. Gabella, M. Houllé, S. Lagarde, P. Martinez, E.

Published 2026-07-28

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CC BY 4.0

Original authors: M. N'Diaye, A. Simonnin, A. Spang, P. Berio, A. Chiavassa, G. Agapito, C. Bailet, G. Carlà, Y. Caujolle, O. Carrión-Gonzalez, G. Chauvin, S. Cuevas, O. Gabella, M. Houllé, S. Lagarde, P. Martinez, E. Pinna, B. Rajpoot, J. Seidel, C. Selmi, A. Vigan, P. Di Marcantonio

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 night sky as a giant, glittering party where the stars are the loudest, brightest guests, shouting so loudly that no one can hear the quiet whispers of the tiny planets dancing around them. For decades, astronomers have been trying to listen to those whispers, but the star's glare is like a blinding flashlight pointed right in their eyes. To solve this, scientists are building the Extremely Large Telescope (ELT), a massive mirror that will act like the ultimate pair of night-vision goggles. But even with a giant mirror, the star's light is too strong. So, they are adding a special "coronagraph," which is essentially a high-tech pair of sunglasses with a tiny, perfect sticker in the middle to block out the star's face while letting the faint light of its neighbors slip through. This paper is about testing the blueprints for those sunglasses before the telescope is even built, using powerful computer simulations to see if they will actually work.

The team behind the ANDES instrument, a super-sensitive camera and spectrograph for the ELT, is designing a specific module to hunt for exoplanets—worlds orbiting other stars. Their goal is to catch the faint glow of these planets, either from the heat they emit or the starlight they reflect, and analyze it to see what their atmospheres are made of. The challenge is immense: they need to block out the star's light so effectively that they can see a planet that is a million times fainter, all while the Earth's atmosphere tries to blur the image. This paper simulates how their new "CORO" module will perform, checking if it can handle the messy reality of the real world, like wind shaking the telescope or the telescope's own tiny imperfections.

The authors ran detailed computer simulations to design and test a "Lyot coronagraph," a classic type of star-blocker that uses a series of masks and stops to filter out light. They found that by carefully sizing the central mask (which blocks the star) and the surrounding ring (which cleans up the leftover starlight), they could theoretically achieve a contrast level of $10^{-3}$ (meaning the star's light is reduced to one-thousandth of its original brightness) at a distance of just 20 milliarcseconds from the star. To put that in perspective, 20 milliarcseconds is the size of a coin seen from 100 kilometers away. The simulations showed that with their specific design—a central mask about 33.4 milliarcseconds wide and a surrounding ring that is 89% of the telescope's diameter—they could preserve most of the planet's light while suppressing the star's glare.

However, the paper is careful to note that these are simulated results, not measurements from a finished machine. The team tested how the system would hold up against various "bugs" or errors that might happen in real life. They discovered the design is surprisingly robust. For instance, even if the telescope's pointing wanders slightly (up to 2 milliarcseconds) or if the focus shifts a tiny bit (up to 30 nanometers), the system still manages to keep the star's glare down to the required level. They also found that the system can tolerate small misalignments in the "Lyot stop" (the ring mask) and even some color-based blurring from the atmosphere, provided they use a special corrector. The paper notes that while they chose a simpler, classical approach for now, alternative concepts like vortex phase masks "could be investigated" before the final design review if they offer clear benefits without adding too much complexity.

The researchers also looked at what this means for finding planets. Using a separate simulation tool called APU, they combined the coronagraph's ability to block starlight with a technique called "molecular mapping," which acts like a super-powered prism to separate the planet's light from the remaining starlight. In these simulations, the combination of the coronagraph and the high-resolution spectrograph could potentially push the detection limit down to a contrast of $10^{-7}$ to $10^{-8}$ (meaning the star's light is reduced to one ten-millionth or one hundred-millionth of its original brightness). This would be a game-changer, potentially allowing the telescope to spot faint, rocky worlds or temperate gas giants that are currently invisible. The paper concludes that while the design looks promising and the error budget seems manageable, these are still preliminary findings. The team plans to refine these simulations and build the actual hardware, with a major design review scheduled for late 2026, hoping that by the 2030s, the ELT will finally be able to take a clear, unblurred photo of an alien world's atmosphere.

Technical Summary: Simulated Performance of the ANDES CORO Module

Problem Statement
The Extremely Large Telescope (ELT) instrument ANDES aims to characterize exoplanet atmospheres through high-resolution spectroscopy (R=100,000) in the visible to near-infrared range. A primary challenge in observing exoplanets is the extreme brightness contrast between the host star and the faint planetary companion. While adaptive optics (AO) corrects for atmospheric turbulence, it leaves residual speckles that limit contrast. To achieve the necessary sensitivity for detecting low-luminosity rocky companions and temperate giant exoplanets, the instrument requires a coronagraphic system capable of suppressing starlight to a contrast level of at least $3 \times 10^{-3}$ (goal $10^{-3}$ ) at an angular separation of 20 mas in the YJH bands (1.0–1.7 µm). This paper addresses the design, optimization, and error budgeting of the Coronagraph (CORO) module integrated into the Single Conjugate Adaptive Optics (SCAO) and Integral Field Unit (IFU) subsystems of ANDES.

Methodology
The authors employed a multi-stage simulation and optimization approach:

Coronagraph Design: A classical Lyot coronagraph (CLC) was selected as the baseline solution due to its simplicity and moderate gain in starlight attenuation. The optical layout includes an entrance pupil, a focal plane mask (FPM), a Lyot stop (pupil plane stop), and a re-imaged focal plane.
Parameter Optimization: Using the ANDES Performance Unfolded (APU) simulation tool, the authors performed a parameter space exploration to optimize the FPM diameter ( $d$ ), and the Lyot stop's outer (OD) and inner (ID) diameters. The optimization aimed to maximize the signal-to-noise ratio (SNR) for an off-axis companion at 20 mas while minimizing residual starlight intensity across the 1.0–1.7 µm band.
Error Budgeting: The study established a preliminary error budget by simulating the impact of various error sources on the contrast performance at 20 mas. These sources included:
- Pointing errors (tilt).
- Defocus errors.
- Chromatic dispersion residuals.
- Lyot stop clocking (angular) and lateral positioning errors.
- Non-common path aberrations (NCPA) modeled as time-evolving phase screens with a $f^{-2}$ power spectral density.
End-to-End Simulation: The APU simulator was used to generate data cubes of images with and without the coronagraph, incorporating SCAO residuals, telluric transmission, and instrument throughput. Post-processing techniques, specifically molecular mapping (cross-correlation), were applied to estimate detection limits for gas giant planets.

Key Contributions and Results

Optimized Configuration: The study identified an optimal configuration for the YJH bands: an FPM diameter of $3.9 \lambda_{ref}/D$ (33.4 mas), a Lyot stop outer diameter of $0.89 D$ , and an inner diameter of $0.35 D$ . This configuration achieves a throughput of 75% for the planet flux at the ELT pupil level and ensures that more than 50% of the maximum planet flux is transmitted at the 20 mas separation.
Contrast Performance: Simulations in median seeing conditions (0.67") demonstrate that the CLC, combined with SCAO, achieves a raw contrast better than $10^{-3}$ at 20 mas across the 1.0–1.7 µm range. The contrast is dominated by atmospheric residuals at shorter wavelengths and diffraction residuals at longer wavelengths.
Error Tolerance and Specifications: The analysis of error sources led to the following preliminary specifications to maintain the $10^{-3}$ $1 0^{- 3}$ contrast goal:
- Pointing: $\le 2$ mas residual tilt.
- Defocus: $\le 30$ nm RMS (longitudinal $\pm 72.5$ µm).
- Chromatic Dispersion: $\le 5$ mas/µm residual (requiring an Atmospheric Dispersion Corrector).
- Lyot Stop Alignment: $\le 1^\circ$ clocking error and $\le 1\%$ of the pupil diameter ( $\pm 101.5$ µm) lateral offset.
- Optical Surface Quality: $\le 30$ nm RMS total NCPA, decomposed into 11.9 nm RMS for low-order modes and 27.5 nm RMS for high-order modes.
Detection Limits: Preliminary end-to-end simulations using APU for L-type gas giant planets (1600 K) suggest that the combination of high-contrast imaging (SCAO + coronagraph) and high-dispersion spectroscopy (molecular mapping) could achieve contrast limits down to $10^{-7}$ . This capability targets the detection of gas giants in emitted light and potentially Earth-like planets in reflected light.

Significance
The paper establishes the feasibility of integrating a coronagraphic module into the ANDES instrument to enable high-contrast, high-resolution spectroscopy of exoplanets. By defining a robust error budget and demonstrating that a classical Lyot design can meet the stringent contrast requirements of $10^{-3}$ at 20 mas, the study provides the necessary technical foundation for the opto-mechanical design of the CORO module. The results indicate that ANDES will be capable of characterizing exoplanet atmospheres, including molecular abundances and isotopic ratios, and potentially detecting biosignatures in Earth-like planets, thereby expanding the scientific reach of the ELT in the 2030s. The work serves as a critical step toward the Preliminary Design Review (PDR) scheduled for late 2026.

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