arXiv subjects

Olivier Absil

Publications and source records attributed to Olivier Absil.

At least 19 recordsLinked to original sources

arXiv2026

Exploring reinforcement learning to enhance focal-plane wavefront control for vortex coronagraphs

High Contrast Imaging (HCI) on ground-based telescopes suffers from phase aberrations on the observed wavefront caused by atmospheric turbulence. Adaptive Optics (AO) systems are adept at correcting these aberrations, but fall short in the correction of non-common path aberrations (NCPAs). NCPAs arise because the wavefront sensor (WFS) measures and corrects a wavefront that is different from that affecting the science images, thus requiring additional intervention. This work makes use of focal-plane wavefront sensing and reinforcement learning (RL) to address the wavefront aberrations caused by NCPAs. The PO4NCPA algorithm utilizes sequential phase diversity to address phase ambiguities and is tested on a simulation designed for the Mid-infrared ELT Imager and Spectrograph (METIS) instrument. In this paper, we present the performance of PO4NCPA with scalar and vector vortex coronagraphs to demonstrate its flexibility.

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arXiv2026

METIS high-contrast imaging simulations: From instrument modelling to science readiness

The Mid-infrared Extremely Large Telescope (ELT) Imager and Spectrograph (METIS) instrument, expected to see first light in early 2030, aims to detect and characterise exoplanets and circumstellar disks through high- contrast imaging (HCI) and spectroscopy. The High-contrast End-to-End Performance Simulator (HEEPS), initially developed to support the design of the METIS HCI modes, has evolved into a crucial tool for the METIS science team to prepare and optimize observations. HEEPS is an open-source Python-based software with a modular architecture, integrating the wavefront Fresnel propagation package PROPER, and HCI image processing with the Vortex Image Processing (VIP) package. Though designed for METIS, its modularity has been applied to other HCI instruments as well. This work presents recent updates to HEEPS, including modelling of the final METIS pupil and Lyot stops, revised quasi-static non-common path aberrations (NCPA) and Talbot effect simulations informed by as-built optical surface errors, and updated METIS Single Conjugated Adaptive Optics (SCAO) simulations. We also discuss advancements in NCPA control strategies focusing on framerate, latency and sensing performance optimization, particularly for mitigating water vapor seeing effects using the asymmetric Lyot wavefront sensor (ALF) algorithm. With these refinements, we present a comprehensive grid of HCI performance simulations for METIS, covering a range of magnitudes in the L, M, and N-bands, and several HCI observing modes. These simulations produce updated 5-sigma sensitivity contrast curves and mock HCI observations, providing key insights on HCI performance for instrument optimization and science observation planning. Our results underscore the key role of end-to-end simulations in bridging instrumental design and scientific readiness in the ELT era.

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arXiv2026

Pre-shipment optical characterization of the SCALES instrument

The Slicer Combined with an Array of Lenslets for Exoplanet Spectroscopy (SCALES) instrument is a 1-5 micron imager and 2-5 micron integral field spectrograph, currently being commissioned on the Keck II Telescope. SCALES is optimized for exoplanet high-contrast imaging and spectroscopic characterization, and will be sensitive to older, colder exoplanets than existing instrumentation. The 12.3" x 12.3" imaging channel is designed to replicate the capabilities of NIRC2, and the low (R~35-200, 2.2" x 2.2" FOV) and medium (R~2500-5000, 0.36" x 0.34" FOV) spectral resolution modes offer new capabilities compared to existing Keck instrumentation. We present preliminary optical performance results from laboratory testing and commissioning of SCALES.

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arXiv2026

The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems VIII: patchy forsterite and enstatite clouds in the atmosphere of VHS 1256 b, retrieval lessons learned and outlook to the future

JWST defines a new era for the data-driven approach of retrieval modelling, which has become a cornerstone tool for the statistical inference of exoplanetary and brown dwarf properties. The Early Release Science program #1386 observations of VHS 1256 b represent a huge jump in data quality, data quantity and spectral coverage for such objects. VHS 1256 b is a young, planetary mass and extremely variable companion that populates the enigmatic L/T cohort of substellar atmospheres. In this first retrieval analysis of the full 1 - 18 micron dataset, we apply the Brewster retrieval framework to the NIRSpec and MIRI spectroscopic observations of VHS 1256 b, exploring a variety of cloud species and structures. Using Delta(BIC) we find that the data is best described by a forsterite (Mg$_{2}$SiO$_{4}$) and enstatite (MgSiO$_{3}$) cloud combination. Our analysis shows a strong preference for patchy silicate cloud coverage, which aligns with VHS 1256 b's extensive and well documented spectral variability. Our retrieval is able to place constraints on the abundances of H$_{2}$O, CO, CO$_{2}$, CH$_{4}$ as well as NH$_{3}$. We also show that the retrieved parameters are sensitive to the data used and the relative signal-to-noise ratios between data from different instruments. We conclude with the next steps for the wider retrieval community to better understand young and cloudy exoplanetary atmospheres.

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arXiv2026

Maximising the mid-infrared high-contrast performance of ELT/METIS despite water vapour seeing

The Mid-infrared ELT Imager and Spectrograph (METIS) will be equipped with a SCAO system delivering Strehl ratios above 90% at L band (3.5 - 4.1 um) and close to 99% at N band (8 - 13 um) on bright stars. Yet, the actual wavefront quality seen by the METIS coronagraphic modules used for high-contrast imaging will be significantly affected by water vapour seeing, which add a strong chromatic component to dry air seeing in the mid-infrared. We analysed two years of VLTI/GRAVITY fringe tracker archives to assess the variability of differential water vapour column density at millisecond timescales on ELT scales. Our analysis suggests that water vapour seeing will add a median wavefront error of about 175 nm rms at N band, consisting mostly of low-order aberrations, with around 150 nm rms of tip-tilt errors. If not corrected, this effect would degrade the achievable sensitivity limits in terms of contrast by more than two magnitudes. To mitigate this effect, we plan to deploy a focal-plane wavefront sensing and control algorithm based on an asymmetric pupil using a deep learning approach. After briefly discussing the practical impacts of focal-plane wavefront control in METIS, we compare the expected high-contrast imaging performance with and without focal-plane wavefront control.

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arXiv2026

Upgrading LBTI/NOMIC with a quadruple annular groove phase mask and GeoSnap detector for imaging nearby, habitable-zone exoplanets

The Large Binocular Telescope Interferometer (LBTI)'s Nulling-Optimized Mid-Infrared Camera (NOMIC) is among the most capable thermal-infrared imaging systems available for high-contrast, high-angular-resolution astronomical observations. Here we describe two in-progress upgrades to LBTI/NOMIC: (1) the design, fabrication, and installation of a quadruple annular groove phase mask (Q-AGPM) coronagraph, and (2) the installation of a 13 micron-cutoff Teledyne GeoSnap array. The Q-AGPM is the first coronagraph to be installed within NOMIC and one of the first optimized for N-band (~11 micron) observations. It places four annular groove phase masks on a single diamond substrate so that, in the LBTI dual-aperture imaging mode, each of the two telescope beams can be chopped between a pair of masks without loss of observing efficiency. The GeoSnap array will replace NOMIC's original AQUARIUS array, delivering higher quantum efficiency, larger well depth, faster and more linear readout, and freedom from the excess low-frequency noise that requires aggressive chopping. Together these upgrades substantially improve the achievable contrast and sensitivity at small angular separations. We also present a high-contrast Fizeau imaging sequence obtained with LBTI's new FFTCam fringe tracker, which confirms the interferometric gain over a single aperture through injection/recovery tests: relative to an equal-time single aperture exposure, the S/N = 3 contrast is a factor of ~2-4 deeper across 0.2-1 arcsec, spanning the contrast- and background-limited regimes. Finally, we describe the role of the upgraded LBTI/NOMIC instrument within the Breakthrough Watch program at the University of Arizona, which aims to perform the deepest observations yet of the habitable zones of the nearest single Sun-like stars.

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arXiv2026

Direct Imaging Discovery of Giant Exoplanet $\beta$ Pictoris d: A Decade-Long Game of Hide-and-Seek

We report the direct imaging discovery of a third exoplanet in the $\beta$ Pictoris system. We detect $\beta$ Pictoris d ($\beta$ Pic d) in non-coronagraphic observations obtained with VLT/ERIS as well as multi-epoch archival datasets from JWST/NIRCam and VLT/SPHERE. Astrometric measurements over an 11-year baseline demonstrate that it is consistent with a gravitationally-bound source with orbital motion. Joint multi-planet orbit fits of all three planets in the system yield a semi-major axis of $26.0^{+2.2}_{-6.1}$ au and inclination $89.0^{+0.7}_{-0.6}$ deg for planet d. $\beta$ Pic d has a larger orbital semi-major axis than the other known planets in the system, but is coplanar with the inner two planets, and its orbit is consistent with sculpting the inner edge of the debris disk. $\beta$ Pic d has a contrast of $\Delta L^{\prime}=12.11\pm0.15$ mag, with colors and luminosity that closely match those of 51 Eri b, another exoplanet in the $\beta$ Pictoris moving group. Its VLT/ERIS and JWST/NIRCam colors are distinct from those of free-floating planetary-mass objects of a similar age and temperature. Its red $F410M-F444W$ color indicates strong CO$_2$ absorption in its atmosphere and suggests significant enhancement in metals compared to free-floating objects. From the ATMO hot-start evolutionary models, we estimate an effective temperature of $600^{+45}_{-60}$ K and mass of $2.4\pm0.6$ $M_{\rm Jup}$, which also closely matches similar estimates for 51 Eri b. $\beta$ Pic d is among the lowest-mass exoplanets imaged from the ground. This discovery highlights the deep sensitivity achievable with ground-based imaging in the mid-infrared and the discovery potential of future high-contrast observations with the Extremely Large Telescope.

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arXiv2026

New substellar candidates identified through deep learning in the F150 sample of the large-scale SHINE direct imaging survey

Context. The SPHERE High-contrast Imaging survey for Exoplanets (SHINE) represents one of the largest direct imaging campaigns, targeting over 400 young, nearby stars with the goal of detecting and characterizing giant exoplanets and brown dwarfs. This dataset offers a unique opportunity to revisit observations using modern, data-driven approaches, potentially uncovering new substellar candidates that may have been overlooked by classical analysis techniques. Aims. Our study focuses on reprocessing and reanalyzing the so-called F150 sample, a well-defined subset of 150 main-sequence stars within 100 pc observed in the H-band with VLT/SPHERE as part of the SHINE survey. Methods. We apply NA-SODINN, a supervised deep learning model specifically tailored for detecting faint planetary signals in angular differential imaging (ADI) sequences. Designed to model local noise properties and capture spatial context, NA-SODINN is effective at distinguishing real companions from residual speckle noise. To translate the model's pixel-wise confidence maps into actionable detections, we introduce a novel F1-score-based thresholding strategy. This principled approach balances sensitivity and specificity, addressing a key limitation in current deep learning-based methods. Results. NA-SODINN recovers all known companions and some of the debris disks in the F150 sample, and identifies 13 new substellar candidates not reported in previous studies: ten detected in both the H2 and H3 bands, and three in only one band. For the ten sources detected in both bands, we use the H2-H3 color-magnitude diagram to perform a first assessment of their nature. Based on this analysis, we identify two ambiguous cases and three photometrically promising candidates. However, in light of the currently available multi-epoch SPHERE data, only the candidate around Smethells 20 remains a strong target for follow-up.

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arXiv2026

Continuing to Advance European High Contrast Imaging Research and Development towards HWO and LIFE

The European Research and Development for Space based High Contrast Imaging II Workshop, held at MPIA in May 2025, advanced Europe strategic coordination in support of future exoplanet imaging missions such as the Habitable Worlds Observatory and the Large Interferometer for Exoplanets mission. Building on the first 2024 workshop, this meeting defined concrete priorities across eight technical areas, including wavefront sensing, coronagraphs, post processing, nulling interferometry, deformable mirrors, detectors, and telescope design. Discussions emphasized Europe strengths in adaptive optics, ground-based facilities, and interferometry, while identifying key gaps, particularly the need for a dedicated European vacuum testbed for high contrast imaging. The community highlighted near infrared or UV coronagraphy as a promising domain for European leadership and called for joint development of advanced data reduction algorithms, detectors, and cross-mission coordination with HWO and LIFE. The workshop outcomes establish a collaborative roadmap to strengthen Europe technological readiness, foster agency partnerships, and ensure its continued leadership in the next generation of space-based exoplanet exploration.

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arXiv2026

Focal plane wavefront control with model-based reinforcement learning

The direct imaging of potentially habitable exoplanets is one prime science case for high-contrast imaging instruments on extremely large telescopes. Most such exoplanets orbit close to their host stars, where their observation is limited by fast-moving atmospheric speckles and quasi-static non-common-path aberrations (NCPA). Conventional NCPA correction methods often use mechanical mirror probes, which compromise performance during operation. This work presents machine-learning-based NCPA control methods that automatically detect and correct both dynamic and static NCPA errors by leveraging sequential phase diversity. We extend previous work in reinforcement learning for AO to focal plane control. A new model-based RL algorithm, Policy Optimization for NCPAs (PO4NCPA), interprets the focal-plane image as input data and, through sequential phase diversity, determines phase corrections that optimize both non-coronagraphic and post-coronagraphic PSFs without prior system knowledge. Further, we demonstrate the effectiveness of this approach by numerically simulating static NCPA errors on a ground-based telescope and an infrared imager affected by water-vapor-induced seeing (dynamic NCPAs). Simulations show that PO4NCPA robustly compensates static and dynamic NCPAs. In static cases, it achieves near-optimal focal-plane light suppression with a coronagraph and near-optimal Strehl without one. With dynamics NCPA, it matches the performance of the modal least-squares reconstruction combined with a 1-step delay integrator in these metrics. The method remains effective for the ELT pupil, vector vortex coronagraph, and under photon and background noise. PO4NCPA is model-free and can be directly applied to standard imaging as well as to any coronagraph. Its sub-millisecond inference times and performance also make it suitable for real-time low-order correction of atmospheric turbulence beyond HCI.

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arXiv2026

The Key to Unlocking Exoplanet Biosignatures: a UK-led IR Spectrograph for the Habitable Worlds Observatory Coronagraph

The detection of life on rocky exoplanets in the habitable zones of nearby stars would be a paradigm-shifting advance, and it is one of the greatest scientific challenges of our time. There is no single spectral feature that is an unambiguous sign of life on a given exoplanet. Instead, the current state-of-the-art approach involves detecting multiple molecular atmospheric features that should not exist together in equilibrium, e.g. simultaneous detection of O$_2$ and CH$_4$. Spectra across a wide wavelength (0.3-1.7 $\mu$m) range are necessary to cover multiple spectral features per molecule of interest and to contextualise the suite of molecular features detected. While the US will lead the optical arm of the Habitable Worlds Observatory (HWO) coronagraph, a UK-led contribution of a near-infrared Integral Field Spectrograph (IFS) for the infrared arm will ensure UK leadership in the flagship scientific goal of HWO - to search for signatures of life on potentially habitable exoplanets.

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arXiv2025

A vision for ground-based astronomy beyond the 2030s: How to build ESO's next big telescope sustainably

Astronomy is the study of the Universe and all the objects that it comprises. Our attention is therefore usually focused beyond Earth, home to the only form of life known today. However, how can we continue to explore the secrets of the Universe, if we stand by and watch our only home burn? We know that there is no Planet B. It is therefore urgent that, as astronomers, we collectively work to protect the Earth, allowing future generations the opportunity to continue to uncover the secrets of the cosmos. As astronomical facilities account for the majority of our community's carbon footprint, we propose guidelines that we hold crucial for the European Southern Observatory (ESO) to consider in the context of the Expanding Horizons programme as it plans a next-generation, transformational facility.

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arXiv2025

Enhanced detection limits in the SHINE F150 survey through the Regime Switching Model. Optimizing thresholds and investigating environmental noise

In high-contrast imaging, a novel detection algorithm for angular differential imaging (ADI) sequences has recently been introduced: the Regime Switching Model (RSM). In this study, we apply the RSM algorithm to analyze the F150 sample from the SHINE high-contrast imaging survey carried out with VLT/SPHERE, aiming to enhance detection limits and identify new exoplanet candidates. Additionally, we investigate how environmental conditions influence post-processed noise distributions and detection thresholds. We generate detection maps and contrast curves for 213 observations in the F150 SHINE sample using the RSM algorithm. A clustering approach based on environmental parameters is used to group observations with similar noise characteristics. We propose two methods for defining radial detection thresholds in the RSM maps: fitting a log-normal distribution to the post-processed noise and maximizing the F1 score. We also assess the performance of various combinations of post-processing techniques within the RSM framework to identify optimal configurations. This study demonstrates the utility of clustering based on observational parameters, effectively distinguishing features like wind-driven halos and low-wind effects. Detection thresholds vary significantly across clusters, differing by up to a factor of 10, highlighting the importance of considering observational environments. Log-normal thresholds provide conservative, noise-aware limits, while F1 score-based thresholds offer observation-specific results, both showing compatibility overall. RSM improves detection limits by an average factor of two at 1arcsec and five at inner working angles compared to standard PCA processing. This study reports more than 30 newly detected signals, including one promising candidate awaiting second-epoch confirmation.

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arXiv2025

MIRAC-5 on the MMT with MAPS: annular groove phase mask N-band coronagraphic upgrade

We describe the coronagraphic upgrade underway for the Mid-Infrared Array Camera-5 (MIRAC-5) to be used with the 6.5-m MMT telescope utilizing the new MMT Adaptive optics exoPlanet characterization System (MAPS). Mid-IR ground-based coronagraphic adaptive-optics-assisted imaging can be a powerful tool for characterizing exoplanet atmospheres and studying protoplanets in formation within circumstellar disks around young stars. In addition to enabling ground-based observations of bright targets in the background limit, high actuator density 1-2 kHz adaptive optics systems can be competitive with JWST in the contrast limit. We have procured an annular groove phase mask (AGPM) and performed preliminary characterization of its on-axis source rejection as a function of wavelength. We present an optimized Lyot Stop design for use with the AGPM using the High-contrast End-to-End Performance Simulator (HEEPS). Future work includes implementing the Quadrant Analysis of Coronagraphic Images for Tip-tilt Sensing (QACITS) control loop algorithm with MAPS. We present the system overview, pupil mask design, and expected performance metrics aligned with our scientific goals, building upon recent advances with MIRAC-5 (Bowens et al. 2025) and MAPS.

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arXiv2025

Panchromatic characterization of the Y0 brown dwarf WISEP J173835.52+273258.9 using JWST/MIRI

Cold brown dwarf atmospheres are good training grounds for analyzing temperate giant planets. WISEP J173835.52+273258.9 (WISE 1738) is an isolated Y0 brown dwarf with a temperature between 350-400 K, at the T-Y transition. While its near-infrared spectrum has been studied, bulk properties and chemistry remain uncertain. We analyze new JWST MIRI medium-resolution spectra (5-18 micron), combined with near-infrared spectra (0.98-2.2 micron) from HST/WFC3 and Gemini/GNIRS, to better constrain WISE 1738's atmosphere and physical parameters. We use Neural Posterior Estimation (NPE) with a cloud-free petitRADTRANS model and evaluate results using posterior checks, coverage, and L-C2ST diagnostics. Our retrieval confirms previous constraints on H2O, CH4, and NH3, and for the first time constrains CO, CO2, and 15NH3. We find evidence of disequilibrium chemistry through CO and CO2 abundances not expected under equilibrium. Estimated properties are temperature 402 (+12,-9) K, log g 4.43 (+0.26,-0.34) cm/s2, mass 13 (+11,-7) M_Jup, radius 1.14 (+0.03,-0.03) R_Jup, and bolometric luminosity -6.52 (+0.05,-0.04) log L/L_sun. Evolutionary models suggest an age between 1 and 4 Gyr, consistent with a 6-hour rotation. We place an upper bound on 15NH3, implying a 3-sigma lower limit on the 14N/15N ratio of 275. We also derive a C/O ratio of 1.35 (+0.39,-0.31) and metallicity of 0.34 (+0.12,-0.11), without accounting for oxygen sequestration.

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arXiv2025

Follow-Up Exploration of the TWA 7 Planet-Disk System with JWST NIRCam

The young M-star TWA 7 hosts a bright and near face-on debris disk, which has been imaged from the optical to the submillimeter. The disk displays multiple complex substructures such as three disk components, a large dust clump, and spiral arms, suggesting the presence of planets to actively sculpt these features. The evidence for planets in this disk was further strengthened with the recent detection of a point-source compatible with a Saturn-mass planet companion using JWST/MIRI at 11 $\mu$m, at the location a planet was predicted to reside based on the disk morphology. In this paper, we present new observations of the TWA 7 system with JWST/NIRCam in the F200W and F444W filters. The disk is detected at both wavelengths and presents many of the same substructures as previously imaged, although we do not robustly detect the southern spiral arm. Furthermore, we detect two faint potential companions in the F444W filter at the 2-3$\sigma$ level. While one of these companions needs further followup to determine its nature, the other one coincides with the location of the planet candidate imaged with MIRI, providing further evidence that this source is a sub-Jupiter mass planet companion rather than a background galaxy. Such discoveries make TWA 7 only the second system, after $\beta$ Pictoris, in which a planet predicted by the debris disk morphology has been detected.

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arXiv2025

GRIP: a generic data reduction package for nulling interferometry

Nulling interferometry is a powerful observing technique to study exoplanets and circumstellar dust at separations too small for direct imaging with single-dish telescopes. With recent photonics developments and the near-future ground-based instrumental projects, it bears the potential to detect young giant planets near the snow lines of their host stars. The observable quantity of a nulling interferometer is called the null depth, its precise measurement and calibration remain challenging against instrument and atmospheric noise. Null self-calibration is a method aiming to model the statistical distribution of the nulled signal. It has proven to be more sensitive and accurate than average-based data reduction methods in nulling interferometry. The variety of existing and upcoming of nullers raises the issue of consistency of the calibration process, structure of the data and the ability to reduce archived data on the long term. It has also led to many different implementations of the Null self-calibration method. In this article, we introduce GRIP: the first open-source toolbox to reduce nulling data with enhanced statistical self-calibration methods from any nulling interferometric instrument within a single and consistent framework. Astrophysical results show good consistency with two published GLINT and LBTI datasets and confirm nulling precision down to a few 10$^{-4}$.

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arXiv2025

MIRI-JWST mid-infrared direct imaging of the debris disk of HD106906

We report MIRI-JWST coronagraphic observations at 11.3 and 15.5 mic of the debris disk around the young star HD 106906. The observations were made to characterize the structure, temperature and mass of the disk through the thermal emission of the dust heated by the central star. Another goal was also to constrain the size distribution of the grains. The data were reduced and calibrated using the JWST pipeline. The analysis was based on a forward-modeling of the images using a multiparameter radiative transfer model coupled to an optical code for coronagraphy processing. The disk is clearly detected at both wavelengths. The slight asymmetry is geometrically consistent with the asymmetry observed in the near-IR, but it is inconsistent the brightness distribution. The observed structure is well reproduced with a model of a disk (or belt) with a critical radius 70 au, a mildly inward-increasing density (index 2) and a steeper decrease outward (index -6). This indication of a filled disk inside the critical radius is inconsistent with sculpting from an inner massive planet. The size distribution of the grains that cause the mid-IR emission is well constrained by the flux ratio at the two wavelengths : 0.45 10 mic and 0.65 10 mic for silicate and graphite grains, respectively. The minimum size is consistent with predictions of blowout through radiative pressure. We derive a mass of the dust that causes the mid-IR emission of 3.3 5.0 E3 Mearth. When the larger grains (up to 1 cm) that cause the millimeter emission are included, we extrapolate this mass to 0.10 0.16 Mearth. We point out to that this is fully consistent with ALMA observations of the disk in terms of dust mass and of its millimeter flux. We estimate the average dust temperature in the planetesimal belt to be 74 K, and a temperature range within the whole disk from 40 to 130 K.

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