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

Olivier Absil; Gilles Orban de Xivry; Prashant Pathak; Abhirami S. Raghu; Benjamin Courtney-Barrer; Roy van Boekel; Thomas Bertram; Markus Feldt; Matthew Kenworthy; Bernhard Brandl

arXiv · 2607.29335

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

Olivier Absil·Gilles Orban de Xivry·Prashant Pathak·Abhirami S. Raghu·Benjamin Courtney-Barrer·Roy van Boekel·Thomas Bertram·Markus Feldt·Matthew Kenworthy·Bernhard Brandl

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Abstract

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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Olivier Absil, Gilles Orban de Xivry, Prashant Pathak, Abhirami S. Raghu, Benjamin Courtney-Barrer, Roy van Boekel, Thomas Bertram, Markus Feldt, Matthew Kenworthy, Bernhard Brandl. 2026-07-31. Maximising the mid-infrared high-contrast performance of ELT/METIS despite water vapour seeing. https://arxiv.org/abs/2607.29335

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