![Time evolution of the vertical gradient of AH across different height layers is shown. The black line overlaid indicates the corresponding PWV in mm.-- [astro-ph.IM]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/10/MidIR.png?fit=1200%2C670&ssl=1)
[astro-ph.IM] Direct imaging and characterization of rocky exoplanets remains one of the central challenges in exoplanetary science. The mid-infrared (mid-IR) offers a favorable high-contrast imaging (HCI) regime, as the planet-star contrast is more moderate than at shorter wavelengths.
However, mid-IR observations with extremely large ground-based telescopes (ELTs) are strongly affected by spatio-temporal fluctuations in atmospheric water vapor (WV), which introduce chromatic wavefront errors that are largely invisible to adaptive optics systems operating at shorter wavelengths.
This poses a significant challenge for mid-IR HCI instruments such as METIS, which aims to detect thermal emission from rocky exoplanets in the N-band (8–13 μ m).
To quantify WV-induced wavefront errors, we analyze K-band fringe-tracker data from the GRAVITY instrument at the VLTI and derive the optical path difference between group-delay and phase-delay measurements. This difference provides a direct measure of the differential WV column density (Σ) above the telescopes.
We examine the dependence of the RMS differential WV column density (σΣ) on precipitable water vapor (PWV), airmass, seeing, and wind speed across multiple VLTI configurations. Among these parameters, PWV shows the strongest correlation with σΣ. We further explore the influence of the vertical PWV distribution on this relationship.
Next, we use power-spectral-density analysis to quantify how increasing PWV modulates Σ variability. The wavefront error associated with this phenomenon on ELT scale is approximately 15nm RMS in the L-band and 175nm RMS in the N-band, relative to the K-band.
These results provide a quantitative framework for assessing and mitigating atmospheric WV effects in next-generation mid-IR HCI instruments.
Conceptual illustration of WV seeing, where blue spheres represent water molecules, and the gray vertical line represent the WV column above the VLTI small AT configuration A0B2D0C1, overlaid on the ELT pupil.
Abhirami S Raghu, Prashant Pathak, Olivier Absil, Benjamin Courtney-Barrer, Gilles Orban de Xivry, Julien Woillez, Alexis Matter, Philippe Bério, Roy van Boekel, Bernhard Brandl
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2610.14801 [astro-ph.IM] (or arXiv:2610.14801v1 [astro-ph.IM] for this version)
https://doi.org/10.48550/arXiv.2610.14801
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Related DOI:
https://doi.org/10.1093/rasti/rzag075
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Submission history
From: Abhirami S Raghu
[v1] Sat, 10 Oct 2026 10:51:48 UTC (7,366 KB)
https://arxiv.org/abs/2610.14801
Astrobiology,
