# Impact of water vapor seeing on mid-infrared high-contrast imaging at ELT scale

**Authors:** Olivier Absil, Christian Delacroix, Gilles Orban de Xivry, Prashant Pathak, Matthew Willson, Philippe Berio, Roy van Boekel, Alexis Matter, Denis Defrere, Leo Burtscher, Julien Woillez, Bernhard Brandl  
**arXiv:** [2210.12412](https://arxiv.org/abs/2210.12412)  
**Date:** 2022-10-22  
**Summary page:** https://www.summarizepaper.com/en/paper/2210.12412v1/

## Key points

- Water vapor has a significant impact on ground-based wavefront quality in the mid-infrared domain for high-contrast imaging at ELT scale
- Water vapor is highly chromatic in the mid-infrared, which means that adaptive optics correction in the visible or near-infrared domain does not ensure high wavefront quality at longer wavelengths
- Additional wavefront error due to water vapor seeing is expected to dominate the wavefront quality budget at N band (8-13 μm), driving the performance of mid-infrared high-contrast imaging modes at ELT scale
- Focal-plane wavefront sensing techniques are being employed by the METIS team to mitigate this effect and significantly improve high contrast imaging performance
- Uncorrected WV seeing affects METIS HCI performance using Kolmogorov statistics with a typical standard deviation of 30 nm rms at L band and 350 nm rms at N band with most power in low spatial frequencies following standard Kolmogorov theory
- Adding time series of turbulent WV phase screens generated from previous sections to a typical sequence of SCAO residuals produced by COMPASS showed that additional WV induced WFE has almost negligible effects on L band but makes a large difference in WFE seen by N band IMG camera of METIS.

## Summary

The impact of water vapor on ground-based wavefront quality in the mid-infrared domain is a significant concern for high-contrast imaging at ELT scale. Unlike dry air, water vapor is highly chromatic, especially in the mid-infrared, which means that adaptive optics correction in the visible or near-infrared domain does not necessarily ensure high wavefront quality at longer wavelengths. In this study, literature measurements of water vapor seeing and infrared interferometric data from the VLTI were used to evaluate the wavefront quality that will be delivered to the METIS mid-infrared camera and spectrograph for ELT after single-conjugate adaptive optics correction in the near-infrared. The team discussed how additional wavefront error due to water vapor seeing is expected to dominate the wavefront quality budget at N band (8-13 μm), driving the performance of mid-infrared high-contrast imaging modes at ELT scale. To mitigate this effect, focal-plane wavefront sensing techniques are being employed by the METIS team. End-to-end simulations show that these techniques can significantly improve high contrast imaging performance. The study also explored how uncorrected WV seeing affects METIS HCI performance using Kolmogorov statistics. The generated turbulent phase screens showed a typical standard deviation of 30 nm rms at L band and 350 nm rms at N band with most power in low spatial frequencies following standard Kolmogorov theory. Adding time series of turbulent WV phase screens generated from previous sections to a typical sequence of SCAO residuals produced by COMPASS showed that additional WV induced WFE has almost negligible effects on L band but makes a large difference in WFE seen by N band IMG camera of METIS. Overall, this study highlights how water vapor seeing can significantly impact ground based wavefront quality and drive mid infrared high contrast imaging modes' performance at ELT scale. However, with appropriate mitigation strategies such as focal plane wavefront sensing techniques, the impact of water vapor seeing can be significantly reduced, leading to improved high contrast imaging performance.