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

TL;DR

This paper evaluates how water vapor variability affects mid-infrared wavefront quality at ELT scale and discusses mitigation strategies to improve high-contrast imaging performance.

Contribution

It provides an analysis of water vapor seeing impact on ELT mid-infrared imaging and proposes wavefront sensing techniques for mitigation.

Findings

Water vapor seeing significantly impacts wavefront quality in the mid-infrared.

Adaptive optics correction in near-infrared does not fully correct mid-infrared wavefront errors.

Wavefront sensing techniques can mitigate water vapor seeing effects.

Abstract

The high-speed variability of the local water vapor content in the Earth atmosphere is a significant contributor to ground-based wavefront quality throughout the infrared domain. Unlike dry air, water vapor is highly chromatic, especially in the mid-infrared. This means that adaptive optics correction in the visible or near-infrared domain does not necessarily ensure a high wavefront quality at longer wavelengths. Here, we use literature measurements of water vapor seeing, and more recent infrared interferometric data from the Very Large Telescope Interferometer (VLTI), to evaluate the wavefront quality that will be delivered to the METIS mid-infrared camera and spectrograph for the Extremely Large Telescope (ELT), operating from 3 to 13 {\mu}m, after single-conjugate adaptive optics correction in the near-infrared. We discuss how the additional wavefront error due to water vapor seeing is expected to dominate the wavefront quality budget at N band (8-13 {\mu}m), and therefore to drive the performance of mid-infrared high-contrast imaging modes at ELT scale. Then we present how the METIS team is planning to mitigate the effect of water vapor seeing using focal-plane wavefront sensing techniques, and show with end-to-end simulations by how much the high-contrast imaging performance can be improved.