  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.
