Simulating the effect of chromatic beam wander on the ELT/METIS high-contrast imaging performance

Mid-infrared European Extremely Large Telescope (ELT) Imager and Spectrograph (METIS), one of the first-generation instruments of the upcoming ELT, is designed toward the direct detection of exoplanets, using high-contrast imaging (HCI) techniques. However, it is well known that noncommon path aberrations (NCPA) limit HCI performance. In particular, quasi-static NCPA creates speckles that evolve on the same time scale as angular differential imaging (ADI) sequences, which are typically used to estimate and subtract the stellar speckle field. It is therefore crucial to model NCPA correctly during the instrument design phase to derive requirements on wavefront control algorithms that minimize their effects. We explore the chromatic beam wander caused by atmospheric refraction on METIS as a source of time-variable NCPA and its impact on HCI performance. We study the vortex coronagraph case, which is expected to set the requirements for NCPA correction. We propagate wavefronts through a model of the full METIS instrument in pupil tracking mode, taking into account the contribution of each optical element to phase aberrations in the presence of chromatic beam wander. We also model the associated amplitude aberrations that develop upon propagation. The beams are propagated within a 4-h interval, from which we extract a 1-h observing sequence close to the zenith as a representative ADI sequence. We show that the time-variable NCPA created by chromatic beam wander already reaches, and sometimes exceeds, the allocated NCPA budget within METIS, even though its effect is smaller than the effect of water vapor seeing at N-band. A closed-loop correction is needed to compensate for their effect, but even so, the HCI performance may be limited by the associated amplitude aberrations, which cannot be corrected within METIS.