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Vortex coronagraphs have been shown to be a promising avenue forhigh-contrast imaging in the close-in environment of stars at thermal infrared(IR) wavelengths. They are included in the baseline design of METIS. To ensuregood performance of these coronagraphs, a precise control of the centering ofthe star image in real time is needed. We previously developed and validatedthe quadrant analysis of coronagraphic images for tip-tilt sensing estimator(QACITS) pointing estimator to address this issue. While this approach is notwavelength-dependent in theory, it was never implemented for mid-IRobservations, which leads to specific challenges and limitations. Here, wepresent the design of the mid-IR vortex coronagraph for the new Earths in the$\alpha$ Cen Region (NEAR) experiment with the VLT/VISIR instrument and assessthe performance of the QACITS estimator for the centering control of the starimage onto the vortex coronagraph. We use simulated data and on-sky dataobtained with VLT/VISIR, which was recently upgraded for observations assistedby adaptive optics in the context of the NEAR experiment. We demonstrate thatthe QACITS-based correction loop is able to control the centering of the starimage onto the NEAR vortex coronagraph with a stability down to $0.015\lambda/D$ rms over 4h in good conditions. These results show that QACITS is arobust approach for precisely controlling in real time the centering of vortexcoronagraphs for mid-IR observations.

Design, pointing control, and on-sky performance of the mid-infrared vortex coronagraph for the VLT/NEAR experiment