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16 September 2020 Design, pointing control, and on-sky performance of the mid-infrared vortex coronagraph for the VLT/NEAR experiment
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Vortex coronagraphs have been shown to be a promising avenue for high-contrast imaging in the close-in environment of stars at thermal infrared (IR) wavelengths. They are included in the baseline design of the mid-infrared extremely large telescope imager and spectrograph. To ensure good performance of these coronagraphs, a precise control of the centering of the star image in real time is needed. We previously developed and validated the quadrant analysis of coronagraphic images for tip-tilt sensing estimator (QACITS) pointing estimator to address this issue. While this approach is not wavelength-dependent in theory, it was never implemented for mid-IR observations, which leads to specific challenges and limitations. Here, we present the design of the mid-IR vortex coronagraph for the “new Earths in the α Cen Region (NEAR) experiment with the Very Large Telescope (VLT)/Very Large Telescope imager and spectrometer for the mid-infrared (VISIR) instrument and assess the performance of the QACITS estimator for the centering control of the star image onto the vortex coronagraph. We use simulated data and on-sky data obtained with VLT/VISIR, which was recently upgraded for observations assisted by adaptive optics in the context of the NEAR experiment. We demonstrate that the QACITS-based correction loop is able to control the centering of the star image onto the NEAR vortex coronagraph with a stability down to 0.015 λ  /  D rms over 4 h in good conditions. These results show that QACITS is a robust approach for precisely controlling in real time the centering of vortex coronagraphs for mid-IR observations.

© 2020 Society of Photo-Optical Instrumentation Engineers (SPIE) 2329-4124/2020/$28.00 © 2020 SPIE
Anne-Lise Maire, Elsa Huby, Olivier Absil, Gérard Zins, Markus Kasper, Christian Delacroix, Serban Leveratto, Mikael Karlsson, Garreth Ruane, Hans-Ulrich Käufl, Gilles Orban de Xivry, Prashant Pathak, Lorenzo Pettazzi, Philippe Duhoux, Johann Kolb, Éric Pantin, A. J. Eldorado Riggs, Ralf Siebenmorgen, and Dimitri Mawet "Design, pointing control, and on-sky performance of the mid-infrared vortex coronagraph for the VLT/NEAR experiment," Journal of Astronomical Telescopes, Instruments, and Systems 6(3), 035003 (16 September 2020).
Received: 15 April 2020; Accepted: 1 September 2020; Published: 16 September 2020

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