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

2020; SPIE; Volume: 6; Issue: 03 Linguagem: Inglês

10.1117/1.jatis.6.3.035003

2329-4221

A.-L. Maire, Elsa Huby, Olivier Absil, G. Zins, M. Kasper, Christian Delacroix, Serban Leveratto, Mikael Karlsson, Garreth Ruane, H. U. Käufl, Gilles Orban de Xivry, Prashant Pathak, Lorenzo Pettazzi, Philippe Duhoux, J. Kolb, É. Pantin, A. J. Eldorado Riggs, R. Siebenmorgen, Dimitri Mawet,

Calibration and Measurement Techniques

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.