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

Abstract

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 0.015λ/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.

Imaging low-mass planets within the habitable zone of α Centauri

“…NEAR explored an instrumental setup and observing strategy designed to push the capabilities of ground-based exoplanet imaging toward mid-infrared wavelengths of ~10 µm. The mid-infrared camera (VISIR: 25 ) on the Very Large Telescope (VLT) was upgraded for NEAR to implement several new technologies, such as a mid-infrared optimized annular groove phase mask (AGPM: 26 ) coronagraph 27 , 28 and shaped-pupil mask 29 , 30 to suppress the starlight. VISIR was moved to unit telescope 4 (UT4/Yepun) of the VLT, which is equipped with a deformable secondary mirror (DSM, 31 ).…”

“…The DSM was used for ∼8 Hz chopping to enable tracking and subtracting the systematic excess low-frequency noise (ELFN) within the Si:As Aquarius detector, which is a major limitation to the sensitivity of mid-infrared imaging 50 , 51 . Downstream of the DSM, the central starlight was reduced by an AGPM coronagraph optimized for performance at mid-infrared wavelengths 26 , 27 and a shaped-pupil mask 28 – 30 designed specifically to limit the spatial extent of the Airy pattern from the off-axis star. The Lyot stop is manufactured out of chromium directly deposited on the NEAR spectral filter, which transmits light from 10 to 12.5 μm.…”

METIS high-contrast imaging: design and expected performance

“…This corresponds to 0.02λ∕D PtV at the L-band. Our previous experience with QACITS at Keck/ NIRC2 58 and our first results obtained in the framework of the NEAR experiment on VLT/ VISIR (Maire et al) 59 show that the closed-loop pointing stability using feedback from the science camera can go down to about 0.01λ∕D rms (i.e., ∼0.025λ∕D PtV) using a simple integrator with a control frequency around 0.1 Hz. While this is slightly larger than the requirement to keep the METIS performance compliant with our goal, it must be noted that speckles associated with temporal frequencies higher than 0.001 Hz will average out to some level during the ADI postprocessing, and that the contribution of frequencies below 0.001 Hz to the total rms pointing error should be significantly smaller than 0.01λ∕D due to the integral control.…”

High-contrast imaging at ten microns: A search for exoplanets around Eps Indi A, Eps Eri, Tau Ceti, Sirius A, and Sirius B

“…The atmospheric data were taken from Paranal Astronomical Site Monitoring. To center the targets on the coronagraph and to minimize the leakage, a dedicated correction using the science images with the aid of the QACITS algorithm was used (for details see Maire et al 2020). We derived a pixel scale of 45.25 mas/pixel using the α Cen campaign data, utilizing the well known orbit of the binary from Kervella et al (2019).…”