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Context. The wind-driven halo is a feature that is observed in images that were delivered by the latest generation of ground-based instruments that are equipped with an extreme adaptive optics system and a coronagraphic device, such as SPHERE at the Very Large Telescope (VLT). This signature appears when the atmospheric turbulence conditions vary faster than the adaptive optics loop can correct for. The wind-driven halo is observed as a radial extension of the point spread function along a distinct direction (this is sometimes referred to as the butterfly pattern). When this is present, it significantly limits the contrast capabilities of the instrument and prevents the extraction of signals at close separation or extended signals such as circumstellar disks. This limitation is consequential because it contaminates the data for a substantial fraction of the time: about 30% of the data produced by the VLT/SPHERE instrument are affected by the wind-driven halo. Aims. This paper reviews the causes of the wind-driven halo and presents a method for analyzing its contribution directly from the scientific images. Its effect on the raw contrast and on the final contrast after post-processing is demonstrated. Methods. We used simulations and on-sky SPHERE data to verify that the parameters extracted with our method can describe the wind-driven halo in the images. We studied the temporal, spatial, and spectral variation of these parameters to point out its deleterious effect on the final contrast. Results. The data-driven analysis we propose provides information to accurately describe the wind-driven halo contribution in the images. This analysis confirms that this is a fundamental limitation of the finally reached contrast performance. Conclusions. With the established procedure, we will analyze a large sample of data delivered by SPHERE in order to propose post-processing techniques that are tailored to removing the wind-driven halo.

The latest generation of high contrast instruments dedicated to exoplanets and circumstellar disks imaging are equipped with extreme adaptive optics and coronagraphs to reach contrasts of up to 10 −4 at a few tenths of arc-seconds in the near infrared. The resulting image shows faint features, only revealed with this combination, such as the wind driven halo. The wind driven halo is due to the lag between the adaptive optics correction and the turbulence speed over the telescope pupil. However we observe an asymmetry of this wind driven halo that was not expected when the instrument was designed. In this letter, we describe and demonstrate the physical origin of this asymmetry and support our explanation by simulating the asymmetry with an end-to-end approach. From this work, we found out that the observed asymmetry is explained by the interference between the AO-lag error and scintillation effects, mainly originating from the fast jet stream layer located at about 12 km in altitude. Now identified and interpreted, this effect can be taken into account for further design of high contrast imaging simulators, next generation or upgrade of high contrast instruments, predictive control algorithms for adaptive optics or image post-processing techniques.

The design of liquid-crystal diffractive phase plate coronagraphs for groundbased and space-based high-contrast imaging systems is limited by the trade-off between spectral bandwidth and polarization leakage. We demonstrate that by combining phase patterns with a polarization grating (PG) pattern directly followed by one or several separate PGs, we can suppress the polarization leakage terms by additional orders of magnitude by diffracting them out of the beam. Using two PGs composed of a single-layer liquid crystal structure in the lab, we demonstrate a leakage suppression of more than an order of magnitude over a bandwidth of 133 nm centered around 532 nm. At this center wavelength we measure a leakage suppression of three orders of magnitude. Furthermore, simulations indicate that a combination of two multi-layered liquid-crystal PGs can suppress leakage to < 10 −5 for 1-2.5 µm and < 10 −10 for 650-800 nm. We introduce multi-grating solutions with three or more gratings that can be designed to have no separation of the two circular polarization states, and offer even deeper suppression of polarization leakage. We present simulations of a triple-grating solution that has < 10 −10 leakage on the first Airy ring from 450 nm to 800 nm. We apply the double-grating concept to the Vector-Vortex coronagraph of charge 4, and demonstrate in the lab that polarization leakage no longer limits the on-axis suppression for ground-based contrast levels. Lastly, we report on the successful installation and first-light results of a double-grating vector Apodizing Phase Plate pupil-plane coronagraph installed at the Large Binocular Telescope. We discuss the implications of these new coronagraph architectures for high-contrast imaging systems on the ground and in space.

Context. The recent discovery of an Earth-mass exoplanet around the nearby star Proxima Centauri provides a prime target for the search for life on planets outside our solar system. Atmospheric characterization of these planets has been proposed by blocking the starlight with a stellar coronagraph and using a high-resolution spectrograph to search for reflected starlight off the planet. Aims. Due to the large flux ratio and small angular separation between Proxima b and its host star (≲10−7 and ≲2.2λ/D respectively; at 750 nm for an 8 m-class telescope) the coronagraph requires high starlight suppression at extremely-low inner working angles. Additionally, it must operate over a broad spectral bandwidth and under residual telescope vibrations. This allows for efficient use of spectroscopic post-processing techniques. We aim to find the global optimum of an integrated coronagraphic integral-field spectrograph. Methods. We present the Single-mode Complex Amplitude Refinement (SCAR) coronagraph that uses a microlens-fed single-mode fiber array in the focal plane downstream from a pupil-plane phase plate. The mode-filtering property of the single-mode fibers allows for the nulling of starlight on the fibers. The phase pattern in the pupil plane is specifically designed to take advantage of this mode-filtering capability. Second-order nulling on the fibers expands the spectral bandwidth and decreases the tip-tilt sensitivity of the coronagraph. Results. The SCAR coronagraph has a low inner working angle (∼1λ/D) at a contrast of < 3 × 10−5 for the six fibers surrounding the star using a sufficiently-good adaptive optics system. It can operate over broad spectral bandwidths (∼20%) and delivers high throughput (> 50% including fiber injection losses). Additionally, it is robust against tip-tilt errors (∼0.1λ/D rms). We present SCAR designs for both an unobstructed and a VLT-like pupil. Conclusions. The SCAR coronagraph is a promising candidate for exoplanet detection and characterization around nearby stars using current high-resolution imaging instruments.