On-sky demonstration of a vector Zernike wavefront sensor in a cascaded adaptive optics system

Abstract

Future high-contrast instruments aim to directly image and characterise Earth-like exoplanets. Achieving the required contrast requires exquisite image quality, and in particular a high-performance correction of the optical aberrations. Hence, future instruments will use adaptive optics (AO) systems operating at increasingly high loop frequencies in order to reduce temporal errors. However, increasing the loop frequency decreases the number of photons available per wavefront-sensor frame, motivating the use of highly sensitive wavefront sensors. Among the available concepts, the Zernike wavefront sensor (ZWFS) approaches the theoretical sensitivity limit of Fourier-filtering wavefront sensors. To extend the limited dynamic range of the classical ZWFS, the vector ZWFS (v-ZWFS) has been proposed as an alternative solution to mitigate this limitation. Although ZWFSs have already been employed for non-common-path aberration compensation and segment phasing, they have not yet been demonstrated in an on-sky closed-loop AO system. We aim to demonstrate the operation as well as the performance of a ZWFS in an on-sky AO loop and to evaluate its potential as a second-stage wavefront sensor in a cascaded AO architecture. We implemented a second-stage AO system, named OZIRIIS, on the PAPYRUS AO platform installed on the 1.52 m telescope of the Observatoire de Haute-Provence. OZIRIIS combines a v-ZWFS and a 97-actuator deformable mirror operating at 400 Hz downstream of the first-stage, pyramid-based AO system. We used one single ZWFS signal for real-time control. The full v-ZWFS was only used a posteriori to analyse the telemetry. We developed a complete calibration strategy based on a numerical model of the sensor to generate synthetic reference signals and interaction matrices for both laboratory and on-sky operation. The performance of the system was assessed through point spread function (PSF) image analysis and wavefront sensor telemetry and was then compared with simulations using a numerical twin of the bench. We present the on-sky demonstration of a v-ZWFS operating within an AO system, together with on-sky closed-loop results obtained using one of the ZWFS signals for real-time control. The second-stage correction provides an improvement in the measured Strehl ratio (SR) of up to 16 percentage points during the observing night presented here. The analysis of the telemetry using the full v-ZWFS to reconstruct residuals reveals the optical-gain effects affecting the ZWFS at a low SR. This demonstrates that the v-ZWFS can improve the dynamic range and performance of the ZWFS in regimes with larger aberrations. We show that an accurate model of the sensor enables the generation of synthetic reference signals and interaction matrices suitable for on-sky operation. We further show that the observed behaviour is consistent with numerical simulations and that the impact of optical gain can be quantified directly from the telemetry.