Optical gains measurement with a gain scheduling camera: On-sky demonstration with PAPYRUS and perspectives

TL;DR

This work demonstrates on-sky validation of a focal-plane assisted PWFS (GSC) to estimate modal optical gains in real time, addressing non-linear PWFS behavior critical to extreme AO for exoplanet imaging. By coupling a PWFS with a gain-scheduling camera and leveraging a convolutional model of the PWFS response, the method estimates per-mode optical gains across the full modal basis at up to 100 Hz on the PAPYRUS AO bench, with strong agreement to high-fidelity simulations. The approach enables operating the PWFS away from the zero-point, improving sensitivity retention and facilitating NCPA compensation, with implications for future XAO systems like PCS. The study includes thorough hardware (FoV, binning, quantisation) and software (FFT-based optimization) considerations, and demonstrates robust, frame-by-frame optical-gain tracking in laboratory and on-sky conditions, marking a significant step toward real-time compensation of PWFS non-linearities in astronomy.

Abstract

Reaching the high angular resolution and contrast level desired for exoplanetary science requires us to equip large telescopes with extreme adaptive optics (XAO) systems to compensate for the effect of the atmospheric turbulence at a very fast rate. This calls for the development of ultra-sensitive wavefront sensors (WFSs), such as Fourier filtering wavefront sensors (FFWFSs), to be operated at low flux, as well as an increase in the XAO loop frame rate. These sensors, which constitute the baseline for current and future XAO systems, exhibit such a high sensitivity at the expense of a non-linear behaviour that must be properly calibrated and compensated for to deliver the required performance. We aim to validate on-sky a recently proposed method that associates the FFWFS with a focal plane detector, the gain scheduling camera (GSC), to estimate in real time the first-order terms of the sensor non-linearities, known as modal optical gains. We implemented a GSC on the adaptive-optics (AO) bench PAPYRUS to be associated with the existing pyramid wavefront sensor (PWFS). We compared experimental results to expected results obtained with a high-fidelity numerical twin of the AO system. We validated experimentally the method both in laboratory and on-sky. We demonstrated the capability of the GSC to accurately estimate the optical gains of the PWFS at 100 Hz, corresponding to the current limit in speed imposed by PAPYRUS hardware, but it could be applied at higher frequencies to enable frame-by-frame optical gains tracking. The presented results exhibit good agreement on the optical gains estimation with respect to numerical simulations reproducing the experimental conditions tested. Our experimental results validate the strategy of coupling a FFWFS with a focal-plane camera to master the non-linearities of the sensor. This demonstrates its attractiveness for future XAO application.

Figures (10)

• Figure 1: (a) Experimental focal-plane camera frame; (b) KL mode 80; (c) impulse response, in symmetrical logarithmic scale for visualisation purpose; (d) reconstructed PFWS response using the convolutional model; (e) experimental linear response, obtained through push-pull process; (f) 1D profile of one sub-pupil of experimental and simulated PWFS signal, with red lines indicating the edge effects due to the convolutional model.
• Figure 2: Experimental measurement of the optical gains using PAPYRUS internal source. The phase offset applied on the deformable mirror shown has an amplitude of 105 nm RMS.
• Figure 3: PAPYRUS AO test bed.
• Figure 4: Schematic of the focal-plane assisted PWFS: combination of a PWFS branch and a GSC branch providing, respectively, the differential intensities and an estimate of optical gains.
• Figure 5: Simulated relative error, $e$, on the OG measurement with respect to the FoV with a reference FoV corresponding to the one of the PWFS (15 arcsec), for seeings of 1.5 arcsec (top), 2.5 arcsec (middle), and 3.5 arcsec (bottom). The dashed black line represents the chosen FoV on PAPYRUS.