CLOSE: a self-regulating, best-performance tracker for modal integrator based AO loops
Résumé
The pyramid wavefront sensor (PWFS), due to its extremely high sensitivity in comparison to the Shack-Hartmann WFS, is the design choice of most single-conjugate adaptive optics (AO) instruments currently being developed for extremely large telescopes (ELTs). This sensitivity benefit is served, however, with several technical drawbacks to overcome, one of which is the intrinsic non-linearity of the sensor. Even in modulated operation, the gradient measurement saturation non-linearity is strongly exacerbated by in-loop phase residuals for typical on-sky regimes, with high spatial frequencies inducing a dramatic sensitivity reduction for the controlled modes. This phenomenon has been dubbed "optical gain", and it was demonstrated that a modal gain compensation on an appropriate control basis provides an adequate mitigation of the sensitivity reduction, and improves the end-to-end performance on ELT SCAO systems across all relevant guide star magnitudes and seeing conditions. Several techniques have been proposed to achieve this nominal performance recovery, however most require offline computation of interaction matrices; or some alterations to the usual AO loop operation. In this paper, we present the CLOSE (Correlation-Locked Optimization StratEgy) algorithm, which achieves determination of the modal gains, in a real-time fashion, through the sole use of the modal decomposition of the WFS measurements. Real-time estimators are implemented to implicitly measure resonance levels of the integrator transfer function, which is used as a proxy to control the modal gains through a second-layer servo loop. Using this method, we achieve compensation of the optical gain in the command law, and a fully automatic optimization of the modal integrator depending on the signal-to-noise level. Simulated end-to-end results are presented, for stationary or quickly varying seeing conditions, all at the scale of the MICADO SCAO design on the ELT. We also discuss the applicability of the CLOSE scheme on other known AO problems.
Origine : Fichiers produits par l'(les) auteur(s)