Status of predictive wavefront control on Keck II adaptive optics bench: on-sky coronagraphic results

TL;DR

This paper reports on the implementation and testing of a predictive wavefront control algorithm on the Keck II adaptive optics system, demonstrating significant contrast improvements in high contrast imaging.

Contribution

The paper introduces a predictive wavefront control algorithm that reduces residual wavefront errors and improves contrast in ground-based high contrast imaging, with on-sky validation at Keck II.

Findings

Residual wavefront error reduced compared to previous methods.

Contrast improved by a factor of 2.03 at 3 λ/D.

Contrast gains up to 3 at larger separations (4-6 λ/D).

Abstract

The behavior of an adaptive optics (AO) system for ground-based high contrast imaging (HCI) dictates the achievable contrast of the instrument. In conditions where the coherence time of the atmosphere is short compared to the speed of the AO system, the servo-lag error becomes the dominate error term of the AO system. While the AO system measures the wavefront error and subsequently applies a correction (taking a total of 1 to 2 milli-seconds), the atmospheric turbulence above the telescope has changed. In addition to reducing the Strehl ratio, the servo-lag error causes a build-up of speckles along the direction of the dominant wind vector in the coronagraphic image, severely limiting the contrast at small angular separations. One strategy to mitigate this problem is to predict the evolution of the turbulence over the delay. Our predictive wavefront control algorithm minimizes the delay in a mean square sense and has been implemented on the Keck II AO bench. In this paper we report on the latest results of our algorithm and discuss updates to the algorithm itself. We explore how to tune various filter parameters on the basis of both daytime laboratory tests and on-sky tests. We show a reduction in residual-mean-square wavefront error for the predictor compare to the leaky integrator implemented on Keck. Finally, we present contrast improvements for both day time and on-sky tests. Using the L-band vortex coronagraph for Keck's NIRC2 instrument, we find a contrast gain of 2.03 at separation of 3~$\lambda/D$ and up to 3 for larger separations (4-6~$\lambda/D$).