Closed-loop focal plane wavefront control with the SCExAO instrument

TL;DR

This paper presents a novel focal plane wavefront control method implemented on the SCExAO instrument, successfully correcting low-order aberrations on-sky, with broad applicability to various telescopic systems due to its low hardware impact.

Contribution

The paper introduces a Fourier-based focal plane wavefront sensor that operates in a closed-loop to correct non-common path errors, demonstrating on-sky success and broad adaptability.

Findings

Successfully controlled low-order modes on-sky at Subaru Telescope

The sensor operates within a specific error range and can bypass saturation issues

The technique is adaptable to various telescope types and configurations

Abstract

This article describes the implementation of a focal plane based wavefront control loop on the high-contrast imaging instrument SCExAO (Subaru Coronagraphic Extreme Adaptive Optics). The sensor relies on the Fourier analysis of conventional focal-plane images acquired after an asymmetric mask is introduced in the pupil of the instrument. This absolute sensor is used here in a closed-loop to compensate the non-common path errors that normally affects any imaging system relying on an upstream adaptive optics system.This specific implementation was used to control low order modes corresponding to eight zernike modes (from focus to spherical). This loop was successfully run on-sky at the Subaru Telescope and is used to offset the SCExAO deformable mirror shape used as a zero-point by the high-order wavefront sensor. The paper precises the range of errors this wavefront sensing approach can operate within and explores the impact of saturation of the data and how it can be bypassed, at a cost in performance. Beyond this application, because of its low hardware impact, APF-WFS can easily be ported in a wide variety of wavefront sensing contexts, for ground- as well space-borne telescopes, and for telescope pupils that can be continuous, segmented or even sparse. The technique is powerful because it measures the wavefront where it really matters, at the level of the science detector.