Controlling petals using fringes: discontinuous wavefront sensing through sparse aperture interferometry at Subaru/SCExAO

TL;DR

This paper introduces a novel wavefront sensing method using sparse aperture interferometry at Subaru/SCExAO to effectively control petaling aberrations caused by telescope aperture discontinuities, improving astronomical imaging quality.

Contribution

The study repurposes SAM interferometers as low-order wavefront sensors to measure and control petaling aberrations, enhancing correction capabilities beyond traditional sensors.

Findings

Successfully measured petaling aberrations on-sky beyond single-wavelength limits.

Demonstrated effective petaling control with dual-band SAM mode.

Improved scientific throughput by reducing PSF core splitting.

Abstract

Low wind and petaling effects, caused by the discontinuous apertures of telescopes, are poorly corrected -- if at all -- by commonly used workhorse wavefront sensors (WFSs). Wavefront petaling breaks the coherence of the point spread function core, splitting it into several side lobes, dramatically shutting off scientific throughput. We demonstrate the re-purposing of non-redundant sparse aperture masking (SAM) interferometers into low-order WFSs complementing the high-order pyramid WFS, on the SCExAO experimental platform at Subaru Telescope. The SAM far-field interferograms formed from a 7-hole mask are used for direct retrieval of petaling aberrations, which are almost invisible to the main AO loop. We implement a visible light dual-band SAM mode, using two disjoint 25 nm wide channels, that we recombine to overcome the one-lambda ambiguity of fringe-tracking techniques. This enables a control over petaling with sufficient capture range yet without conflicting with coronagraphic modes in the near-infrared. We present on-sky engineering results demonstrating that the design is able to measure petaling well beyond the range of a single-wavelength equivalent design.