On-sky demonstration of low-order wavefront sensing and control with focal plane phase mask coronagraphs

TL;DR

This paper demonstrates a low-order wavefront sensing technique using a Lyot-based sensor with phase mask coronagraphs, achieving precise pointing control on the Subaru Telescope for improved exoplanet imaging.

Contribution

The paper introduces and validates a Lyot-based low-order wavefront sensor (LLOWFS) for phase mask coronagraphs, enabling real-time correction of low-order aberrations both in laboratory and on-sky conditions.

Findings

Achieved 0.02 mas pointing residual in laboratory tests.

Achieved 0.15 mas pointing residual on sky.

Validated correction of non-common path errors with 0.23 mas residual.

Abstract

The ability to characterize exoplanets by spectroscopy of their atmospheres requires direct imaging techniques to isolate planet signal from the bright stellar glare. One of the limitations with the direct detection of exoplanets, either with ground- or space-based coronagraphs, is pointing errors and other low-order wavefront aberrations. The coronagraphic detection sensitivity at the diffraction limit therefore depends on how well low-order aberrations upstream of the focal plane mask are corrected. To prevent starlight leakage at the inner working angle of a phase mask coronagraph, we have introduced a Lyot-based low-order wavefront sensor (LLOWFS), which senses aberrations using the rejected starlight diffracted at the Lyot plane. In this paper, we present the implementation, testing and results of LLOWFS on the Subaru Coronagraphic Extreme Adaptive Optics system (SCExAO) at the Subaru Telescope. We have controlled thirty-five Zernike modes of a H-band vector vortex coronagraph in the laboratory and ten Zernike modes on sky with an integrator control law. We demonstrated a closed-loop pointing residual of 0.02 mas in the laboratory and 0.15 mas on sky for data sampled using the minimal 2-second exposure time of the science camera. We have also integrated the LLOWFS in the visible high-order control loop of SCExAO, which in closed-loop operation has validated the correction of the non-common path pointing errors between the infrared science channel and the visible wavefront sensing channel with pointing residual of 0.23 mas on sky.