Vortex coronagraph: revisiting the phase retrieval properties via Zernike analysis

TL;DR

This paper revisits the phase retrieval properties of vortex coronagraphs using Zernike polynomial analysis, providing a second-order expansion that enhances understanding and suggests practical wavefront sensing applications.

Contribution

It introduces a second-order Zernike-based formalism to analyze vortex phase mask behavior, improving phase retrieval understanding and enabling unambiguous focal-plane wavefront sensing.

Findings

Vortex phase masks modify phase retrieval properties compared to normal imaging.

Scalar vortex coronagraph images can be used for unambiguous wavefront sensing.

The formalism aligns well with numerical simulations and informs practical instrument design.

Abstract

High contrast imaging (HCI) is fundamentally limited by wavefront aberrations, and the ability to perform wavefront sensing from focal plane images is key to reach the full potential of ground and space-based instruments. Vortex focal plane mask coupled with downstream pupil (Lyot) stop stands as one of the best small-angle coronagraphs, but is also sensitive to low-order aberrations. Here, we revisit the behavior of the vortex phase mask, from entrance pupil down to the final detector plane, with Zernike polynomials as input phase aberrations. In particular we develop a second-order expansion that allows us to analyze the phase retrieval properties in a more intuitive and accurate way than previously proposed. With this formalism, we show how the azimuthal vortex modulation modifies the phase retrieval properties compared to normal imaging. In particular, our results suggest that images obtained with a scalar vortex coronagraph can be used for unambiguous focal-plane wavefront sensing in any practical situation. We compare our results with numerical simulations and discuss practical implementation in coronagraphic instruments.