Vector speckle grid: instantaneous incoherent speckle grid for high-precision astrometry and photometry in high-contrast imaging

TL;DR

This paper introduces vector speckle grids (VSGs), a novel method that uses polarization modulation to create incoherent speckle references, significantly enhancing the precision of high-contrast imaging for exoplanet studies.

Contribution

The paper proposes and analytically demonstrates the effectiveness of VSGs, which improve speckle grid incoherence and measurement precision in high-contrast imaging.

Findings

VSGs are completely incoherent for unpolarized light.

Simulation shows VSGs reduce photometric error to 0.3-0.8%.

Astrometric error is reduced to 3-10×10^{-3} λ/D, a 5-fold improvement.

Abstract

Photometric and astrometric monitoring of directly imaged exoplanets will deliver unique insights into their rotational periods, the distribution of cloud structures, weather, and orbital parameters. As the host star is occulted by the coronagraph, a speckle grid (SG) is introduced to serve as astrometric and photometric reference. Speckle grids are implemented as diffractive pupil-plane optics that generate artificial speckles at known location and brightness. Their performance is limited by the underlying speckle halo caused by evolving uncorrected wavefront errors. The speckle halo will interfere with the coherent SGs, affecting their photometric and astrometric precision. Our aim is to show that by imposing opposite amplitude or phase modulation on the opposite polarization states, a SG can be instantaneously incoherent with the underlying halo, greatly increasing the precision. We refer to these as vector speckle grids (VSGs). We derive analytically the mechanism by which the incoherency arises and explore the performance gain in idealised simulations under various atmospheric conditions. We show that the VSG is completely incoherent for unpolarized light and that the fundamental limiting factor is the cross-talk between the speckles in the grid. In simulation, we find that for short-exposure images the VSG reaches a $\sim$0.3-0.8\% photometric error and $\sim$$3-10\cdot10^{-3}$ $\lambda/D$ astrometric error, which is a performance increase of a factor $\sim$20 and $\sim$5, respectively. Furthermore, we outline how VSGs could be implemented using liquid-crystal technology to impose the geometric phase on the circular polarization states. The VSG is a promising new method for generating a photometric and astrometric reference SG that has a greatly increased astrometric and photometric precision.