Asymmetries in adaptive optics point spread functions

TL;DR

This paper develops an analytic model explaining the origin of asymmetries in adaptive optics PSFs caused by high-altitude turbulence scintillation and wavefront errors, validated with observations and simulations.

Contribution

It introduces a novel analytic model linking PSF asymmetries to turbulence altitude, wind speed, and AO system parameters, enhancing understanding of AO image distortions.

Findings

Asymmetry strongest with slow wind near the coronagraphic mask

Brighter PSF lobe points opposite to wind direction

Model validated with Gemini observations and AO simulations

Abstract

An explanation for the origin of asymmetry along the preferential axis of the PSF of an AO system is developed. When phase errors from high altitude turbulence scintillate due to Fresnel propagation, wavefront amplitude errors may be spatially offset from residual phase errors. These correlated errors appear as asymmetry in the image plane under the Fraunhofer condition. In an analytic model with an open-loop AO system, the strength of the asymmetry is calculated for a single mode of phase aberration, which generalizes to two dimensions under a Fourier decomposition of the complex illumination. Other parameters included are the spatial offset of the AO correction, which is the wind velocity in the frozen flow regime multiplied by the effective AO time delay, and propagation distance or altitude of the turbulent layer. In this model, the asymmetry is strongest when the wind is slow and nearest to the coronagraphic mask when the turbulent layer is far away, such as when the telescope is pointing low towards the horizon. A great emphasis is made about the fact that the brighter asymmetric lobe of the PSF points in the opposite direction as the wind, which is consistent analytically with the clarification that the image plane electric field distribution is actually the inverse Fourier transform of the aperture plane. Validation of this understanding is made with observations taken from the Gemini Planet Imager, as well as being reproducible in end-to-end AO simulations.