Asymmetries in adaptive optics point spread functions
Abstract
An explanation for the origin of asymmetry along the preferential axis of the point spread function (PSF) of an AO system is developed. When phase errors from highaltitude 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 openloop 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 toward 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 endtoend AO simulations.
 Publication:

Journal of Astronomical Telescopes, Instruments, and Systems
 Pub Date:
 October 2019
 DOI:
 10.1117/1.JATIS.5.4.049003
 arXiv:
 arXiv:1909.12981
 Bibcode:
 2019JATIS...5d9003M
 Keywords:

 Astrophysics  Instrumentation and Methods for Astrophysics;
 Astrophysics  Earth and Planetary Astrophysics
 EPrint:
 28 Pages, 13 Figures, Accepted to JATIS