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 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 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 end-to-end AO simulations.
Highlights
The advancement of adaptive optics (AO) as a technology has enabled significant progress in astrophysics
The analysis in this paper expands on our previous work,[7] which demonstrated the origin of azimuthal asymmetry in the point spread function (PSF) as a consequence of the time lag error, to explore asymmetry along the preferential axis introduced by scintillation
Scintillation has previously been identified as a performance limiting factor in high-contrast imaging,[16] and here we have demonstrated the severity of this effect, visible in the form of PSF asymmetry
Summary
The advancement of adaptive optics (AO) as a technology has enabled significant progress in astrophysics. Spectral measurements of individual giant planets constrain evolutionary and atmospheric models[4] of these objects, paving the way toward characterization of extrasolar terrestrial planets For this generation of instruments and the understanding the point spread function (PSF) of AO instruments on giant telescopes will be important for the development of algorithms optimized in the search for planets.[5,6] The analysis in this paper expands on our previous work,[7] which demonstrated the origin of azimuthal asymmetry in the PSF as a consequence of the time lag error, to explore asymmetry along the preferential axis introduced by scintillation. This paper concludes with a brief discussion about the importance of this effect in the context of improving AO systems performance from design to postprocessing
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