The Asymmetric Pupil Fourier Wavefront Sensor

Abstract

ABSTRACT. This article introduces a novel wavefront sensing approach that relies on the Fourier analysis of a single conventional direct image. In the high Strehl ratio regime, the relation between the phase measured in the Fourier plane and the wavefront errors in the pupil can be linearized, as was shown in a previous work that introduced the notion of generalized closure-phase, or kernel-phase. The technique, to be usable as presented requires two conditions to be met: (1) the wavefront errors must be kept small (of the order of one radian or less), and (2) the pupil must include some asymmetry, which can be introduced with a mask, for the problem to become solvable. Simulations show that this asymmetric pupil Fourier wavefront sensing or APF-WFS technique can improve the Strehl ratio from 50% to over 90% in just a few iterations, with excellent photon noise sensitivity properties, suggesting that on-sky close loop APF-WFS is possible with an extreme adaptive optics system.Online material: color figures NON-COMMON PATH ERROR AND EXTREME AOContrast limits for the direct imaging of extrasolar planets from ground based adaptive optics (AO) observations are currently set by the presence of static and slow-varying aberrations in the optical path that leads to the science instrument (Marois et al. 2003). Because some of these aberrations are not sensed by the wavefront sensor, something called the non-common path error, they are responsible for the presence of long lasting speckles in the image. Since extrasolar planets are faint unresolved sources, it is impossible to discriminate them among these speckles in one single frame. The family of differential imaging techniques is aimed at calibrating out some of these static aberrations, in postprocessing by using either sky rotation (angular differential imaging, or ADI), polarization differential imaging (PDI), or wavelength dependence of the speckles (spectral differential imaging or SDI). Of these, ADI (Marois et al. 2006) has been successful in most notably producing the image of the planetary system around HR 8799 (Marois et al. 2008).A new generation of high-contrast instruments using an approach of improved wavefront control called extreme adaptive optics (XAO), aims at producing higher contrast raw images, by including additional high-density wavefront control devices, fed by advanced wavefront sensing techniques to actively control the wavefront during the observations. In all cases, the primary goal of the high-order deformable mirror is to calibrate the non-common path error between the conventional AO system and the science camera, and then to try and help creating a high-contrast region in the image, by actively modulating the speckles in the field, for instance using speckle nulling .The control of the non-common path error is an important element of any XAO system, which requires the implementation of dedicated hardware or special acquisition procedures: the Gemini Planet Imager (Wallace et al. 2010) and the Project P1640 (Zhai et al. 2012) for instance...

Closed-loop focal plane wavefront control with the SCExAO instrument

“…Following on from the conceptual study proposed by Martinache (2013), this paper described the implementation of the asymmetric pupil Fourier wavefront sensor as one of the wavefront control loops of the SCExAO instrument. This approach has proven capable of repeatedly accounting for the non-common path error that affects the instrument after a new telescope pointing and provides an updated zero-point for the upstream pyramid wavefront sensor currently implemented inside SCExAO.…”

“…When observing a point source, for which Φ 0 = 0 (or if the object is known), this relation can be inverted if one introduces an asymmetry in the pupil (Martinache 2013). A direct focal plane image, with only a small amount of additional diffraction generated by the pupil asymmetry (see Fig.…”

“…The geometry of the asymmetric feature of the mask used for this work is not the result of an optimization and simply follows the shape used in the concept paper of Martinache (2013). A smaller asymmetric feature would be expected to result in a lower sensitivy but a systematic study of the sensitivity impact of the geometry of the asymmetry has yet to be done.…”

“…The case featured in Martinache (2013) was somewhat idealized. Working on monochromatic images, and with a perfect DM, it was able to exactly generate the wavefront correction determined by the analysis.…”

“…While this problem is generally degenerate, one way to break this degeneracy proved to be simple, and involved masking a small but non-negligible fraction of the pupil to introduce some level of asymmetry. The principles of this asymmetric pupil Fourier wavefront sensor (APF-WFS) were described by Martinache (2013), and exploited by Pope et al (2014) to show how it could be used, for instance, to cophase a segmented mirror. This paper further expands on the possible applications of this wavefront sensor, since it has now been implemented as part of the SCExAO instrument (Jovanovic et al 2015b), A&A 593, A33 (2016) Fig.…”