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Published December 2019 | Accepted Version + Published
Journal Article Open

Focal-plane wavefront sensing with the vector-Apodizing Phase Plate


Context. One of the key limitations of the direct imaging of exoplanets at small angular separations are quasi-static speckles that originate from evolving non-common path aberrations (NCPA) in the optical train downstream of the instrument's main wavefront sensor split-off. Aims. In this article we show that the vector-Apodizing Phase Plate (vAPP) coronagraph can be designed such that the coronagraphic point spread functions (PSFs) can act as wavefront sensors to measure and correct the (quasi-)static aberrations without dedicated wavefront sensing holograms or modulation by the deformable mirror. The absolute wavefront retrieval is performed with a nonlinear algorithm. Methods. The focal-plane wavefront sensing (FPWFS) performance of the vAPP and the algorithm are evaluated via numerical simulations to test various photon and read noise levels, the sensitivity to the 100 lowest Zernike modes, and the maximum wavefront error (WFE) that can be accurately estimated in one iteration. We apply these methods to the vAPP within SCExAO, first with the internal source and subsequently on-sky. Results. In idealized simulations we show that for 107 photons the root mean square (RMS) WFE can be reduced to ~ λ/1000, which is 1 nm RMS in the context of the SCExAO system. We find that the maximum WFE that can be corrected in one iteration is ~ λ/8 RMS or ~200 nm RMS (SCExAO). Furthermore, we demonstrate the SCExAO vAPP capabilities by measuring and controlling the 30 lowest Zernike modes with the internal source and on-sky. On-sky, we report a raw contrast improvement of a factor ~2 between 2 and 4 λ/D after five iterations of closed-loop correction. When artificially introducing 150 nm RMS WFE, the algorithm corrects it within five iterations of closed-loop operation. Conclusions. FPWFS with the vAPP coronagraphic PSFs is a powerful technique since it integrates coronagraphy and wavefront sensing, eliminating the need for additional probes and thus resulting in a 100% science duty cycle and maximum throughput for the target.

Additional Information

© 2019 ESO. Received: 11 June 2019; Accepted: 10 September 2019. Published online 26 November 2019. The authors thank the referee for comments on the manuscript that significantly improved the presentation of the work. The authors warmly thank S.Y. Haffert and E.H. Por for useful discussion on focal-plane wavefront sensing. The research of Steven P. Bos, David S. Doelman, and Frans Snik leading to these results has received funding from the European Research Council under ERC Starting Grant agreement 678194 (FALCONER). The development of SCExAO was supported by the JSPS (Grant-in-Aid for Research #23340051, #26220704, #23103002); the Astrobiology Center (ABC) of the National Institutes of Natural Sciences, Japan; the Mt Cuba Foundation; and the directors contingency fund at Subaru Telescope. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This research made use of HCIPy, an open-source object-oriented framework written in Python for performing end-to-end simulations of high-contrast imaging instruments (Por et al. 2018). This research used the following Python libraries: Scipy (Jones et al. 2014), Numpy (Walt et al. 2011), and Matplotlib (Hunter 2007).

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Published - aa36062-19.pdf

Accepted Version - 1909.08317.pdf


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August 19, 2023
October 18, 2023