Abstract
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 non-linear 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.
Highlights
The exploration of circumstellar environments at small angular separations by means of direct imaging is crucial for the detection and characterization of exoplanets
The bottom row shows the result of a closedloop correction where we introduced and estimated a 300 nm rms wavefront error (WFE) with 50 Zernike modes, which should be correctable according to Fig. 8
The physical model for non-linear wavefront estimation developed in Sect. 3 was tested in idealized simulations (Sect. 4), confirming that the vector-Apodizing Phase Plate (vAPP) currently installed at Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) can sense the even modes, but is more sensitive to odd modes
Summary
The exploration of circumstellar environments at small angular separations by means of direct imaging is crucial for the detection and characterization of exoplanets. It operates by placing a small hole in the pupilplane Lyot stop outside of the geometric pupil where the scattered starlight is located This hole creates a reference beam that generates high spatial frequency fringes in the focal-plane image, which can be used to determine the full electric field. The authors overcame this problem by using the vAPP (Miller et al 2018), where the bright field of one coronagraphic PSF covers the dark hole of the other This has been shown to work in the laboratory (Miller 2018), but there is still a significant null space for most vAPP designs; it is insensitive to the even pupil phase modes
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