Abstract
The estimation and compensation of quasi-static aberrations is mandatory to reach the ultimate performance of high-contrast imaging systems. COFFEE is a focal plane wave-front sensing method that consists in the extension of phase diversity to high-contrast imaging systems. Based on a Bayesian approach, it estimates the quasi-static aberrations from two focal plane images recorded from the scientific camera itself. In this paper, we present COFFEE's extension which allows an estimation of low and high order aberrations with nanometric precision for any coronagraphic device. The performance is evaluated by realistic simulations, performed in the SPHERE instrument framework. We develop a myopic estimation that allows us to take into account an imperfect knowledge on the used diversity phase. Lastly, we evaluate COFFEE's performance in a compensation process, to optimize the contrast on the detector, and show it allows one to reach the 10(-6) contrast required by SPHERE at a few resolution elements from the star. Notably, we present a non-linear energy minimization method which can be used to reach very high contrast levels (better than 10(7) in a SPHERE-like context).
Highlights
Exoplanet imaging is one of the most challenging areas in today’s astronomy
Unlike other energy minimization methods [8,9,10,11,12], the one we propose does not rely on the calibration of an interaction matrix, which is sensitive to the position of the coronagraphic image on the detector
The electric field in the detector plane ΨD is obtained by propagating ΨA through each plane of the coronagraphic imaging system: the signal is first focused on the coronagraphic focal plane mask M ; the electric field is propagated trough the Lyot Stop pupil Pd(r) (Pd(r) = Π (2r/Dd) with Dd the Lyot Stop pupil diameter)
Summary
Exoplanet imaging is one of the most challenging areas in today’s astronomy. The observation of an extremely faint object (the planet) very close to a bright source (the host star) requires the use of an extreme adaptive optics (XAO) system coupled with a high-contrast imaging technique such as coronagraphy. We note that this approach, based on the analysis of fringed speckles, requires a 2 oversampling of the coronagraphic images to properly sample the interference fringes These techniques aim at minimizing the energy in a chosen area (“Dark Hole”), leading to a contrast optimization on the detector in a closed loop process. In the SPHERE baseline design, quasi-static aberrations are measured with conventional phase diversity [6] (no coronagraph), which is unable to sense high-order aberrations to a nanometric level As a result, this high contrast imaging instrument performance will be limited by high-order phase aberrations and not by amplitude aberrations (amplitude variations in a pupil plane). The latter are not considered in the simulations presented and are currently not estimated by COFFEE
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