Coronagraphic phase diversity: performance study and laboratory demonstration

Abstract

Context. The final performance of current and future instruments dedicated to exoplanet detection and characterization (such as SPHERE on the European Very Large Telescope, GPI on Gemini North, or future instruments on Extremely Large Telescopes) is limited by uncorrected quasi-static aberrations. These aberrations create long-lived speckles in the scientific image plane, which can easily be mistaken for planets. Aims. Common adaptive optics systems require dedicated components to perform wave-front analysis. The ultimate wave-front measurement performance is thus limited by the unavoidable differential aberrations between the wave-front sensor and the scientific camera. To reach the level of detectivity required by high-contrast imaging, these differential aberrations must be estimated and compensated for. In this paper, we characterize and experimentally validate a wave-front sensing method that relies on focal-plane data. Methods. Our method, called COFFEE (for COronagraphic Focal-plane wave-Front Estimation for Exoplanet detection), is based on a Bayesian approach, and it consists in an extension of phase diversity to high-contrast imaging. It estimates the differential aberrations using only two focal-plane coronagraphic images recorded from the scientific camera itself. Results. We first present a thorough characterization of COFFEE's performance by means of numerical simulations. This characterization is then compared with an experimental validation of COFFEE using an in-house adaptive optics bench and an apodized Roddier & Roddier phase mask coronagraph. An excellent match between experimental results and the theoretical study is found. Lastly, we present a preliminary validation of COFFEE's ability to compensate for the aberrations upstream of a coronagraph.

High-order myopic coronagraphic phase diversity (COFFEE) for wave-front control in high-contrast imaging systems

“…As described in [15], the two main error sources of the first version of COFFEE have been shown both by simulations and experimentally to be aliasing and modelling error. The former was due to the use of a Zernike basis and prevented COFFEE from estimating high order aberrations.…”

“…Most of the notations of this article are coherent with [14,15]. We consider a coronagraphic imaging system made of four successive planes denoted by A (circular entrance pupil of diameter D u ), B (coronagraphic focal plane), C (Lyot Stop), and D (detector plane).…”

“…In a previous study [15], we presented COFFEE's early performance and limitations, detailed below, as well as its sensitivity to a realistic experimental environment. In this paper, we present a high order extension of COFFEE and its performance evaluation in a compensation process, in the framework of the quasi-static aberration calibration of a ground-based instrument such as SPHERE.…”

“…In this paper, we present a high order extension of COFFEE and its performance evaluation in a compensation process, in the framework of the quasi-static aberration calibration of a ground-based instrument such as SPHERE. Section 2 presents the modifications that allows COFFEE to overcome its previous limitations [15], allowing an estimation of high order aberrations with nanometric precision for any coronagraphic device. Section 3 presents the noise sensitivity of this extended version of COFFEE using realistic SPHERE-like simulations.…”

Compensation of high-order quasi-static aberrations on SPHERE with the coronagraphic phase diversity (COFFEE)

“…Section 4.1 presents the application of the pseudo-closed loop (PCL) process proposed in Paul et al (2013b) to SPHERE, which is then used in Sect. 4.2 to optimize the contrast on IRDIS.…”