Proceedings Article | 28 July 2014
KEYWORDS: Coronagraphy, Sensors, Optical spheres, Imaging systems, Calibration, Planets, Spatial light modulators, Cameras, Exoplanetary science, Image acquisition
The final performance of current and future instruments dedicated to exoplanet detection and characterization (such as
SPHERE on the VLT, GPI on Gemini North or future instruments on the ELTs) is limited by intensity residuals in the
scientific image plane, which originate in uncorrected optical aberrations. After correction of the atmospheric turbulence,
the main contribution to these residuals comes from the quasi-static aberrations introduced upstream of the coronagraph
which create long-lived speckles in the detector plane that can easily be mistaken for a planet. In order to reach very high
contrast such as the ones required to image earth-like planets, these aberrations needs to be compensated for. We have
recently proposed a dedicated focal-plane wave-font sensor called COFFEE (for COronagraphic Focal-plane wave-Front
Estimation for Exoplanet detection), which consists in an extension of conventional phase diversity to a coronagraphic
system: aberrations both upstream and downstream of the coronagraph are estimated using two coronagraphic focal-plane
images, recorded from the scientific camera itself, without any differential aberration. Such a system has been successfully
validated on the SPHERE instrument, where COFFEEs estimation has been used to compensate for the phase aberration
upstream of the coronagraph, leading to a contrast optimization in the whole focal plane area controlled by the AO loop.
If compensating for phase aberrations only was acceptable to reach levels of contrast of 106, it will no longer be the case
for instruments that aim at imaging earth-like planets. Such targets, which would be the ones of a planet-finder instrument
integrated on an ELT, require a level of contrast better than 109. To reach this level, neglecting amplitude aberrations
(inhomogeneous intensity in the pupil, Fresnel propagation effect) is no longer possible. In this communication, we
present an extension of COFFEE able to perform a simultaneous estimation of both phase and amplitude aberration from
three focal plane images. After a theoretical presentation, we present a study of its performances. Notably, we analyze
the contrast that can be achieved in a compensation process when this estimation method is combined with our non-linear
dark hole method, demonstrating that the nanometric precision estimation that can be achieved with COFFEE allow one
to reach very high contrast levels. It is worth mentioning that both estimation (COFFEE) and compensation (the nonlinear
dark hole) methods are model based, and thus easily adaptable to a broad class of coronagraphic device. Lastly, we
validate our complex field estimator on the LAM (Laboratoire dAstrophysique de Marseille) XAO test bench, described
in this communication. We introduce calibrated phase and amplitude aberration in the entrance pupil plane. Then, we
demonstrate the ability of our extended version of COFFEE to estimate both phase and amplitude aberration from three
coronagraphic focal plane images that differs from a known aberration.
