Imaging low-mass planets within the habitable zone of α Centauri.

Giant exoplanets on wide orbits have been directly imaged around young stars. If the thermal background in the mid-infrared can be mitigated, then exoplanets with lower masses can also be imaged. Here we present a ground-based mid-infrared observing approach that enables imaging low-mass temperate exoplanets around nearby stars, and in particular within the closest stellar system, α Centauri. Based on 75-80% of the best quality images from 100 h of cumulative observations, we demonstrate sensitivity to warm sub-Neptune-sized planets throughout much of the habitable zone of α Centauri A. This is an order of magnitude more sensitive than state-of-the-art exoplanet imaging mass detection limits. We also discuss a possible exoplanet or exozodiacal disk detection around α Centauri A. However, an instrumental artifact of unknown origin cannot be ruled out. These results demonstrate the feasibility of imaging rocky habitable-zone exoplanets with current and upcoming telescopes.

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

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).

High-order adaptive optics requirements for direct detection of extrasolar planets: Application to the SPHERE instrument.

The detection of extrasolar planets implies an extremely high-contrast, long-exposure imaging capability at near infrared and probably visible wavelengths. We present here the core of any Planet Finder instrument, that is, the extreme adaptive optics (XAO) subsystem. The level of AO correction directly impacts the exposure time required for planet detection. In addition, the capacity of the AO system to calibrate all the instrument static defects ultimately limits detectivity. Hence, the extreme AO system has to adjust for the perturbations induced by the atmospheric turbulence, as well as for the internal aberrations of the instrument itself. We propose a feasibility study for an extreme AO system in the frame of the SPHERE (Spectro-Polarimetry High-contrast Exoplanet Research) instrument, which is currently under design and should equip one of the four VLT 8-m telescopes in 2010.

Surface characterization of micro-optical components by Foucault's knife-edge method: the case of a micromirror array.

As micro-optical components are introduced into optical systems, accurate surface characterization becomes important. We describe a method for quantitative evaluation of surface deformations based on Foucault's knife-edge test. By measurement of local slopes, the surface shape of each mirror in a micromirror array has been reconstructed with a subnanometer accuracy. In addition to low-order deformation (tilt, curvature, astigmatism), each mirror is seen to be palm-tree shaped. This may be explained by strain relaxation in the fabrication process. Measurement on a conventional concave mirror confirms our method.

Illustration of the use of multimode deformable plane mirrors to record high-resolution concave gratings: results for the Cosmic Origins Spectrograph gratings of the Hubble Space Telescope.

To illustrate the efficiency of using a deformable plane mirror to record holographic gratings, we have computed the three gratings for the Cosmic Origins Spectrograph. Their working conditions are severe, since they have to correct the residual spherical aberration of the Hubble Space Telescope. Nevertheless, all images obtained are largely diffraction limited with regard to the resolution.