Speckle control

Contrast limits for the detection of extrasolar planets currently are set by the presence of static and slow-varying aberrations in the optical path that leads to the science instrument. These aberrations, due to the non-common path error between the wavefront sensor and the science camera are responsible for the presence of long lasting speckles in the image. Because the planets one tries to detect are unresolved sources, they cannot be distinguished from these speckles. One family of techniques, called differential imaging, actually manages to calibrate out these static aberrations, by using sky rotation (Angular Differential Imaging, or ADI), polarization (PDI) or wavelength dependence of the speckles (Spectral Differential Imaging or SDI). Of these, ADI seems very well adapted to the problem of the detection of extrasolar planets, and has been successful, most notably the image of the planetary system around HR 8799 (Marois et al, 2008). ADI uses the rotation of the sky that naturally happens while tracking with an alt-azimuthal telescope around transit. The position of static and slowly varying speckles, tied to the diffraction by the pupil remains stable over long timescales, while the image of planetary companions will rotate around the one of the host star.

The rotation of the field only leads to sufficient linear displacement for orbital separations of the order of one arcsecond. And in practice, below 0.5 arc second, the performance of ADI quickly degrades. One way to complement ADI toward small angular separation, is to use a deformable mirror (DM) to modulate speckles and introduce the diversity that will distinguish them from genuine structures like planets and lumps in disks. This type of technique is regularly used for high contrast experiments (Guyon et al, 2010; ...), and appear as the technique of choice for a space borne mission dedicated to the direct imaging of high contrast planets.

Close-loop systems relying on an acute knowledge of the complex amplitude response matrix of the system exist. Such a close loop system can produce raw images with high contrast performance, but only operates well in the controlled laboratory environment. Nevertheless, post-processing of the data acquired during close-loop experiments, used in concert with the knowledge of the wavefront corrections applied by the DM can push contrast limits up to two orders of magnitude.

The latter approach is actually quite robust, and appears applicable beyond the pampered environment of the laboratory. We demonstrate here its applicability in the presence of atmospheric turbulence and partial adaptive optics (AO) correction.