Speckle noise reduction techniques for high-dynamic range imaging

High-dynamic range imaging from space in the visible light range, aiming, in particular, at the detection of terrestrial exoplanets, necessitates not only the use of a coronagraph, but also of adaptive optics to correct optical defects in real time. Indeed, these defects scatter light and give birth to speckles in the image plane. Speckles can be cancelled by driving a deformable mirror to measure and compensate wavefront aberrations. In a first approach, targeted speckle nulling, speckles are cancelled iteratively by starting with the brightest ones. This first method has demonstrated a contrast better than 109 in laboratory. In a second approach, zonal speckle nulling, the total energy of speckles is minimized in a given zone of the image plane. This second method has the advantage of tackling simultaneously all speckles from the targeted zone, but it still needs better experimental demonstration. To cite this article: P. Bordé, W. Traub, C. R. Physique 8 (2007).