Tuesday, June 25, 2013

1306.5561 (Johan Mazoyer et al.)

Estimation and correction of wavefront aberrations upstream a FQPM coronagraph: laboratory results using the Self-Coherent Camera    [PDF]

Johan Mazoyer, Pierre Baudoz, Raphaël Galicher, Marion Mas, Gérard Rousset
Direct imaging of exoplanets requires very high contrast levels, obtained using coronagraphs. But, residual quasi-static aberrations create speckles in the focal plane downstream the coronagraph which mask the planet. This issue appears in ground-based instruments as well as in space telescopes.Active correction of these wavefront errors using a deformable mirror upstream of the coronagraph is mandatory, but conventional adaptive optics are limited by differential path aberrations. Dedicated techniques have to be implemented to measure phase and amplitude errors directly in the science focal plane.First, we propose a method to estimate phase and amplitude aberrations upstream a coronagraph from the speckle complex field in the downstream focal plane. Then, we present the Self-Coherent Camera, which uses the coherence of light to spatially encode the focal plane speckles and retrieve the associated complex field. We are therefore able to estimate and compensate in closed loop for the aberrations upstream the coronagraph. Numerical simulations as well as laboratory tests using a four quadrant phase mask and a 32x32 actuator deformable mirror have been conducted.We demonstrate in laboratory our capability to achieve a stable closed loop and compensate for phase and amplitude quasi-static aberrations. We determine the optimum parameter values to be selected to implement our technique. Contrasts better than 10^{-6} between 2 and 12 \lambda/D and even 3.10^{-7} (RMS) between 7 and 11 \lambda/D are reached in the focal plane. It seems that the contrast level is mainly limited both by amplitude defects created by the surface of the deformable mirror and by the dynamic of the detector. These results are promising for a future application to a dedicated space mission for exoplanet characterization. A number of possible improvements have been identified.
View original: http://arxiv.org/abs/1306.5561

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