Self-coherent camera as a focal plane wavefront sensor: simulations

Abstract

Context. Direct detection of exoplanets requires high dynamic range imaging. Coronagraphs could be the solution, but their performance in space is limited by wavefront errors (manufacturing errors on optics, temperature variations, etc.), which create quasi-static stellar speckles in the final image. Aims. Several solutions have been suggested for tackling this speckle noise. Differential imaging techniques substract a reference image to the coronagraphic residue in a post-processing imaging. Other techniques attempt to actively correct wavefront errors using a deformable mirror. In that case, wavefront aberrations have to be measured in the science image to extremely high accuracy. Methods. We propose the self-coherent camera sequentially used as a focal-plane wavefront sensor for active correction and differential imaging. For both uses, stellar speckles are spatially encoded in the science image so that differential aberrations are strongly minimized. The encoding is based on the principle of light incoherence between the hosting star and its environment. Results. In this paper, we first discuss one intrinsic limitation of deformable mirrors. Then, several parameters of the self-coherent camera are studied in detail. We also propose an easy and robust design to associate the self-coherent camera with a coronagraph that uses a Lyot stop. Finally, we discuss the case of the association with a four-quadrant phase mask and numerically demonstrate that such a device enables detection of Earth-like planets under realistic conditions. Conclusions. The parametric study of the technique lets us believe it can be implemented quite easily in future instruments dedicated to direct imaging of exoplanets.

Estimation and correction of wavefront aberrations using the self-coherent camera: laboratory results

“…We simulated FQPM coronagraphs in this paper using the method described in Mas et al (2012). This coronagraph is completely insensitive to some aberrations, for instance to one of the astigmatism aberrations (Galicher 2009;Galicher et al 2010). Because these aberrations introduce no aberration inside the Lyot pupil, we are unable to estimate them.…”

High-contrast imaging in polychromatic light with the self-coherent camera

“…This light creates an Airy pattern (Galicher et al 2011) in the image plane after the Lyot, which, because it is not created by wavefront errors, is scarcely corrected by the DM. Galicher et al (2010) also pointed out specific problems in the SCC estimation at broad bandwidths. In addition to the smearing of the speckles, the interference fringes have wavelength-dependent periods (λ/ ξ 0 ).…”

“…Several solutions have been investigated to enhance the correction zone (Galicher et al 2010). The association of an SCC with a Wynne compensator would prevent the blurring of the fringes.…”

“…In previous papers (Galicher et al 2010;Mazoyer et al 2013a), I − was used to retrieve the phase and amplitude aberrations in the pupil plane upstream of the coronagraph and minimize them. This method was dependent on a model of the coronagraph.…”

“…Among these, the self-coherent camera (SCC, Baudoz et al 2006) uses the coherence between the focal plane speckles and the stellar light rejected by the coronagraph. This technique has already reached high contrast levels in simulations (Galicher et al 2010) and experimentally (Mazoyer et al 2013a) in monochromatic light. This paper presents the latest laboratory performance obtained with the SCC.…”