Abstract
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. 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. 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. 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 the detection of Earth-like planets under realistic conditions. The parametric study of the technique lets us believe it can be implemented quite easily in future instruments dedicated to direct imaging of exoplanets.
Highlights
Exoplanets are typically 107 to 1010 fainter than their host and are often located within a fraction of an arcsecond from their star
In Galicher et al (2008), we numerically demonstrated that, applying step B after step A, a selfcoherent camera associated with a 32 × 32 deformable mirror and a perfect coronagraph detects earths
We considered a single telescope associated with a perfect coronagraph and a deformable mirror
Summary
Exoplanets are typically 107 to 1010 fainter than their host and are often located within a fraction of an arcsecond from their star. Performance is limited by wavefront errors in the upstream beam for all these coronagraphs and the final focal plane image shows stellar speckles. The effect of most of these aberrations can be corrected by adaptive optics (AO) or eXtreme AO (XAO, Vérinaud et al 2008) but the uncorrected part generates quasi-static speckles, which limit the image contrast (Cavarroc et al 2006; Macintosh et al 2005) To reduce this speckle noise, differential imaging techniques attempt to subtract a reference image of the stellar speckles from the science image (star plus companion). – Step A, wavefront estimation and correction: estimate phase and amplitude errors from the focal plane image and correct for them using a deformable mirror – Step B, companion detection: record the science image when the best correction is achieved and post-process that image to overcome the DM limitation (Sect. 2.3)
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