Abstract
Context. Several exoplanet direct imaging instruments will soon be in operation. They use an extreme adaptive optics (XAO) system to correct the atmospheric turbulence and provide a highly-corrected beam to a near-infrared (NIR) coronagraph for starlight suppression. The performance of the coronagraph is however limited by the non-common path aberrations (NCPA) due to the differential wavefront errors existing between the visible XAO sensing path and the NIR science path, leading to residual speckles in the coronagraphic image. Aims. Several approaches have been developed in the past few years to accurately calibrate the NCPA, correct the quasi-static speckles and allow the observation of exoplanets at least 1e6 fainter than their host star. We propose an approach based on the Zernike phase-contrast method for the measurements of the NCPA between the optical path seen by the visible XAO wavefront sensor and that seen by the near-IR coronagraph. Methods. This approach uses a focal plane phase mask of size {\lambda}/D, where {\lambda} and D denote the wavelength and the telescope aperture diameter, respectively, to measure the quasi-static aberrations in the upstream pupil plane by encoding them into intensity variations in the downstream pupil image. We develop a rigorous formalism, leading to highly accurate measurement of the NCPA, in a quasi-linear way during the observation. Results. For a static phase map of standard deviation 44 nm rms at {\lambda} = 1.625 {\mu}m (0.026 {\lambda}), we estimate a possible reduction of the chromatic NCPA by a factor ranging from 3 to 10 in the presence of AO residuals compared with the expected performance of a typical current-generation system. This would allow a reduction of the level of quasi-static speckles in the detected images by a factor 10 to 100 hence, correspondingly improving the capacity to observe exoplanets.
Highlights
Following the recent images of the exoplanets HR 8799 b, c, d (Marois et al 2008; Currie et al 2011a; Soummer et al 2011; Galicher et al 2011) and β Pic b (Lagrange et al 2009, 2010; Currie et al 2011b; Bonnefoy et al 2011), the astronomical community has high expectations for future discoveries and studies in comparative exoplanetology
The decade will see the emergence of new exoplanet imagers, such as EPICS (Kasper et al 2008), for the future European Extremely Large Telescope (E-ELT, Gilmozzi & Spyromilio 2008) on the ground and coronagraphic telescopes in space (Guyon et al 2010b; Trauger et al 2010; Boccaletti et al 2012), for the study and spectral analysis of extrasolar planets from 107 to 1010 times fainter than their host stars at a few hundredths of an arcsecond
Postprocessing methods will attenuate the speckles present in the coronagraphic image due to ripples in the residual wavefront after adaptive optics correction to make further improvements, necessary to reduce these residual speckles which are quasistatic wavefront deformations due to non-common path aberrations (NCPA) between the optical path seen by the visible wavefront sensor and that seen by the NIR coronagraph
Summary
Following the recent images of the exoplanets HR 8799 b, c, d (Marois et al 2008; Currie et al 2011a; Soummer et al 2011; Galicher et al 2011) and β Pic b (Lagrange et al 2009, 2010; Currie et al 2011b; Bonnefoy et al 2011), the astronomical community has high expectations for future discoveries and studies in comparative exoplanetology. It is noteworthy that the Zernike sensor can be used with any aperture shape and pupil obscuration (shadow of the secondary mirror and spiders, gaps between segments, etc.) and that it is sensitive to phasing errors in segmented telescope systems It constitutes a promising wavefront-sensing option for future space and ground-based instruments like EPICS, receiving light through segmented primary collectors such as the European Extremely Large Telescope. We discuss the application of the Zernike sensor to an exoplanet direct imaging instrument in Sect. 6, proposing an error budget in order to estimate the ultimate performance limits of the system
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