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 starl
ight 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.
For direct imaging of exoplanets, a stellar coronagraph helps to remove the image of an observed bright star by attenuating the diffraction effects caused by the telescope aperture of diameter D. The Dual Zone Phase Mask (DZPM) coronagraph constitute
s a promising concept since it theoretically offers a small inner working angle (IWA sim lambda_0/D), good achromaticity and high starlight rejection, typically reaching a 1e6 contrast at 5 lambda_0/D from the star over a spectral bandwidth Deltalambda/lambda_0 of 25% (similar to H-band). This last value proves to be encouraging for broadband imaging of young and warm Jupiter-like planets. Contrast levels higher than 1e6 are however required for the observation of older and/or less massive companions over a finite spectral bandwidth. An achromatization improvement of the DZPM coronagraph is therefore mandatory to reach such performance. In its design, the DZPM coronagraph uses a grey (or achromatic) apodization. We propose to replace it by a colored apodization to increase the performance of this coronagraphic system over a large spectral range. This innovative concept, called Colored Apodizer Phase Mask (CAPM) coronagraph, is defined with some design parameters optimized to reach the best contrast in the exoplanet search area. Once this done, we study the performance of the CAPM coronagraph in the presence of different errors to evaluate the sensitivity of our concept. A 2.5 mag contrast gain is estimated from the performance provided by the CAPM coronagraph with respect to that of the DZPM coronagraph. A 2.2e-8 intensity level at 5 lambda_0/D separation is then theoretically achieved with the CAPM coronagraph in the presence of a clear circular aperture and a 25% bandwidth. In addition, our studies show that our concept is less sensitive to low than high-order aberrations for a given value of rms wavefront errors.
