Scintillation effects on a high-contrast imaging instrument for a direct detection of exo-planets

A study of the scintillation effects on the PSF halo of the high-contrast imaging instrument (CHEOPS) for direct exo-planet detection from the ground is presented. The fundamental goal of our analysis is to quantify the perturbations induced by the amplitude (scintillation) variations compared to those induced by the phase variations of a perturbed wavefront. Simulations of amplitude and phase screens are obtained for different seeing conditions and for a wavefront propagating at different zenith angles. For all cases a set of simulations of the PSFs in the ideal mirror-limited case (perfect AO-system) and an estimation of the detection limit Am vs. angular separation obtained with and without scintillation are presented. The whole study is made in I-band (lambda = 0.9 mum) i.e. the centered wavelength of the CHEOPS polarimetric imager. A maximum loss of contrast (obtained with and without scintillation) of similar to 25 % over a FOV of 5 arcsec is found in the speckle noise-limited regime and of similar to 18 % in the photon noise-limited regime. Results are discussed and conditions in which the scintillation effects cannot be neglected are investigated.