New space missions dedicated to exoplanet imaging will rely on coronagraphs to address the high contrast between the stars and their environments. In order to avoid the image of planets to be lost in post-coronagraphic starlight residuals, high precision wavefront sensing and control is a key element to these missions. We present recent results of simultaneous post-coronagraphic phase and amplitude sensing obtained on the THD bench using the coronagraphic phase diversity. We also present results of simulation studies on the non-linear dark hole technique to assess the main limitations of this technique. Finally, we present a first experimental validation of its principle and corroborate expectations on its speed of convergence. These results suggest that the non-linear dark hole is a good candidate for wave-front control for future space-based exoplanet imaging missions, where fast techniques to produce deep dark holes are of paramount importance.
The final performance of current and future instruments dedicated to exoplanet detection and characterization
is limited by intensity residuals in the scientific image plane, which originate in uncorrected optical aberrations.
In order to reach very high contrasts, these aberrations needs to be compensated for. We have proposed a focalplane
wave-font sensor called COFFEE (for COronagraphic Focal-plane wave-Front Estimation for Exoplanet
detection), which consists in an extension of conventional phase diversity to a coronagraphic system. In this
communication, we study the extension of COFFEE to the joint estimation of the phase and the amplitude in
the context of space-based coronagraphic instruments: we optimize the diversity phase in order to minimize the
reconstruction error; we also propose and optimize a novel low-amplitude high-frequency diversity that should
allow the phase-diverse images to still be used for science. Lastly, we perform a first experimental validation of
COFFEE in the very high, space-like contrast conditions of the THD bench and show that COFFEE is able to
distinguish between phase and amplitude aberrations.
The resolution of coronagraphic high contrast exoplanet imaging devices such as SPHERE is limited by quasistatic aberrations. These aberrations produce speckles that can be mistaken for planets in the image. In order to design instruments, correct quasi-static aberrations or analyze data, the expression of the point spread function of a coronagraphic telescope in the presence of residual turbulence is useful. We have derived an analytic formula for this point spread function. We explain physically its structure, we validate it by numerical simulations and we show that it is computationally efficient.