ZELDA, a Zernike wavefront sensor for the fine measurement of quasi-static aberrations in coronagraphic systems: concept studies and results with VLT/SPHERE

The high-contrast imaging instruments VLT/SPHERE and GPI have been routinely observing gas giant planets, brown dwarfs, and debris disks around nearby stars since 2013-2014. In these facilities, low-wind effects or differential aberrations between the extreme Adaptive Optics sensing path and the science path represent critical limitations for the observation of exoplanets orbiting their host star with a contrast ratio larger than 106 at small separations. To circumvent this problem, we proposed ZELDA, a Zernike wavefront sensor to measure these quasistatic aberrations at a nanometric level. A prototype was installed on VLT/SPHERE during its integration in Chile. We recently performed measurements on an internal source with ZELDA in the presence of Zernike or Fourier modes introduced with the deformable mirror of the instrument. In this communication, we present the results of our experiment and report on the contrast gain obtained with a first ZELDA-based wavefront correction. We finally discuss the suitability of such a solution for a possible upgrade of VLT/SPHERE and for its use with future E-ELT instruments or space missions with high-contrast capabilities (e.g. WFIRST-AFTA, HDST).