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Warm or massive gas giant planets, brown dwarfs, and debris disks aroundnearby stars are now routinely observed by dedicated high-contrast imaginginstruments on large, ground-based observatories. These facilities includeextreme adaptive optics (ExAO) and state-of-the-art coronagraphy to achieveunprecedented sensitivities for exoplanet detection and spectralcharacterization. However, differential aberrations between the ExAO sensingpath and the science path represent a critical limitation for the detection ofgiant planets with a contrast lower than a few $10^{-6}$ at very smallseparations (<0.3\as) from their host star. In our previous work, we proposed awavefront sensor based on Zernike phase contrast methods to circumvent thisissue and measure these quasi-static aberrations at a nanometric level. Wepresent the design, manufacturing and testing of ZELDA, a prototype that wasinstalled on VLT/SPHERE during its reintegration in Chile. Using the internallight source of the instrument, we performed measurements in the presence ofZernike or Fourier modes introduced with the deformable mirror. Ourexperimental and simulation results are consistent, confirming the ability ofour sensor to measure small aberrations (<50 nm rms) with nanometric accuracy.We then corrected the long-lived non-common path aberrations in SPHERE based onZELDA measurements. We estimated a contrast gain of 10 in the coronagraphicimage at 0.2\as, reaching the raw contrast limit set by the coronagraph in theinstrument. The simplicity of the design and its phase reconstruction algorithmmakes ZELDA an excellent candidate for the on-line measurements of quasi-staticaberrations during the observations. The implementation of a ZELDA-basedsensing path on the current and future facilities (ELTs, future space missions)could ease the observation of the cold gaseous or massive rocky planets aroundnearby stars.

astronomy and astrophysics

Calibration of quasi-static aberrations in exoplanet direct-imaging  instruments with a Zernike phase-mask sensor. II. Concept validation with  ZELDA on VLT/SPHERE