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Coronagraphic imaging of exoplanets using ground-based instruments on largetelescopes is intrinsically limited by speckles induced by uncorrectedaberrations. These aberrations originate from the imperfect correction of theatmosphere by an extreme adaptive optics system; from static optical defects;or from small opto-mechanical variations due to changes in temperature,pressure, or gravity vector. More than the speckles themselves, the performanceof high-contrast imagers is ultimately limited by their temporal stability,since most post-processing techniques rely on difference of images acquired atdifferent points in time. Identifying the origin of the aberrations and thetimescales involved is therefore crucial to understanding the fundamentallimits of dedicated high-contrast instruments. We previously demonstrated theuse of a Zernike wavefront sensor called ZELDA for sensing non-common pathaberrations (NCPA) in VLT/SPHERE. We now use ZELDA to investigate the stabilityof the instrumental aberrations using 5 long sequences of measurements obtainedat high cadence on the internal source. Our study reveals two regimes ofdecorrelation of the NCPA. The first, with a characteristic timescale of a fewseconds and an amplitude of a few nanometers, is induced by a fast internalturbulence within the enclosure. The second is a slow quasi-lineardecorrelation on the order of a few $10^{-3}$ nm rms/s that acts on timescalesfrom minutes to hours. We use coronagraphic image reconstruction to demonstratethat these two NCPA contributions have a measurable impact on differences ofimages, and that the fast internal turbulence is a dominating term over to theslow linear decorrelation. We also use dedicated sequences where the derotatorand atmospheric dispersion compensators emulate a real observation todemonstrate the importance of performing observations symmetric around themeridian.

astronomy and astrophysics

Calibration of quasi-static aberrations in exoplanet direct-imaging  instruments with a Zernike phase-mask sensor. IV. Temporal stability of  non-common path aberrations in VLT/SPHERE