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The major source of noise in high-contrast imaging is the presence of slowlyevolving speckles that do not average with time. The temporal stability of thepoint-spread-function (PSF) is therefore critical to reach a high contrast withextreme adaptive optics (xAO) instruments. Understanding on which timescalesthe PSF evolves and what are the critical parameters driving the specklevariability allow to design an optimal observing strategy and data reductiontechnique to calibrate instrumental aberrations and reveal faint astrophysicalsources. We have obtained a series of 52 min, AO-corrected, coronagraphicallyocculted, high-cadence (1.6Hz), H-band images of the star HR 3484 with theSPHERE (Spectro-Polarimeter High-contrast Exoplanet REsearch instrument on theVLT. This is a unique data set from an xAO instrument to study its stability ontimescales as short as one second and as long as several tens of minutes. Wefind different temporal regimes of decorrelation. We show that residuals fromthe atmospheric turbulence induce a fast, partial decorrelation of the PSF overa few seconds, before a transition to a regime with a linear decorrelation withtime, at a rate of several tens parts per million per second (ppm/s). Weanalyze the spatial dependence of this decorrelation, within the well-correctedradius of the adaptive optics system and show that the linear decorrelation isfaster at short separations. Last, we investigate the influence of the distanceto the meridian on the decorrelation.

Speckle lifetime in XAO coronagraphic images: temporal evolution of SPHERE coronagraphic images