  Coronagraphic imaging of exoplanets using ground-based instruments on large
telescopes is intrinsically limited by speckles induced by uncorrected
aberrations. These aberrations originate from the imperfect correction of the
atmosphere 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 performance
of high-contrast imagers is ultimately limited by their temporal stability,
since most post-processing techniques rely on difference of images acquired at
different points in time. Identifying the origin of the aberrations and the
timescales involved is therefore crucial to understanding the fundamental
limits of dedicated high-contrast instruments. We previously demonstrated the
use of a Zernike wavefront sensor called ZELDA for sensing non-common path
aberrations (NCPA) in VLT/SPHERE. We now use ZELDA to investigate the stability
of the instrumental aberrations using 5 long sequences of measurements obtained
at high cadence on the internal source. Our study reveals two regimes of
decorrelation of the NCPA. The first, with a characteristic timescale of a few
seconds and an amplitude of a few nanometers, is induced by a fast internal
turbulence within the enclosure. The second is a slow quasi-linear
decorrelation on the order of a few $10^{-3}$ nm rms/s that acts on timescales
from minutes to hours. We use coronagraphic image reconstruction to demonstrate
that these two NCPA contributions have a measurable impact on differences of
images, and that the fast internal turbulence is a dominating term over to the
slow linear decorrelation. We also use dedicated sequences where the derotator
and atmospheric dispersion compensators emulate a real observation to
demonstrate the importance of performing observations symmetric around the
meridian.
