Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor

Coronagraphic imaging of exoplanets and circumstellar environments using ground-based instruments on large telescopes is intrinsically limited by speckles induced uncorrected aberrations. These aberrations originate from the imperfect correction atmosphere an extreme adaptive optics system; static optical defects; or small opto-mechanical variations due to changes in temperature, pressure, gravity vector. More than themselves, performance high-contrast imagers ultimately their temporal stability, since most post-processing techniques rely difference images acquired at different points time. Identifying origin timescales involved therefore crucial understanding fundamental limits dedicated instruments. In previous works we demonstrated use a Zernike wavefront sensor called ZELDA for sensing non-common path (NCPA) VLT/SPHERE instrument. We now investigate stability instrumental five long sequences measurements obtained high cadence internal calibration source. Our study reveals two regimes decorrelation NCPA. The first, with characteristic timescale few seconds amplitude nanometers, fast turbulence within enclosure. second slow quasi-linear order 10 −3 nmrms s −1 that acts minutes hours. coronagraphic image reconstruction demonstrate these NCPA contributions have measurable impact differences images, dominating term over linear decorrelation. also where derotator atmospheric dispersion compensators emulate real observation importance performing observations symmetric around meridian, which minimizes speckle decorrelation, maximizes sensitivity point sources images.