Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor. III. On-sky validation in VLT/SPHERE. (arXiv:1907.11241v1 [astro-ph.IM])
<a href="http://arxiv.org/find/astro-ph/1/au:+Vigan_A/0/1/0/all/0/1">A. Vigan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+NDiaye_M/0/1/0/all/0/1">M. N'Diaye</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dohlen_K/0/1/0/all/0/1">K. Dohlen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sauvage_J/0/1/0/all/0/1">J.-F. Sauvage</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Milli_J/0/1/0/all/0/1">J. Milli</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zins_G/0/1/0/all/0/1">G. Zins</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Petit_C/0/1/0/all/0/1">C. Petit</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wahhaj_Z/0/1/0/all/0/1">Z. Wahhaj</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cantalloube_F/0/1/0/all/0/1">F. Cantalloube</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Caillat_A/0/1/0/all/0/1">A. Caillat</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Costille_A/0/1/0/all/0/1">A. Costille</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Merrer_J/0/1/0/all/0/1">J. Le Merrer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Carlotti_A/0/1/0/all/0/1">A. Carlotti</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Beuzit_J/0/1/0/all/0/1">J.-L. Beuzit</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mouillet_D/0/1/0/all/0/1">D. Mouillet</a>
Second-generation exoplanet imagers using extreme adaptive optics and
coronagraphy have demonstrated their great potential for studying close
circumstellar environments and for detecting new companions and helping to
understand their physical properties. However, at very small angular
separation, their performance in contrast is limited by several factors:
diffraction by the complex telescope pupil not perfectly canceled by the
coronagraph, residual dynamic wavefront errors, chromatic wavefront errors, and
wavefront errors resulting from noncommon path aberrations (NCPAs). In a
previous work, we demonstrated the use of a Zernike wavefront sensor called
ZELDA for sensing NCPAs in VLT/SPHERE and their compensation. In the present
work, we move to the next step with the on-sky validation of NCPA compensation
with ZELDA. We start by reproducing previous results on the internal source and
show that the amount of aberration integrated between 1 and 15 cycles/pupil is
decreased by a factor of five, which translates into a gain in raw contrast of
between 2 and 3 below 300 mas. On sky, we demonstrate that NCPA compensation
works in closed loop, leading to an attenuation of the amount of aberration by
a factor of approximately two. However, we identify a loss of sensitivity for
the sensor that is only partly explained by the difference in Strehl ratio
between the internal and on-sky measurements. Coronagraphic imaging on sky is
improved in raw contrast by a factor of 2.5 at most in the ExAO-corrected
region. We use coronagraphic image reconstruction based on a detailed model of
the instrument to demonstrate that both internal and on-sky raw contrasts can
be precisely explained, and we establish that the observed performance after
NCPA compensation is no longer limited by an improper compensation for
aberration but by the current apodized-pupil Lyot coronagraph design.
[abridged]
Second-generation exoplanet imagers using extreme adaptive optics and
coronagraphy have demonstrated their great potential for studying close
circumstellar environments and for detecting new companions and helping to
understand their physical properties. However, at very small angular
separation, their performance in contrast is limited by several factors:
diffraction by the complex telescope pupil not perfectly canceled by the
coronagraph, residual dynamic wavefront errors, chromatic wavefront errors, and
wavefront errors resulting from noncommon path aberrations (NCPAs). In a
previous work, we demonstrated the use of a Zernike wavefront sensor called
ZELDA for sensing NCPAs in VLT/SPHERE and their compensation. In the present
work, we move to the next step with the on-sky validation of NCPA compensation
with ZELDA. We start by reproducing previous results on the internal source and
show that the amount of aberration integrated between 1 and 15 cycles/pupil is
decreased by a factor of five, which translates into a gain in raw contrast of
between 2 and 3 below 300 mas. On sky, we demonstrate that NCPA compensation
works in closed loop, leading to an attenuation of the amount of aberration by
a factor of approximately two. However, we identify a loss of sensitivity for
the sensor that is only partly explained by the difference in Strehl ratio
between the internal and on-sky measurements. Coronagraphic imaging on sky is
improved in raw contrast by a factor of 2.5 at most in the ExAO-corrected
region. We use coronagraphic image reconstruction based on a detailed model of
the instrument to demonstrate that both internal and on-sky raw contrasts can
be precisely explained, and we establish that the observed performance after
NCPA compensation is no longer limited by an improper compensation for
aberration but by the current apodized-pupil Lyot coronagraph design.
[abridged]
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