Abstract
Second-generation exoplanet imagers using extreme adaptive optics (ExAO) 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 (central obscuration and spiders) not perfectly canceled by the coronagraph, residual dynamic wavefront errors, chromatic wavefront errors, and wavefront errors resulting from noncommon path aberrations (NCPAs). These latter are differential aberrations between the visible wavefront sensing path of the ExAO system and the near-infrared science path in which the coronagraph is located. In a previous work, we demonstrated the use of a Zernike wavefront sensor called ZELDA for sensing NCPAs in the VLT/SPHERE exoplanet imager and their compensation with the high-order deformable mirror of the instrument. These early tests on the internal light source led to encouraging results for the attenuation of the quasi-static speckles at very small separation. In the present work, we move to the next step with the on-sky validation of NCPA compensation with ZELDA. With an improved procedure for the compensation of NCPAs, we start by reproducing previous results on the internal source. We show that the amount of aberration integrated between 1 and 15 cycles/pupil (c/p) is decreased by a factor of approximately five, which translates into a gain in raw contrast of between 2 and 3 at separations 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. Our simulations show that the impact of ExAO residuals on ZELDA measurements is negligible for integration times beyond a few tenths of a second. 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. We finally conclude that a coronagraph upgrade combined to a proper NCPA compensation scheme could easily bring a gain in raw contrast of a factor of two to three below 200 mas.
Highlights
With the advent of the second-generation, high-contrast instruments on the ground in 2013–2014, unprecedented performance has been obtained on the images of nearby stars with contrasts down to 10−6 at separations beyond 300 mas in the near-infrared (NIR) band, leading to the observation of various circumstellar disks and the discovery of new young gas giant planets (e.g., Macintosh et al 2015; Chauvin et al 2017a; Keppler et al 2018)
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)
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
Summary
With the advent of the second-generation, high-contrast instruments on the ground in 2013–2014, unprecedented performance has been obtained on the images of nearby stars with contrasts down to 10−6 at separations beyond 300 mas in the near-infrared (NIR) band, leading to the observation of various circumstellar disks (e.g., de Boer et al 2016; Lagrange et al 2016; Currie et al 2017; Feldt et al 2017; Goebel et al 2018; Esposito et al 2018) and the discovery of new young gas giant planets (e.g., Macintosh et al 2015; Chauvin et al 2017a; Keppler et al 2018). The ZELDA sensor is based on phase-contrast techniques, originally proposed by Zernike (1934), to measure NCPAs in high-contrast imaging instruments with nanometric accuracy This sensor uses a focal plane phase mask to produce interference between a reference wave created by the mask and the phase errors present in the system. The second strategy is obviously more efficient and avoids dealing with problems such as integration times and variable observing conditions, it requires a complete understanding of the behavior of the instrument when switching between the internal calibration source and a star It relies on the presence of a calibration source sufficiently close to the entrance of the instrument, which is the case in VLT/SPHERE, but even so the light will not see the telescope mirrors as there is rarely (if ever) an appropriate light source in the telescope itself. We quantify the impact of NCPA correction on coronagraphic data and discuss prospects for further exoplanet direct imaging and spectroscopy observations
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