On-sky compensation of non-common path aberrations with the ZELDA wavefront sensor in VLT/SPHERE

TL;DR

This paper demonstrates the on-sky application of the ZELDA wavefront sensor in VLT/SPHERE to measure and compensate non-common path aberrations, aiming to improve high-contrast imaging for exoplanet detection.

Contribution

It presents recent on-sky measurements showing the potential of ZELDA for NCPA compensation during observations, building on previous internal calibration results.

Findings

Contrast gain of 10 at 0.2 arcseconds achieved in internal tests.

Initial on-sky tests did not show contrast improvement.

Recent measurements indicate potential for on-sky NCPA correction.

Abstract

Circumstellar environments are now routinely observed by dedicated high-contrast imagers on large, ground-based observatories. These facilities combine extreme adaptive optics and coronagraphy to achieve unprecedented sensitivities for exoplanet detection and spectral characterization. However, non-common path aberrations (NCPA) in these coronagraphic systems represent a critical limitation for the detection of giant planets with a contrast lower than a few $10^{-6}$ at very small separations ($<$0.3$^{\prime\prime}$) from their host star. In 2013 we proposed ZELDA, a Zernike wavefront sensor to measure these residual quasi-static phase aberrations and a prototype was installed in SPHERE, the exoplanet imager for the VLT. In 2016, we demonstrated the ability of our sensor to provide a nanometric calibration and compensation for these aberrations on an internal source in the instrument, resulting in a contrast gain of 10 at 0.2$^{\prime\prime}$ in coronagraphic images. However, initial on-sky tests in 2017 did not show a tangible gain in contrast when calibrating the NCPA internally and then applying the correction on sky. In this communication, we present recent on-sky measurements to demonstrate the potential of our sensor for the NCPA compensation during observations and quantify the contrast gain in coronagraphic data.