SPHERE adaptive optics performance for faint targets

Abstract

Context. High-contrast imaging is a powerful technique for detecting and characterizing planetary companions at orbital separations ≳100 mas from their parent stars. Aims. We aim to study the limiting magnitude of the VLT/SPHERE adaptive optics (AO) system and the corresponding instrument performance for faint targets (G ≥ 11.0 mag). Methods. We computed the coronagraphic H-band raw contrast and the full width at half maximum (FWHM) of the non-coronagraphic point spread function (PSF), for a total of 111 different stars observed between 2016 and 2022 with IRDIS. For this, we processed a large number of individual frames that were obtained under different atmospheric conditions. We then compared the resulting raw contrast and the PSF shape as a function of the visible wave front sensor (WFS) instant flux, which scales with the G-band stellar magnitude. We repeated this analysis for the top 10% (TCAT10) and top 30% (TCAT38) best turbulence conditions on Cerro Paranal. Results. We found a strong decrease in the coronagraphic contrast for stars fainter than G ∼ 12.5 mag, even under the best atmospheric conditions. In this regime, the AO correction is dominated by the read-out noise of the WFS detector. In particular we found roughly a factor of 10 decrease in the raw contrast ratio between stars with G ∼ 12.5 and G ∼ 14.0 mag. Similarly, we observed a sharp increase in the FWHM of the non-coronagraphic PSF beyond G ∼ 12.5 mag, and a corresponding decrease in the strehl ratio from ∼0.5 to ∼0.2 for the faintest stars. The decrease in the contrast ratio and PSF sharpness is slightly more pronounced for TCAT38 than for TCAT18.