Author:
Jones M. I.,Milli J.,Blanchard I.,Wahhaj Z.,De Rosa R. J.,Romero C.,Ihanec N.
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.
Subject
Space and Planetary Science,Astronomy and Astrophysics
Cited by
5 articles.
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