Abstract
Context. The wind-driven halo is a feature that is observed in images that were delivered by the latest generation of ground-based instruments that are equipped with an extreme adaptive optics system and a coronagraphic device, such as SPHERE at the Very Large Telescope (VLT). This signature appears when the atmospheric turbulence conditions vary faster than the adaptive optics loop can correct for. The wind-driven halo is observed as a radial extension of the point spread function along a distinct direction (this is sometimes referred to as the butterfly pattern). When this is present, it significantly limits the contrast capabilities of the instrument and prevents the extraction of signals at close separation or extended signals such as circumstellar disks. This limitation is consequential because it contaminates the data for a substantial fraction of the time: about 30% of the data produced by the VLT/SPHERE instrument are affected by the wind-driven halo. Aims. This paper reviews the causes of the wind-driven halo and presents a method for analyzing its contribution directly from the scientific images. Its effect on the raw contrast and on the final contrast after post-processing is demonstrated. Methods. We used simulations and on-sky SPHERE data to verify that the parameters extracted with our method can describe the wind-driven halo in the images. We studied the temporal, spatial, and spectral variation of these parameters to point out its deleterious effect on the final contrast. Results. The data-driven analysis we propose provides information to accurately describe the wind-driven halo contribution in the images. This analysis confirms that this is a fundamental limitation of the finally reached contrast performance. Conclusions. With the established procedure, we will analyze a large sample of data delivered by SPHERE in order to propose post-processing techniques that are tailored to removing the wind-driven halo.
Highlights
The latest generation of instruments dedicated to exoplanet and circumstellar disk imaging have in the past five years enabled a huge step in high-contrast imaging (HCI) of the close environment of nearby stars
A classical on-axis single-conjugated adaptive optics (AO) system is composed of three main components: (i) the wavefront sensor (WFS), which analyzes the incoming phase distortion, (ii) the real-time computer (RTC), which based on the WFS measurement calculates the command that is to be sent to the phase corrector, and (iii) the deformable mirrors (DM), which corrects for the phase distortion
The AO-loop delay τAO is a pure delay defined as the addition of the equivalent delays from the various processes involved between taking a measurement of the atmospheric disturbance via the WFS and commanding the DM : the WFS delay, the RTC delay, the digital-to-analog conversion delay at the DM amplifier, the DM positioning delay, and at last an overall data transfer delay
Summary
The latest generation of instruments dedicated to exoplanet and circumstellar disk imaging have in the past five years enabled a huge step in high-contrast imaging (HCI) of the close environment of nearby stars. By equipping 8 m class telescopes with dedicated instruments combining extreme adaptive optics (AO) systems using high-density deformable mirrors (DM) with specific coronagraphs and advanced post-processing techniques, instruments such as VLT/SPHERE (Beuzit et al 2019), Gemini/GPI (Macintosh et al 2008), and Subaru/SCExAO (Jovanovic et al 2015) successfully addressed this challenge After achieving such high resolution and contrast, new limitations are detected in the focal-plane images that were not visible with the first generation of HCI instruments such as VLT/NaCo (Rousset et al 2003), Gemini/NICI (Artigau et al 2008), or Keck/NIRC2 (McLean & Chaffee 2000). From these analyses, we conclude that the current post-processing techniques based on differential imaging are not capable of fully removing the wind-driven halo, and the contrast performance is decreased by an order of magnitude in the AO-corrected area
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