Laboratory Demonstration of Spatial Linear Dark-Field Control for Imaging Extrasolar Planets in Reflected Light

Abstract

Imaging planets in reflected light, a key focus of future NASA missions and extremely large telescopes, requires advanced wavefront control to maintain a deep, temporally correlated null of stellar halo-i.e., a dark hole (DH)at just several diffraction beam widths. Using the Ames Coronagraph Experiment testbed, we present the first laboratory tests of Spatial Linear Dark Field Control (LDFC) approaching raw contrasts (∼5 × 10 −7 ) and separations (1.5-5.2λ/D) needed to image Jovian planets around Sun-like stars with space-borne coronagraphs like WFIRST-CGI and image exo-Earths around low-mass stars with future ground-based 30 m class telescopes. In four separate experiments and for a range of different perturbations, LDFC largely restores (to within a factor of 1.2-1.7) and maintains a DH whose contrast is degraded by phase errors by an order of magnitude. Our implementation of classical speckle nulling requires a factor of 2-5 more iterations and 20-50 deformable mirror (DM) commands to reach contrasts obtained by spatial LDFC. Our results provide a promising path forward to maintaining DHs without relying on DM probing and in the low-flux regime, which may improve the duty cycle of high-contrast imaging instruments, increase the temporal correlation of speckles, and thus enhance our ability to image true solar system analogues in the next two decades.

On-sky performance and recent results from the Subaru coronagraphic extreme adaptive optics system

“…Laboratory tests of LDFC at raw contrasts (∼ 5×10 −7 ) relevant for imaging reflected-light planets from ground-based telescopes demonstrated its ability to correct for a range of phase perturbations with improved efficiency relative to classical speckle nulling. 62 Tests of LDFC at slightly milder contrasts using SCExAO's internal source and simulated turbulence as well as preliminary on-sky tests were extremely promising (K. Miller et al 2020 submitted; S. Bos 2021 in prep. ).…”

Imaging exoplanets with coronagraphic instruments

“…The reader can find more information on this subject in these two recent reviews [34,189]. In the present review, we quickly recall the principle and limitations of these techniques (Section 8.1) and, we present some of the specific observational strategies (Section 8.2) and postprocessing algorithms (Section 8.3).…”

“…No assumptions are needed when using spatial modulation of the speckle intensity [160,[204][205][206]. For a more complete description of each strategy, the reader can refer to [189].…”

Computational complexities of image plane algorithms for high contrast imaging in space telescopes

“…Linear dark-field control (LDFC) is a closed-loop approach that aims at “freezing” the bright speckle field in one part of the image, thus maintaining a fixed speckle pattern at the dark hole as well 20 , 44 . In practice, there might be wavefront disturbances that cancel out in the bright but not the dark field, hence LDFC does not perfectly “freeze” the speckles.…”