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Published September 1, 2017 | public
Journal Article

A Demonstration of a Versatile Low-order Wavefront Sensor Tested on Multiple Coronographs


Detecting faint companions in close proximity to stars is one of the major goals of current/planned ground- and space-based high-contrast imaging instruments. High-performance coronagraphs can suppress the diffraction features and gain access to companions at small angular separation. However, the uncontrolled pointing errors degrade the coronagraphic performance by leaking starlight around the coronagraphic focal-plane mask, preventing the detection of companions at small separations. A Lyot-stop low-order wavefront sensor (LLOWFS) was therefore introduced to calibrate and measure these aberrations for focal-plane phase mask coronagraphs. This sensor quantifies the variations in wavefront error decomposed into a few Zernike modes by reimaging the diffracted starlight rejected by a reflective Lyot stop. The technique was tested with several coronagraphs on the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system at the Subaru Telescope. The wavefront was decomposed into 15 and 35 Zernike modes with an occulting and focal-plane phase mask coronagraph, respectively, which were used to drive a closed-loop correction in the laboratory. Using a 2000-actuator deformable mirror, a closed-loop pointing stability between 10−3–10^(−4) λ/D was achieved in the laboratory in H-band, with sub nanometer residuals for the other Zernike modes (Noll index >4). On-sky, the low-order control of 10+ Zernike modes for the phase-induced amplitude apodization and the vector vortex coronagraphs was demonstrated, with a closed-loop pointing stability of 10^(-4) λ /D under good seeing and 10^(-3) λ/D under moderate seeing conditions readily achievable.

Additional Information

© 2017 The Astronomical Society of the Pacific. Received 2016 November 30; accepted 2017 May 26; published 2017 July 28. This work was performed on the SCExAO instrument at the Subaru Telescope as a part of a PhD thesis on the LLOWFS (Singh 2015). The SCExAO team thanks the AO188 scientists and engineers for operating the AO system and diagnosing the issues faced during the observations. We gratefully acknowledge the support and help from the Subaru Observatory staff. The SCExAO team also thanks the GEPI/Observatoire de Paris for lending the FQPM coronagraph. This research was partly supported by a Grant-in-Aid for Science Research in a Priority Area from MEXT, Japan and also by the Astrobiology Center (ABC) of the National Institutes of Natural Sciences, Japan, and the directors contingency fund at Subaru Telescope. The authors acknowledge support from the JSPS (Grant-in-Aid for Research #23340051, #26220704, and #23103002). G.S. would also like to acknowledge her appointment to the NASA Postdoctoral Program at the Jet Propulsion Laboratory, California Institute of Technology, administered by Universities Space Research Association under contract with NASA. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We were most fortunate to have had the opportunity to conduct observations from this mountain.

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