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On-sky performance of the QACITS pointing control technique with the Keck/NIRC2 vortex coronagraph

Huby, E. and Bottom, M. and Femenia, B. and Ngo, H. and Mawet, D. and Serabyn, E. and Absil, O. (2017) On-sky performance of the QACITS pointing control technique with the Keck/NIRC2 vortex coronagraph. Astronomy & Astrophysics, 600 . Art. No. A46. ISSN 0004-6361. doi:10.1051/0004-6361/201630232.

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Context. A vortex coronagraph is now available for high contrast observations with the Keck/NIRC2 instrument at L band. The vortex coronagraph uses a vortex phase mask in a focal plane and a Lyot stop in a downstream pupil plane to provide high contrast at small angular separations from the observed host star. Aims. Reaching the optimal performance of the coronagraph requires fine control of the wavefront incident on the phase mask. In particular, centering errors can lead to significant stellar light leakage that degrades the contrast performance and prevents the observation of faint planetary companions around the observed stars. It is thus critical to correct for the possible slow drift of the star image from the phase mask center, generally due to mechanical flexures induced by temperature and/or gravity field variation, or to misalignment between the optics that rotate in pupil tracking mode. Methods. A control loop based on the QACITS algorithm for the vortex coronagraph has been developed and deployed for the Keck/NIRC2 instrument. This algorithm executes the entire observing sequence, including the calibration steps, initial centering of the star on the vortex center, and stabilisation during the acquisition of science frames. Results. On-sky data show that the QACITS control loop stabilizes the position of the star image down to 2.4 mas rms at a frequency of about 0.02 Hz. However, the accuracy of the estimator is probably limited by a systematic error due to a misalignment of the Lyot stop with respect to the entrance pupil, estimated to be on the order of 4.5 mas. A method to reduce the amplitude of this bias down to 1 mas is proposed. Conclusions. The QACITS control loop has been successfully implemented and provides a robust method to center and stabilize the star image on the vortex mask. In addition, QACITS ensures a repeatable pointing quality and significantly improves the observing efficiency compared to manual operations. It is now routinely used for vortex coronagraph observations at Keck/NIRC2, providing contrast and angular resolution capabilities suited for exoplanet and disk imaging.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Bottom, M.0000-0003-1341-5531
Ngo, H.0000-0001-5172-4859
Mawet, D.0000-0002-8895-4735
Absil, O.0000-0002-4006-6237
Additional Information:© ESO 2017. Received: 12 December 2016. Accepted: 20 January 2017. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (ERC Grant Agreement No. 337569) and from the French Community of Belgium through an ARC grant for Concerted Research Action.
Group:Astronomy Department
Funding AgencyGrant Number
European Research Council (ERC)337569
Communauté française de Belgique – Actions de recherche concertéesUNSPECIFIED
Subject Keywords:instrumentation: adaptive optics – techniques: high angular resolution – methods: observational
Record Number:CaltechAUTHORS:20170602-063112801
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Official Citation:On-sky performance of the QACITS pointing control technique with the Keck/NIRC2 vortex coronagraph E. Huby, M. Bottom, B. Femenia, H. Ngo, D. Mawet, E. Serabyn and O. Absil A&A, 600 (2017) A46 DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:77903
Deposited By: Ruth Sustaita
Deposited On:02 Jun 2017 18:36
Last Modified:15 Nov 2021 17:34

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