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Characterizing the Performance of the NIRC2 Vortex Coronagraph at W. M. Keck Observatory

Xuan, W. Jerry and Mawet, Dimitri and Ngo, Henry and Ruane, Garreth and Bailey, Vanessa P. and Choquet, Élodie and Absil, Olivier and Alvarez, Carlos and Bryan, Marta and Cook, Therese and Femenía Castellá, Bruno and Gomez Gonzalez, Carlos A. and Huby, Elsa and Knutson, Heather A. and Matthews, Keith and Ragland, Sam and Serabyn, Eugene and Zawol, Zoë (2018) Characterizing the Performance of the NIRC2 Vortex Coronagraph at W. M. Keck Observatory. Astronomical Journal, 156 (4). Art. No. 156. ISSN 0004-6256. doi:10.3847/1538-3881/aadae6.

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The NIRC2 vortex coronagraph is an instrument on Keck II designed to directly image exoplanets and circumstellar disks at mid-infrared bands L' (3.4–4.1 μm) and M_s (4.55–4.8 μm). We analyze imaging data and corresponding adaptive optics telemetry, observing conditions, and other metadata over a three-year time period to characterize the performance of the instrument and predict the detection limits of future observations. We systematically process images from 359 observations of 304 unique stars to subtract residual starlight (i.e., the coronagraphic point-spread function) of the target star using two methods: angular differential imaging (ADI) and reference star differential imaging (RDI). We find that for the typical parallactic angle (PA) rotation of our data set (~10°), RDI provides gains over ADI for angular separations smaller than 0farcs25. Furthermore, we find a power-law relation between the angular separation from the host star and the minimum PA rotation required for ADI to outperform RDI, with a power-law index of −1.18 ± 0.08. Finally, we use random forest models to estimate ADI and RDI post-processed detection limits a priori. These models, which we provide publicly on a website, explain 70%–80% of the variance in ADI detection limits and 30%–50% of the variance in RDI detection limits. Averaged over a range of angular separations, our models predict both ADI and RDI contrast to within a factor of 2. These results illuminate important factors in high-contrast imaging observations with the NIRC2 vortex coronagraph, help improve observing strategies, and inform future upgrades to the hardware.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Xuan, W. Jerry0000-0002-6618-1137
Mawet, Dimitri0000-0002-8895-4735
Ngo, Henry0000-0001-5172-4859
Ruane, Garreth0000-0003-4769-1665
Bailey, Vanessa P.0000-0002-5407-2806
Choquet, Élodie0000-0002-9173-0740
Absil, Olivier0000-0002-4006-6237
Bryan, Marta0000-0002-6076-5967
Gomez Gonzalez, Carlos A.0000-0003-2050-1710
Knutson, Heather A.0000-0002-0822-3095
Additional Information:© 2018. The American Astronomical Society. Received 2018 May 28; revised 2018 August 2; accepted 2018 August 14; published 2018 September 14. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration (NASA). The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This work was funded, in part, by a Summer Undergraduate Research Fellowship (SURF) from California Institute of Technology. W.J.X. would like to thank Johanna Hardin at Pomona College for her advice on implementing the statistical models. G.R. is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1602444. E.C. acknowledges support from NASA through Hubble Fellowship grant HF2-51355 awarded by STScI, which is operated by AURA, Inc. for NASA under contract NAS5-26555. O.A. is an F.R.S-FNRS research associate. V.B. acknowledges government sponsorship; this research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Facility: Keck II (NIRC2). - Software: VIP (Gomez Gonzalez et al. 2017), QACITS (Huby et al. 2015, 2017), Astropy (The Astropy Collaboration et al. 2018), Matplotlib (Hunter 2007), caret (Kuhn 2008), scikit-image (van der Walt et al. 2014), Mongo (
Group:Astronomy Department
Funding AgencyGrant Number
W. M. Keck FoundationUNSPECIFIED
Caltech Summer Undergraduate Research Fellowship (SURF)UNSPECIFIED
NSF Astronomy and Astrophysics FellowshipAST-1602444
NASA Hubble FellowshipHF2-51355
Fonds de la Recherche Scientifique (FNRS)UNSPECIFIED
Subject Keywords:instrumentation: adaptive optics; planets and satellites: detection
Issue or Number:4
Record Number:CaltechAUTHORS:20180914-100923902
Persistent URL:
Official Citation:W. Jerry Xuan et al 2018 AJ 156 156
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:89641
Deposited By: George Porter
Deposited On:14 Sep 2018 17:29
Last Modified:16 Nov 2021 00:36

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