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Microarcsecond Astrometry with MCAO Using a Diffractive Mask

Published online by Cambridge University Press:  29 April 2014

S. Mark Ammons
Affiliation:
Lawrence Livermore National Laboratory Physics Division L-210 7000 East Ave., Livermore, CA 94550 email: ammons1@llnl.gov
Eduardo A. Bendek
Affiliation:
University of Arizona
Olivier Guyon
Affiliation:
University of Arizona National Astronomical Observatory of Japan, Subaru Telescope
Bruce Macintosh
Affiliation:
Lawrence Livermore National Laboratory Physics Division L-210 7000 East Ave., Livermore, CA 94550 email: ammons1@llnl.gov
Dmitry Savransky
Affiliation:
Lawrence Livermore National Laboratory Physics Division L-210 7000 East Ave., Livermore, CA 94550 email: ammons1@llnl.gov
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Abstract

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We present a new ground-based technique to detect or follow-up long-period, potentially habitable exoplanets via precise relative astrometry of host stars using Multi-Conjugate Adaptive Optics (MCAO) on 8 meter telescopes equipped with diffractive masks. MCAO improves relative astrometry both by cancellation of high-altitude atmospheric layers, which induce dynamic focal-plane distortions, and the improvement of centroiding precision with sharper PSFs. However, mass determination of habitable exoplanets requires multi-year reference grid stability of ~1–10 μas or nanometer-level stability on the long-term average of out-of-pupil phase errors, which is difficult to achieve with MCAO (e.g., Meyer et al. 2011). The diffractive pupil technique calibrates dynamic distortion via extended diffraction spikes generated by a dotted primary mirror, which are referenced against a grid of background stars (Guyon et al. 2012). The diffractive grid provides three benefits to relative astrometry: (1) increased dynamic range, permitting observation of V < 10 stars without saturation; (2) calibration of dynamic distortion; and (3) a spectrum of the target star, which can be used to calibrate the magnitude of differential atmospheric refraction to the microarcsecond level. A diffractive 8-meter telescope with diffraction-limited MCAO in K-band reaches < 3–5 μas relative astrometric error per coordinate perpendicular to the zenith vector in one hour on a bright target star in fields of moderate stellar density (~10–40 stars arcmin−2). We present preliminary on-sky results of a test of the diffractive mask on the Nickel telescope at Lick Observatory.

Type
Contributed Papers
Copyright
Copyright © International Astronomical Union 2014 

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