Microarcsecond astrometry with MCAO using a diffractive mask

S. Mark Ammons, Eduardo A. Bendek, Olivier Guyon, Bruce Macintosh, Dmitry Savransky

Research output: Chapter in Book/Report/Conference proceedingConference contribution

1 Scopus citations


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.

Original languageEnglish (US)
Title of host publicationFormation, Detection, and Characterization of Extrasolar Habitable Planets
PublisherCambridge University Press
Number of pages6
ISBN (Print)9781107033825
StatePublished - Aug 2012

Publication series

NameProceedings of the International Astronomical Union
ISSN (Print)1743-9213
ISSN (Electronic)1743-9221


  • astrometry
  • instrumentation: adaptive optics
  • stars: planetary systems
  • techniques:high angular resolution

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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