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Proceedings Paper

On-sky low order non-common path correction of the GPI calibration unit
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Paper Abstract

The Gemini Planet Imager (GPI) entered on-sky commissioning phase, and had its First Light at the Gemini South telescope in November 2013. Meanwhile, the fast loops for atmospheric correction of the Extreme Adaptive Optics (XAO) system have been closed on many dozen stars at different magnitudes (I=4-8), elevation angles and a variety of seeing conditions, and a stable loop performance was achieved from the beginning. Ultimate contrast performance requires a very low residual wavefront error (design goal 60 nm RMS), and optimization of the planet finding instrument on different ends has just begun to deepen and widen its dark hole region. Laboratory raw contrast benchmarks are in the order of 10-6 or smaller. In the telescope environment and in standard operations new challenges are faced (changing gravity, temperature, vibrations) that are tackled by a variety of techniques such as Kalman filtering, open-loop models to keep alignment to within 5 mas, speckle nulling, and a calibration unit (CAL). The CAL unit was especially designed by the Jet Propulsion Laboratory to control slowly varying wavefront errors at the focal plane of the apodized Lyot coronagraph by the means of two wavefront sensors: 1) a 7x7 low order Shack-Hartmann SH wavefront sensor (LOWFS), and 2) a special Mach-Zehnder interferometer for mid-order spatial frequencies (HOWFS) - atypical in that the beam is split in the focal plane via a pinhole but recombined in the pupil plane with a beamsplitter. The original design goal aimed for sensing and correcting on a level of a few nm which is extremely challenging in a telescope environment. This paper focuses on non-common path low order wavefront correction as achieved through the CAL unit on sky. We will present the obtained results as well as explain challenges that we are facing.

Paper Details

Date Published: 7 August 2014
PDF: 9 pages
Proc. SPIE 9148, Adaptive Optics Systems IV, 91485Q (7 August 2014); doi: 10.1117/12.2056661
Show Author Affiliations
Markus Hartung, Gemini Observatory (Chile)
Bruce Macintosh, Kavli Institute for Particle Astrophysics and Cosmology, Stanford Univ. (United States)
Paul Langlois, Gemini Observatory (Chile)
Naru Sadakuni, Gemini Observatory (Chile)
Don Gavel, Lick Observatory, Univ. of California, Santa Cruz (United States)
J. Kent Wallace, Jet Propulsion Lab. (United States)
Dave Palmer, Lawrence Livermore National Lab. (United States)
Lisa Poyneer, Lawrence Livermore National Lab. (United States)
Dmitry Savransky, Cornell Univ. (United States)
Sandrine Thomas, NASA Ames Research Ctr. (United States)
Daren Dillon, Lick Observatory, Univ. of California, Santa Cruz (United States)
Jennifer Dunn, NRC - Herzburg Institute of Astrophysics (Canada)
Pascale Hibon, Gemini Observatory (Chile)
Fredrik Rantakyrö, Gemini Observatory (Chile)
Stephen Goodsell, Gemini Observatory (Chile)

Published in SPIE Proceedings Vol. 9148:
Adaptive Optics Systems IV
Enrico Marchetti; Laird M. Close; Jean-Pierre Véran, Editor(s)

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