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Published July 21, 2014 | Published
Book Section - Chapter Open

A near-infrared tip-tilt sensor for the Keck I laser guide star adaptive optics system


The sky coverage and performance of laser guide star (LGS) adaptive optics (AO) systems is limited by the natural guide star (NGS) used for low order correction. This limitation can be dramatically reduced by measuring the tip and tilt of the NGS in the near-infrared where the NGS is partially corrected by the LGS AO system and where stars are generally several magnitudes brighter than at visible wavelengths. We present the design of a near-infrared tip-tilt sensor that has recently been integrated with the Keck I telescope's LGS AO system along with some initial on-sky results. The implementation involved modifications to the AO bench, real-time control system, and higher level controls and operations software that will also be discussed. The tip-tilt sensor is a H2RG-based near-infrared camera with 0.05 arc second pixels. Low noise at high sample rates is achieved by only reading a small region of interest, from 2×2 to 16×16 pixels, centered on an NGS anywhere in the 100 arc second diameter field. The sensor operates at either Ks or H-band using light reflected by a choice of dichroic beamsplitters located in front of the OSIRIS integral field spectrograph.

Additional Information

© 2014 SPIE. This material is based in part upon work supported by the National Science Foundation under Grant No. AST-1007058. We would like to acknowledge the technical contributions of Jon Chock, Liz Chock, Shui Kwok, Sudha Laven, Scott Lilley, Jeff Mader, Olivier Martin, Bill Randolph, Kevin Tsubota and Ed Wetherell at WMKO, Ernest Croner at Caltech, and Antonin Bouchez at GMT. We would also like to thank our science team of Tommaso Treu at University of California (UC) Santa Barbara and Mark Morris at UC Los Angeles. The W. M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. 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 Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

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