Near-infrared tip-tilt sensing at Keck: System architecture and on-sky performance

Abstract

The sky coverage and performance of laser guide star (LGS) adaptive optics (AO) systems is limited by the natural guidestar (NGS) used for low order correction. This limitation can be reduced by measuring image motion of the NGS in thenear-infrared where it is partially corrected by the LGS AO system and where stars are generally several magnitudesbrighter than at visible wavelengths. We have integrated a near-infrared tip-tilt sensor with the Keck I telescope’s LGSAO system and recently began offering it for science use. The implementation involved modifications to the AO bench,real-time control system, higher-level controls and operations software. The tip-tilt sensor is a H2RG-based near-infraredcamera with 0.05 arcsecond pixels. Low noise at high sample rates is achieved by only reading a small region of interest,from 2x2 to 16x16 pixels, centered on an NGS anywhere in an 100 arc second diameter field. The sensor operates ateither Ks or H-band using light reflected by a choice of dichroic beam-splitters located in front of the OSIRIS integralfield spectrograph. This work presents an overview of the completed system along with on-sky performance results.Lessons learned and efforts to extend the capabilities and further optimize the system are also discussed.