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Infrared tip-tilt sensors (IR TTSs) have been deployed on three differentlaser guide star adaptive optics (AO) systems on three different telescopes.These IR TTS benefit from the high-order loop PSF sharpening in the nearinfrared, hence they provide a low tip-tilt residual and a good sky coverage.Nevertheless, these IR TTS are challenging and their use in AO is limited. Inthis paper, we outline existing IR TTS, provide on-sky performance results anddescribe our experience using IR TTS and along with plans for the near future.The second part of the paper deals with unresolved challenges for IR TTS. Theseinclude algorithms and loop stability in the low Strehl regime, using IR TTSsto measure higher-order modes and guiding on multiple guide stars withdifferent magnitudes.

Infrared tip-tilt sensing: on-sky experience, lessons learned and unsolved problems