Keck I LGS-AO system scheme (Wizinowich et al. (2006), van Dam et al. (2006)). The high-order (all modes expect TT) loop is operated with the light coming from the LGSs, while the low-order loop is operated with NGS light. In the low-order loop, the TT and slow focus (Z₄) sensing can be both performed using images provided by TRICK. The current approach for slow focus sensing is to use the LBWFS. The measured focus values are then used to compensate for the focus error by moving the FCS. OSIRIS is the science camera.