Attenuation of tonal disturbances on the Keck 10-meter telecope

In this paper, we describe a new design for the pointing control system of the Keck Planet Imager and Characterizer (KPIC) instrument at the W. M. Keck Observatory. KPIC is a single-mode fiber fed instrument that sits behind the Keck II Telescope's facility AO system. Though the AO system handles the bulk of the wavefront and tip/tilt pointing correction, KPIC provides a second-stage tip/tilt correction to ensure optimal coupling into the fibers, especially for the fiber nulling mode which places stringent requirements on the pointing residuals. In designing the new controller, several challenges had to be overcome, including developing models of the tip/tilt mirror (TTM) response and tip/tilt pointing disturbances. A Schroeder stimulus signal was used to identify the TTM plant model as well as the camera latency. The tip/tilt disturbances were characterized with centroid measurements in open loop (AO system active but KPIC TTM held fixed), which revealed both broadband atmospheric disturbance energy as well as tonal energy coming from structural vibrations of the telescope and possibly line noise from the facility power. Due to the limited bandwidth achievable with the system sample rate and sensing latency, this paper focuses on attenuation of the tonal energy, which adds a significant amount to the overall RMS jitter. To do this, a novel control technique is developed, which we term "feedback assist", which adds robust notches to the disturbance sensitivity at the known tonal frequencies. This technique borrows some results from the adaptive control literature. In particular, it conjugates the phase of the secondary path transfer function using passive ringers. Simulations of our feedback assist control technique demonstrate a 20 to 25% improvement in the RMS jitter in both axes relative to a feedback-only design. The on-sky results showed a combined 16% improvement between the two axes relative to feedback alone over an ensemble of three trials with feedback-only and three trials with feedback assist. Future work will address the attenuation of the broadband atmospheric disturbances using adaptive estimation techniques.