Development of a cold chopper for TAO/MIMIZUKU: Repetitive Control

In ground-based mid-infrared observations the background radiation must be removed. Chopping is a background removal method requiring fast switching of the observation field. For MIMIZUKU, the mid-infrared instrument for the TAO telescope, we have developed a cold chopper which switches the observing field by tilting a movable mirror inside MIMIZUKU, instead of tilting the large secondary mirror. We require a short transition time, sufficient amplitude, high frequency and steadyness for observation in the chopper movement. With Repetitive Control we significantly increase performance by iteratively improving a feedforward trajectory and continously adapting to changes in the nonlinear dynamics. This allows for much shorter transition time (<30 ms) and more freedom in the design of a feedback controller. Furthermore, repetitive disturbances originating from the cryo-cooler can be countered thus improving stability on sky. Controller design, stabilisation, choice of reference trajectory, real-time computability and performance trade-offs are subjects in this research.