Disturbance Observer-Based Control of Tip-Tilt Mirror for Mitigating Telescope Vibrations

In the ground-based astronomical telescope, the imaging quality is usually affected by tip-tilt vibrations induced by the telescope structure. Long exposure time of the image sensor is usually required for a good signal-to-noise ratio, which brings the amount of time delays into the control loop of the tip-tilt mirror. Thus, a low control bandwidth is insufficient to compensate telescope vibrations. In this paper, an improved disturbance observer (DOB) control based on only the image sensor is proposed to reject structural vibrations. This new DOB controller is added into the original loop in the tip-tilt mirror control system, so the closed-loop performance can be optimized by designing an appropriate Q-filter. An improved Q-filter is presented to reject large-magnitude vibrations and also to reduce amplifications induced by this controller at nondisturbance frequencies. This proposed controller can achieve a satisfying result in vibration mitigations when vibration frequencies are provided to design the Q-filter. Furthermore, in the tilt-tilt mirror control system, this improved DOB is not restricted by an accurate model such that noise in the loop cannot provide serious influence on vibration mitigation. Both simulations and extensive experiments are carried out to demonstrate that the DOB can earn an extra improvement on the control performance in comparisons with the conventional feedback controller.