Research On Wavefront Tip-tilt Correction Technology In Adaptive Optics System

Tip-tilt aberrations’ correction is the prerequisite for the adaptive optics system to image stabilization and obtain high resolution images. With the diffraction limited resolution increasing of the large telescopes, the requirement for the tip-tilt correction accuracy increases, that is to say the wave front tip-tilt error rejection bandwidth should be enhanced. So, the research in this thesis focuses on increasing the tip-tilt error rejection bandwidth of the adaptive optics system.Through quantitatively analyzing the wave front tip-tilt aberrations’ amplitude and frequency characteristics in adaptive optics for 1~2 meter telescopes, we obtain that the atmospheric turbulence inducing tip-tilt aberrations decide the tip-tilt correction bandwidth requirement. When an adaptive optics system is used for low orbit satellites imaging, the changing velocity of the tip-tilt aberrations will be intensified by the satellites movement, and the tip-tilt frequency will increase to be 30 Hz. Therefore, the-3d B error rejection bandwidth of the tip-tilt correction system should be about 80 Hz.For the problem of energy loss with the traditional light-splitting tip-tilt detection method, a single Shake-Hartmann wave front sensor is used for both tip-tilt and other high order aberrations’ detection and the separate tip-tilt sensor is saved. The tip-tilt detection and correction is divided into two stages, which not only increases the energy efficiency but also simplifies the adaptive optics system.A Neural Network is used to model the inverse hysteresis of the TTM which is added to the feed-forward loop of the tip-tilt correction loop to compensate the hysteresis. With hysteresis compensation the TTM static hysteresis non-linearity is reduced from 17% to 5% and the open-loop TTM positioning accuracy is increasing by more than 70%.Based on the precise response model of the TTM and WFS, the tip-tilt residual aberration signal detected by the WFS is modified and the time delay of the tip-tilt correction system is compensated. This method highly increased the tip-tilt correction ability and the-3dB error rejection bandwidth increased from 61 Hz to 77 Hz.With a high sampling frequency TTM position feedback loop, the phase lag decreased. With this double feedback loop control strategy which consists of the WFS based tip-tilt correction loop and the TTM position feedback loop, the tip-tilt correction system-3dB error rejection bandwidth increased to 84 Hz. That can satisfy the requirement for the 1~2 meter telescope’s high precision tracking for the satellites imaging at a low orbit.This research and its achievements can prompt the development of the large telescope in obtaining a high resolution image of space targets.