Adaptive optics for small telescope systems

Abstract

Small telescope systems are commonly used in various applications such as astronomy, Earth observation and free-space optical communication. All those applications are susceptible to atmospheric turbulences, which limit the achievable performance. AO is used for correcting atmospheric turbulences in reflective telescope systems, by measuring the wavefront distortions and compensating them with deformable mirrors, thus restoring clear and sharp images. AO systems are highly complex and therefore hardly used in small telescopes (< 0.5 m), but nevertheless enable significant improvements in imaging quality for this telescope class.In this thesis an AO system designed for a small telescope system is implemented and evaluated. A combination of a dedicated tip-tilt compensation system together with a higher order AO system is used to maximize the disturbance rejection. To measure aberrations, a Quad Photo Diode (QPD) sensor and a Shack Hartmann wavefront sensor (WFS) are used. The tip-tilt compensation system is implemented with a Fast Steering Mirror (FSM) and a QPD. Using PID control a -3 dB closed loop bandwidth of 1.045 kHz is achieved. The higher order compensation system uses a WFS and a Deformable Mirror (DM). An Integral controller is implemented resulting in a closed loop bandwidth of 42.6 Hz. The two proposed subsystems are used standalone as well in two combinations to analyze the achievable performance. Furthermore, the influence of a fast tip-tilt compensation system on higher order aberrations is analyzed and the dynamics of the DM are evaluated.The AO system is characterized using a custom-made optical turbulence generator (OTG) oriented at the measured disturbance spectrum of a typical LEO object tracklet. This provides a reproducible disturbance to characterize the performance of the individual compensation approaches. The wavefront error is recorded using the WFS. In comparison to the uncompensated case the QPD-FSM loop reduces the wavefront error by a factor of 6.02 and WFS-DM by 4.56. With both systems combined an error reduction by a factor 10.88 is achieved, resulting in a clear and stabilized spot on the camera.