Adaptive optics system based on the Southwell geometry and Improvement on control stability

An adaptive optics (AO) system has advantages of avoiding waffle modes and ease to assemble, when the lenslets of the Shack-Hartmann wavefront sensor (SH-WFS) and the actuators of the deformable mirror (DM) are configured according to the Southwell geometry. However, an AO system which follows the Southwell geometry suffers from the calibration difficulty and from the poor control stability. In this paper, an AO system based on the Southwell configuration is built up and experimentally demonstrated. The calibration problem is solved by adopting a DM which has an appropriate inter-actuator coupling, so that the movement of an actuator can be measured by the neighboring lenslets. The control stability is evaluated by the error propagators, and then, the control stability can be improved by filtering out smaller singular values of the response matrix. It is also shown that a tradeoff between the bandwidth error and the fitting error of an AO system can be made according to the error propagator. Finally, the performance measurement experiments indicate the reasonable ranges for the number of reserved singular values of the response matrix and the integrator gain, i.e. from 81 to 89, and from 0.2 to 0.5, respectively.