Upgrading the Controller of the Fast Tip-Tilt Tertiary Mirror for the SOAR Telescope

The SOAR telescope fast tip-tilt tertiary mirror, was delivered by the Goodrich Optical and Space Systems Division, Danbury, CT, and integrated into the SOAR optical system in 2004. It consist of a plane, light weighted 655×470 mm elliptical mirror, controllable over a range of ±1 mrad, in two axes, with a required position loop bandwidth of 50 Hz. It operates using the signal from a fast read-out guide camera to generate position commands, in an outer loop fashion. The original tertiary mirror controller consisted of several analog circuit boards, incorporating the position control loop compensation, and power amplifiers. This system was limited by the difficulty of making any modifications, to optimize the control loop, and meet the required bandwidth. The analog controller was replaced with a digital controller based on a National Instruments Compact RIO/FPGA device. This allows the full optimization of the control system, and also allows closing the torque (acceleration) loop using the optical feedback of the guide signal alone, which should result in even higher performance. This paper will describe the models, design, and performance tests, of the new digital control system.