The SOAR Telescope developed by NOAO and sited on Cerro Pachon, Chile is a 4.1-meter Ritchey-Chretien design incorporating active optics (AO). The AO system is composed of PC-hosted control software, a solid primary mirror supported by 120 electro-mechanical actuators, a lightweighted 600 mm secondary mirror supported by a six-degree-of-freedom hexapod mechanism, and a lightweighted 600 mm tertiary mirror controllable over a range of +-100 μrad in two axes with a bandwidth of 50 Hz. The tertiary mirror assembly is in turn mounted on an azimuthal bearing that allows the output bundle of the telescope to be directed to one of five science instruments located at nasymth and bent-cassegrain foci. This paper discusses the active tertiary mirror assembly from the perspective of a control system designer. After a brief overview to establish the tertiary mirror's place in the overall AO system architecture, the paper presents the requirements that drove the design and some of the design's salient electrical and mechanical features. A model representing electrical and mechanical aspects of the mirror and controller is presented and observed performance metrics such as frequency response, NEA, and measures of servo robustness are compared with values predicted by this model. The paper discusses a number of the design challenges that arose from the requirement to control a massive load with great precision and over a relatively large bandwidth and concludes with "lessons learned."
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