Abstract: For segmented large mirrors to be used effectively for astronomy they must be actively aligned and controlled to extreme levels of precision. The figure control problem for a spaceborne far-IR telescope, the Precision Segmented Reflector Project Focus Moderate Mission Telescope, is considered. A two-stage approach is proposed. A figure initialization controller is used to achieve initial phasing and alignment of the telescope using an imaging science detector. A figure maintenance controller keeps the telescope aligned during normal operation using a laser metrology optical truss sensor system. Performance of any figure control system is subject to limits on the controllability of the wavefront. Maintenance controllers are additionally limited by considerations of the observability of the wavefront from the maintenance sensor. Preliminary results for the figure initialization controller are reported. A `Wavefront compensation' method for figure maintenance control that minimizes wavefront errors due to misalignment errors is presented. !13
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