David Greenwald (The Boeing Company)
Keywords: Adaptive Optics, Imaging
Abstract:
Since first light in 1997, the Advanced Electro-Optical System (AEOS) telescope at the Maui Space Surveillance Site has used an active system for figure control that applies forces on the primary mirror and positions the secondary mirror to minimize wavefront aberrations. Periodically a wavefront optimization loop is closed with a Shack-Hartmann WaveFront Sensor (WFS), 84 primary mirror force actuators and three secondary mirror translation actuators. This optimization loop is used with a series of stellar targets to find coefficients for each force or position in a sine and cosine of elevation model. During normal telescope operation when the WFS is not in use, this elevation angle dependant model is used to control the primary mirror forces and secondary mirror positions. Recently the system was upgraded with new computers, electronics and algorithms. The primary goal of the upgrade was to replace obsolete and no longer maintainable hardware with secondary goals of reducing the effort required to update the wavefront model, and improving the final operational wavefront performance. This paper discusses the algorithms implemented to achieve the secondary goals and initial performance results. In order to eliminate erroneous data from the WFS, the processing algorithms were modified to dynamically assign pixels on the WFS camera to lenslets, and closed loop tracking of the gimbal was implemented using a camera that shares the focal plane with the WFS. These changes permit the elimination of human operator review from the wavefront optimization loop. The original system collected data for either a single star or a series of stars and then replaced either the constant or the complete model at the end of a data collection session. In the revised system, each wavefront measurement is used for a Kalman update to the model. Operationally, the Kalman updates allow data to be collected intermittently as time is available between other telescope tasks. By combining the relative measurement uncertainty estimated from the high spatial frequency content of each WFS measurement with the model uncertainty to compute the contribution of each measurement to the model update, the resulting model is expected to be more accurate than models generated by the old system that assumed all star observations at a given elevation angle were equally accurate. Additionally, a control loop for modal amplitude was added to generate the force commands to each actuator in real time, mitigating the wavefront degradation due to the relatively common failure of individual force actuators.
Date of Conference: September 11-14, 2012
Track: Adaptive Optics and Imaging