Wavefront Compensation Segmented Mirror Sensing and Control
- Created: Tuesday, 01 May 2012
Six degrees of freedom can be sensed at each segment edge.
The primary mirror of very large submillimeter-wave telescopes will necessarily be segmented into many separate mirror panels. These panels must be continuously co-phased to keep the telescope wavefront error less than a small fraction of a wavelength, to ten microns RMS (root mean square) or less. This performance must be maintained continuously across the full aperture of the telescope, in all pointing conditions, and in a variable thermal environment.A wavefront compensation segmented mirror sensing and control system, consisting of optical edge sensors, Wavefront Compensation Estimator/Controller Soft ware, and segment position actuators is proposed. Optical edge sensors are placed two per each segment-to-segment edge to continuously measure changes in segment state. Segment position actuators (three per segment) are used to move the panels. A computer control system uses the edge sensor measurements to estimate the state of all of the segments and to predict the wavefront error; segment actuator commands are computed that minimize the wavefront error.
Translational or rotational motions of one segment relative to the other cause lateral displacement of the light beam, which is measured by the imaging sensor. For high accuracy, the collimator uses a shaped mask, such as one or more slits, so that the light beam forms a pattern on the sensor that permits sensing accuracy of better than 0.1 micron in two axes: in the z or local surface normal direction, and in the y direction parallel to the mirror surface and perpendicular to the beam direction.
Using a coaligned pair of sensors, with the location of the detector and collimated light source interchanged, four degrees of freedom can be sensed: transverse x and y displacements, as well as two bending angles (pitch and yaw). In this approach, each optical edge sensor head has a collimator and an imager, placing one sensor head on each side of a segment gap, with two parallel light beams crossing the gap.
Two sets of optical edge sensors are used per segment-to-segment edge, separated by a finite distance along the segment edge, for four optical heads, each with an imager and a collimator. By orienting the beam direction of one edge sensor pair to be +45º away from the segment edge direction, and the other sensor pair to be oriented –45º away from the segment edge direction, all six degrees of freedom of relative motion between the segments can be measured with some redundancy.
The software resides in a computer that receives each of the optical edge sensor signals, as well as telescope pointing commands. It feeds back the edge sensor signals to keep the primary mirror figure within specification. It uses a feed-forward control to compensate for global effects such as decollimation of the primary and secondary mirrors due to gravity sag as the telescope pointing changes to track science objects.
Three segment position actuators will be provided per segment to enable controlled motions in the piston, tip, and tilt degrees of freedom. These actuators are driven by the software, providing the optical changes needed to keep the telescope phased.
A novel aspect of this design is the angled optical edge sensor configuration. By angling the light beam of each edge sensor pair at + and –45º, a full four degrees of freedom can be sensed at each segment edge by each sensor pair. This configuration results in full observability of the segment optical state, and is crucial in achieving the needed performance.
The software incorporates a structural/ optical model of the telescope in a least-squares or Kalman filter-based estimator/ controller, which processes the optical edge sensor signals in a low-bandwidth control loop. The estimator produces an estimate of the optical state of the mirror, and predicts the resulting wavefront error, balancing current against previous measurements in a least-squares optimization. The controller calculates the segment actuator commands that will minimize not the sensor signals, but the predicted wavefront error. This formulation allows the controller to compensate for the optical effects of motions (such as lateral sag of the segments) that are not directly actuated. The result is far better performance than could be achieved using a conventional sensor-nulling approach.
This work was done by David C. Redding, John Z. Lou, Andrew Kissil, Charles M. Bradford, David Woody, and Stephen Padin of Caltech for NASA’s Jet Propulsion Laboratory.
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