Original Date: 04/26/1999
Revision Date: 01/18/2007
Best Practice : Phased Array Mirror Extendible Large Aperture
(equipment: Wyco 6000 Interferometer, Blue Line Quad Cell Wavefront Sensor, Lateral Effect Diode Wavefront Sensor)
The Phased Array Mirror Extendible Large Aperture (PAMELA) is the first telescope to have a fully-adaptive segmented mirror. This mirror consists of 36 hexagonal segments, each measuring seven centimeters across flats. Each mirror segment is tripod-mounted on three voice coil actuators which provide automatic tip, tilt, and piston adjustments of each segment relative to its neighbors. In 1993, Marshall Space Flight Center (MSFC) acquired the PAMELA from the Command Sciences Corporation in Tucson, Arizona, and determined that the telescope was functional but needed to be qualified.
MSFC decided to use the PAMELA as a development test bed. To achieve this goal, the Center made numerous innovative improvements to the optical performance of the telescope. As received, some individual mirror segments exhibited peak-to-valley surface irregularities of up to 0.375 wave, as measured by a Wyco 6000 Interferometer. Consequently, MSFC’s Optics Group removed all mirror segments and measured their radii of curvature. Eight segments were chosen to be refigured.
After reassembling the telescope, the Group discovered that actuator displacements as small as 0.005 inch could introduce one-wave, peak-to-valley, surface perturbations. Stress analyses of the mirror segments, as mounted on their substrates, were performed to verify that stresses during actuator movements were below yield or ultimate. The Group reasoned that since the segments were easily distorted, this characteristic could be used for flatness adjustment if a precision, repeatable method of adjustment could be devised. A 0.024-inch thick ring gasket was installed between the three actuators and the mirror segment substrate. This gasket acted as a stiff spring when the actuators were reinstalled, allowing precise tip and tilt adjustments at each actuator dependent on mounting screw torque. This adjustment capability reduced mirror segment flatness errors from 0.488 to 0.038 wave, approximately a 13:1 improvement.
The resonant frequency of the mirror segment, as mounted on its substrate and tripod-supported by the three voice coil actuators, was found to be in the 60 to 70 Hertz (Hz) range. This resonance presented a stability problem to the mirror segment positioning controller. A small, viscoelastic, multilayer, cantilever beam was constructed and added to each interface of the voice coil pistons with the mirror segment substrate. The resonance range of the mirror segment was effectively damped at 9.4%. In addition, this optimally damped system could be easily tuned and adjusted with the existing controller gain.
Additionally, the introduction of a Blue Line Quad Cell Wavefront Sensor calibration device permitted linear calibration to be achieved, with a noise floor improvement of 40 times better than the original Lateral Effect Diode Wavefront Sensor. Today, the image sensor spot intensity profile of the six-phased PAMELA has now been demonstrated as closely approaching theoretical reference accuracy.
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