Adaptive secondary mirror for LBT and its capacitive sensors: how can we calibrate them?
2008
The Adaptive Secondary Mirror (ASM) for LBT arrived in its final characterization phase. In order to use
its large stroke performances and high order correction capability, we needed to develop a fast and accurate
calibration procedure which use optical measurements and correlates them with signals of capacitive sensors co-located
with the actuators. Each capacitive sensor gives a signal related to the local gap between the Reference
Body and the Thin Shell of the ASM. The main issue in this implementation is the inverse relationship between
mirror position (quantity to be measured) and capacitance (sensed quantity), and the evaluation of the effective
reference and stray capacitance for each actuators that affects the calibration parameters. We developed a new
method to correlate interferograms optically describing the mirror shape and the shell pistoning: moreover,
with a non-linear data fit, we solved the unknown calibration parameters to find the linearization formula to be
implemented in the system, giving a optimal solution to the capacitive sensor calibration problem.
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