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21 March 1997 Real-time modal control implementation for adaptive optics
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Proceedings Volume 2871, Optical Telescopes of Today and Tomorrow; (1997)
Event: Optical Telescopes of Today and Tomorrow, 1996, Landskrona/Hven, Sweden
This paper presents the electronics, computing hardware, and computing software currently being built to provide real time modal control for a laser guide star adaptive optics system. This approach offers advantages in the control of unobserved modes, the elimination of unwanted modes (e.g. tip and tilt) and automatically handles the case of low resolution lens arrays. In our two step modal implementation, the input vector of gradients is first decomposed into Zernike polynomial modes by performing a least squares. The number of modes is assumed to be less than or equal to the number of actuators. The mode coefficients are then available for collection and analysis or for the application of modal weights. The control loop integrators are at this point in the algorithm. To calculate the DM drive signals, the mode coefficients are converted to the zonal signals via a matrix multiply. At closed loop bandwidths slightly below maximum, it will be possible to do the full two part multiply in real time. Thus the modal weights may be changed quickly without recalculating the full matrix. When the number of gradients measured is less than the number of actuators, the integration in the control loop will be done on the lower resolution grid to avoid growth of unobserved modes. These low resolution data will then be interpolated to yield the DM drive signals.
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Allan Wirth, Joseph Navetta, and Bruce Martin Levine "Real-time modal control implementation for adaptive optics", Proc. SPIE 2871, Optical Telescopes of Today and Tomorrow, (21 March 1997);

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