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25 October 2004 MACAO-VLTI piston issue: achieving the interferometry requirements
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MACAO-VLTI is a set of four adaptive optics systems dedicated to interferometry with the ESO 8 meter telescopes in Paranal, Chile. One of the most important requirements for the MACAO-VLTI is to keep the piston variations of the bimorph deformable mirror below 25 nm RMS in a time window of 48 msec. For this purpose, a piston removal algorithm has been developed, that uses a pre-calibrated set of voltages to compensate the natural piston of each influence function. This pre-calibration constitutes a critical laboratory measurement of the influence functions. Using Hadamard matrices, a (64 x 64) Shack-Hartman sensor and a capacitive sensor located at the center of the mirror (back-side), an accuracy better than 1% has been reached to characterize them. Various configurations were investigated to minimize the dynamical residual piston: the control matrix, the loop speed and the loop gain. Particular attention was paid to the influence functions non-linearities. An original indirect method was developed to measure the residual piston in real-time. We present here the methods and results obtained so far.
© (2004) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Liviu Ivanescu, Robin Arsenault, Enrico Fedrigo, Markus E. Kasper, Sylvain Oberti, Jerome Paufique, and Stefan Stroebele "MACAO-VLTI piston issue: achieving the interferometry requirements", Proc. SPIE 5490, Advancements in Adaptive Optics, (25 October 2004);

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