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.
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Liviu Ivanescu, Robin Arsenault, Enrico Fedrigo, Markus E. Kasper, Sylvain Oberti, Jerome Paufique, Stefan Stroebele, "MACAO-VLTI piston issue: achieving the interferometry requirements", Proc. SPIE 5490, Advancements in Adaptive Optics, (25 October 2004); doi: 10.1117/12.551854; https://doi.org/10.1117/12.551854


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