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TCS (TCS)
punturo - 18:53 Friday 04 December 2009 (25836) Print this report
Mirror resonant mode identification
As underlined in the past, the main problem in the method that measures the temperature of the mirror through its resonant modes is the fact that, using the dark fringe signal, it is possible to erroneously identify who is who. In fact, for example, the input mirrors have resonant modes close in frequency and, if commissioners play too much with the TCS the peaks could overlap and swap. This was not crucial in the past, since the mirror identification monitor was just used offline. Now, as described here (Etalon servos) a servo loop is realized on that monitor (that it is running on the PC of my office !!!!!) and a mistake in the mirror identification could cause a wrong temperature correction. In all the VSR2 the resonant mode frequencies of the input mirrors have been very well separated, but to improve the safety something can be done. In the attached figure, first plot, the WI (5585-5586Hz) drum mode and the NI (5587-5588Hz) drum mode (in the dark fringe) are plotted for 4 different dates (4/09 - 4/10 - 4/11 - 4/12/2009). In the second plot the corresponding signal in Pr_B5_d2 is reported. In this second channel the NI signals is higher and its larger amplitude could be used, in the next version of the monitor, to make the peak identification more sicure.
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