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Message ID: 17087     Entry time: Wed Aug 17 10:27:49 2022
Author: Cici 
Type: Update 
Category: General 
Subject: Locking X-arm AUX laser 

TL;DR: Got the x-arm aux laser locked again and took more data - my fit on my transfer functions need improvement and my new method for finding coherence doesn't work so I went back to the first way! See attached file for an example of data runs with poor fits. First one has the questionable coherence data, second one has more logical coherence. (ignore the dashed lines.)

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  • The aux laser on the x-arm was still off after the power shutdown, so Paco and I turned it back on, and realigned the oplev of the ETMX - initial position was P = -0.0420, Y = -5.5391.
  • Locked the x-arm and took another few runs - was calculating coherence by I/Q demodulation of the buffers and then recombining the I/Q factors and then taking scipy.signal.coherence(), but for some reason this was giving me coherence values exclusively above 0.99, which seemed suspicious. When I calculated it the way I had before, by just taking s.s.coherence() of the buffers, I got a coherence around 1 except for in noisy areas of the data where it dropped more significantly, and seemed to be more correlated to the data. So I'll go back to using that way.
  • I also think my fits are not great - my standard error of the fits (calculated using the coherence as weight, see Table 9.6 of Random Data by Piersol and Bendat for the formula I'm using) are enormous. Now that I have a good idea that the UGF is between 1 - 15 kHz, I'm going to restrict my frequency band and try to fit just around where the UGF would be. 

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To do:

  • Reduce frequency band and take more data
  • Get fit with better standard error, use that error to calculate the uncertainty in the UGF!
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