Rana, Alexa
We repeated a similar procedure as to alog 14022 but this time for L2P. At low frequency, i.e. 0.01Hz we found the optimal gain to be 0.0106. As Stefan mentioned in his alog, the L2P coupling seems to fall off faster than 1/f^4 at high frequnecies (ie about 3Hz). Therefore at high frequencies we want our filter to be essentially zero. The attached screen shot shows the original filter (FM1 red trace) and two new filters (FM2 blue trace, FM3 green trace). FM2 has a gain at low frequency to match our measurement. The phase is flipped so that the gain in the filter module can remain -1. Then the filter falls off as 1/f^2. FM3 is intended to provide the user an option to include a notch at the resonance at 0.7Hz as seen in Stefan's L2P TF. The other two resonances at 1.1Hz and 0.5Hz seem to cancel with the P2P TF, so we did not include these.
As a reference, the FM2 filter: zpk([],[0.766044+i*0.642788;0.766044-i*0.642788],-0.0106,"n") and FM3 filter: notch(0.749,10,30)
After we turned on this filter and locked the michelson on the dark fringe, we quickly noticed that the PIT motion was actually worse. This was due to two things. First, I had not saved and properly loaded the filter, so the sign was off. Second, my phase at mircoseism was falling off from 180 too quickly. You can see this in the first screenshot by comparing it to FM1. With these corrections, the PIT motion appears to be better.
For reference FM2: zpk([],[0.098242+i*1.12291;0.098242-i*1.12291],-0.0106,"n")
Tonight I will run L2P transfer functions to examine the residual motion. These measurements will start at 1am (and require that the BS oplevs are turned off).
I have to apologized that I accidentally had the PRMI locked in the first a couple of minutes of Alexa's scheduled measurement. As soon as I noticed it, I switched the LSC and oplev damping servos off, but this must have screwed up some data points around 10 Hz.