Sheila, Chris, Stefan In order to tackle the changing sensing matrix problem today we tried to use the soft-hard degree of freedoms. The theory is that the beam overlap is much more sensitive to the hard DoF, thus this should be the one that is easily sensed independent of the sensing-matrix-of-the-day. Then a gain hierarchy could be used to lock down everything. Thus we started with calculation the theoretical hard-soft output matrix. Details can be found at https://gwic.ligo.org/thesisprize/2011/dooley_thesis.pdf For reference, the desin RoCs are ETM: 2245m, ITM: 1934m, giving (with L=3994.5m) g_ETM =-0.779, g_ITM =-1.065. We messed up the initial calculation and worked with ITM = .571 .495 * SOFT ETM -.495 .571 HARD for yaw, and opposite off-diagonal sign for pitch. while we should have used ITM = 1.000 -0.867 * SOFT ETM 0.867 1.000 HARD We verified that the corresponding modes were mostly hard/soft by observing the power drop for a fixed offset in soft/hard. The hard mode has the bigger power drop. Next we engaged DoF's one at a time, starting with the hard modes. For yaw, DoF2 (hard mode) we inverted the last measured sensing matrix and converted it into the soft/hard basis. We got SOFT = 5.14 11.7 * WFS A HARD -.272 1.04 WFS B The hard loop easily closed that way. For the pitch DoF's we first tried the same scheme, but noticed that the hard mode ended up much stronger in what should have been the soft signal. So instead we used the diagonal matrix WFS A -> hard, WFS B -> soft SOFT = 0 1 * WFS A HARD 1 0 WFS B Again closing that loop was straight forward. Next we closed the soft loops with low gain. That seemed straight forward. But then we started noticing that we saturated the PUM with slow signals because the top-mass bleed-off was too slow. (We moved the bleed-off filters to the unused UIM filters to keep the reliev script working.) Before we could fix the saturation issue we noticed that the cavity transmitted spot on the camera did no longer line up with the straight shot beam once all loops were on. So we manually realigned the cavity and tried to reengage the WFS, loop-by-loop. Yaw hard worked fine, but to our surprise pitch hard didn't work at all - the error signal behaviour was completely different. So our tentative conclusion is that our "sensing matrix of the day" doesn't seem provide a clean signal for the supposedly easy hard pitch mode.
(Sheila, Stefan, Chris)
We again closed 3 WFS loops (the hard pitch loop, and both yaw loops), after decentering the beam on the WFS heads. In this state, I compared Rich's new blend filters (Tbetter) and the standard (Tcrappy), finding a modest but repeatable improvement.
The reason we investigated the centering was that, with the beam spot well centered, the response of individual WFS quadrants to pitch drives on ETMX and ITMX was bogus. All quadrants responded with the same phase, as if it had been a length drive. We then suspected that an ugly beam profile on the WFS could make the DC light levels on the quadrants unreliable for centering. Moving the beam spot down on WFS_A, sure enough, there was a "happy place" where a usable error signal for the hard pitch DOF was recovered. We will check the beam quality on the WFS again to see if it can explain this.
The attached spectra show the ALS COMM noise with the ISI RY blends on Tcrappy (refs 10/11) and Tbetter (refs 8/9). Also, Rich advised us of a problem with the RZ blends, so those were left on Start for all measurements.