Evan, Sheila, Cheryl
After realizing that T0900511 suggests using REFL 45 for SRC control and only using AS36 for the BS (not any SRC controls), we decided to have a look at the REFL 45 WFS tonight. Indeed, what we see is rather consistent with what is predicted T0900511.
We have known for a while that the phasing of the refl 45 WFS is wrong (we are subtracting a lof of our signals), so we would like to fix that before attempting to use them in an additional loop.
First we disengaged the PRC2 loop (combination of REFL 9+45 A+B to PR3) at 23 Watts, we reverted the phases of the refl wfs to the ones we found in 20811 . We checked that these are still phases where the CHARD signal is in the Q phase, and we checked the transfer function from a CHARD PIT excitation to each quadrant. The phases for the individual quadratures are consistent with a pitch signal.
We also drove PR3 and SR3 PIT and measured the transfer function to each quadrant of refl 45. The measurement was a little tricky for PR3. At first we drove at 8 Hz, and got a signal that looked mostly like a length sigal (same phase on all 4 quadrants). We added a resG filter (which is now loaded in PRM M3 ISCINF), which changed the quadrant TF phases to be somewhat PITCH like. We then drove at 3 Hz to be well within the bandwidth of all our LSC loops, and got quadrant TF phases that would prodce a pitch signal for a pitch drive.
We then looked at the DC signals and moved optics around to see what gave us a good signal. The matrix that you would guess based on T0900511 seems like a good one: REFL 45 A + B I to PR3, REFL 45 A - B to SR3, and we have CHARD signal in REFL 45 A + B Q.
After a lockloss that I caused by moving optics too far, Cheryl brought the IFO back up and we engaged the ASC with the new REFL 45 phases, and a matrix that did not include any refl 9 signals for PR3. This seemed OK although the turn on was a bit rough. At 10 Watts and 15 Watts things also seemed fine, but at 20 Watts we had our usual runaway, with POP 90 increasing.
I left a few alingment offsets different after working on ASC, so SDF was red. We accepted SR3 and PR3 alignment values. We thought about not monitoring these channels, but they don't change every lock.