[Jenne, Kiwamu]
Since we kept losing lock at Noise Tunings, on the third time around I went to step through the state line-by-line. But, we lost lock in the same way at the first line: DC coupling the ISS second loop.
Kiwamu pointed out that since the light level on PDA changed yesterday (alog 30871), we might need to change the offset in the 2nd loop. This caused us to realize that since we had forgotten to SDF-accept the offset we picked on Monday (alog 30817), there just wasn't any offset at all. In the end, Kiwamu and I chose -0.25 for the offset H1:PSL-ISS_SECONDLOOP_AC_COUPLING_SERVO_OFFSET, since that is the value that kept the diffracted power constant when the 2nd loop was closed. Note that on Monday we changed the way the AC coupling is turned off, so that this offset doesn't integrate up forever by turning it off at the output rather than the input. This is in guardian.
After this, we noticed that all of the power seemed to be on 2 of the inner loop PDs, rather than balanced between all 4. This is certainly a result of my moving IM3 earlier today (alog 30910). We found Robert's alog 29583, to remind ourselves which picomotor was which, and moved motor #1 on the HAM2+oplev controller, which is labelled PSL ISS QPD/PD (PM1). This is the motor closest to the ISS array. We used Large steps at a Sprint, since this was not in-loop. The positions for the picomotor started at (0,0) for (X,Y), and ended at (-1900,0).
Although the QPD was at about 0.8 in yaw before yesterday's alignment shenanigans, we went for center. As Robert pointed out in his alog, center on the QPD maximizes (and roughly equalizes) the power on all 4 diodes. Kiwamu and I checked that this was better also by driving a line in IM3 and seeing that it did couple to IMC power when the beam was falling off the diodes, and then didn't couple to IMC input power when the beam was centered. As one might expect since we didn't have to move in pitch, driving a pitch line didn't cause problems either time.
At next acquisition we had no problems with the ISS, and are now sitting in NomLowNoise.
Not the best design choice, since the AC coupling button also turns off the 3rd loop. Why do we need this offset now, when it wasn't needed in the past? On the other hand, we forgot to recalibrated the ISS PD Norm, when we changed the light level on PDA. This needs to be close to 1, when we are at 2W.