I started to repeat the measurement described in llo alog 28797 while we were waiting for LLO to come back up around 20:05 UTC. Since LLO is back, I have stopped and put things back to normal, and Cheryl is running A2L before we go back to observing. (This was about 45 minutes of commisioning).
I got as far as tuning an excitation and finding that an offset of around 0.3 in POSY may be about right, but that we need to wait a long time for the OMC ASC loops and the slow kappa C calculation to settle when making these measurements. The template with the excitation is saved at sheila.dwyer/OMC/OMC_alignment_jitter_check.xml We will continue this Wednesday.
Set the gain of PI modes 27 to zero next time you do this measurement.
As Cheryl noted, mode 27 rang up during your work today (mode 19 was just bleed over from 27). Since we're using OMC DCPD as the error signal for this mode, driving OMC ASC loops changes the phase as seen by this mode such that we must've been driving up the PI; mechanical drive up was real, as it was seen by the TransMon QPD.
This mode is nominally stable after the thermal transient, so as long as you're an hour or so into a lock, you can just set the gain of mode 27 to zero during OMC commissioning.
I also took about 10 minutes with the interferometer locked on RF just now to put some offsets into the OMC alignment loops. These dither loops are very slow, it takes about 3 minutes for them to react to an offset change. For POS X introducing an offset of 0.3 decreases the transmitted power to 88%, for POS Y an offset of 0.3 decreases it to 95% of the power for the normal alingment. I didn't get to check the ANG loops, but it seems like they will need larger offsets (2 or 3).