Summary:

1. The two arm lengths can be controlled reliably in the CARM/DARM basis using ALS error signals.
2. With a CARM offset to keep the arm cavitites off resonance, the PRMI can be locked using 3f error signals.
3. On attempting to reduce the CARM offset, I see a drop in the POP22 buildup in the PRC (correlated with the arm powers increasing). Not entirely clear why this is happening.

I ran some sensing measurements at various CARM offsets to check if the PRCL-->REFL33 and MICH-->REFL165 signals were being rotated out of the sensing quadrature as I lowered the CARM offset - there was no evidence of this happening. See Attachment #2. Other possibilities:

• CARM offset dependant offsets in the MICH/PRCL error points?
• Check the RAM due to the EOM? Perhaps the pointing / polarization control into the EOM got degraded.
• Angular stability of the PRC is still pretty poor, getting the angular feedforward back up and running would help the duty cycle enormously.

The IMC went into some crazy state so I'm calling it for the night, need to think about what could be happening and take a closer look at more signals during the CARM offset reduction period for some clues...