Jeff K, Betsy
While the SR2 M3 stage coil had previously been balanced using coil sensors, we tried to use the AS_C PD to repeat the measurement. AS_C is an in-vacuum PD on HAM6 behind SRM. Keita helped me center the beam on the PD with SR2 bias. We then used the SR2 LOCK-IN to drive the SR2 in the pringle mode at 5Hz, with 100k aplitude, at varying coil imbalance states. Unfortunately we could not see much change in the response when the M3 coils were balanced or unbalanced. Attached shows the unchanged peaks between the 2 states of coil unbalanace. SR2 pointing was restored to how we found it when we started.
Things to try next:
- Try on SR3
- Try using AS_B PD - a nearby WFS which does not have a lense in front of it - which maybe caused us a problem
For the record, the two states of balance in the plot are a "fresh" start, with the COILOUTF gains all set to unity, vs those "balanced" values found by Borja (see LHO aLOG 13229). Betsy had tried sweeping both the pitch and yaw imbalance by 20% in either direction and saw an inconsistent story at best -- however, she was sure to continually check for coherence between the drive and response channels and ensure that the IFO configuration was stable enough to provide light for the QPD. We also had WFS_AS_A_DC and WFS_AS_B_DC up with the plan to check if they were any more or less valid measures of the P and Y from SR2. Though there was signal, the assessment of their use for balancing was not as systematically studied as it was for AS_C. Very strange that this sensor didn't work out, which is why we'll try other QPDs/WFS and also look at driving other suspensions.