Fabrice, Hugh and I had a look at the arm cavity signals with sensor correction on and off this afternoon.  The arm cavity locking was not stable because there was no feedback to the top mass, (it turns out the PRMI guardian was reseting the XARM gain to 0).  Anyway, I learned something usefull....

With the sensor correction on, the ETM Stage 2 X drive is in phase with our refl controls signal as well as the slow feedback from the COMM PLL to the IMC VCO, using up about 1/4 of the range of each.  With the sensor correction off (Which Fabrice did by setting H1:ISI-ETMX_ST2_SENSORCOR_X_MATCH_GAIN to 0 from the normal setting of 1.15) this signal is greatly reduced, and we use less of the range of both VCOs.  However, the OPLev sees more pitch motion.  Fabrice says that we can also use a compromise sensor correction that would give less benefit in pitch and also introduce less longitudnal motion. 

Once Daniel and I got the top mass feedback going, the loops are all fine.  Our drive to the top mass clearly introduces pitch motion on the optic, so we need to diagonlaize the drive to the ETM, which Arnaud and Jeff agreed to help us with. 

Right now the arm cavity has been locked for about an hour, I will leave it that way for the night, with the alignment dithers, and top mass feedback on.  The COMM PLL automation is also running overnight. We have low winds (below 50th percentile) and low microseism tonight.

I also started a rudimentary LSC_XARM Guardian just to manage the feedback to the top mass, so we will not shoot ourselves in the foot as much in the future. It's not working yet, but coming soon.