The MC alignment servo wasn't great in the last 1 hour or so as it kept disturbing the MC lock. It was found to be due to some offsets in the MC trans QPD signals.
I put some values to cancel the offsets and then the lock became stable.
This is a first aid. So we need to take a closer look at the QPD signals and also probably the spot position on the QPD.
The symptom was that every time the alignment servo was engaged, at the beginning the amount of the transmitted light went to 27000 counts, which is good.
However, then the amount of the transmitted light slowly decreased in a time scale of ~ 20 sec or so, ending up with destruction of the MC lock.
According to the time scale I suspected that the servos using the trans QPD signals were doing something bad because their control width had been designed to be slow and slower than the rest of the servo loops.
I switched off the servos, called C1:IOO-TRANS_PIT and C1:IOO-TRANS_YAW and found the MC stayed locked stably with 27000 counts of the transmitted light.
Leaving the trans QPD servos off, I zeroed the offsets and then switched them on. It worked.
The values below are the current offset that I put.
C1:IOO-MC2_TRANS_PIT_OFFSET = -0.115203
C1:IOO-MC2_TRANS_YAW_OFFSET = -0.0323576