Robert in his LHO alog 68053 had suggested to change ITMX and ITMY HV ESD bias to 0V and 60V respectively (from -428V for both the suspension), to reduce the voltage noise.

After talking to Jeff Kissel yesterday, I decided to tackle one optic at a time, and selected ITMY to make the changes. This is because it is the easiest among the two and can be done at any time (and not during any particular ISC_LOCK state).

For ITMY I have changed the HV ESD bias from -428V to +60V and have also accepted the changes in the SDF. See screenshot of ITMY medm screen attached which shows the H1:SUS-ITMY_L3_ESDAMON_DC_OUTMON to be 60V. This was done by changing H1:SUS-ITMY_L3_LOCK_BIAS_GAIN from +1.0 to -0.0141 (in L3 locking filter medm screen).

I am also attached some before pictures (for -428V) for reference - L3_lock with gain of +1.0 and L3 ESD mon filter output showing -428V.

Earlier I also trended a few channels in ndscope to realized that the above changes in ITMY can be done at any time since ISC_Lock doesn't use ITMY HV ESD during locking.

Next up is ITMX.