As a test, I lowered H1:OMC-ASC_MASTER_GAIN from 0.05 to 0.02 at 17:35 UTC during calibration sweeps. We have been having frequency glitches at 10-15 Hz for the last few weeks, I was wondering if reducing the control signal sent to the OMC would reduce these glitches.
Lowering the gain of the OMC ASC did reduce the control signal, although the rms of the QPD error signals don't seem to change much (attachment). Based on the DMT Omega monitor over the last half hour the low frequency glitches do seem to be reduced, This time doesn't show up on the summary page because the ISC_LOCK state was NLN_CAL_MEAS, not the nominal state.
I haven't put this into the gaurdian yet, but Travis has accepted the change in SDF for this observing segment. If the low frequency glitches do look better we can make this change permanent.
Update: Based on the summary pages there were fewer low frequency glitches while the gain was lowered, but the wind is also dying down simultaneously, so I've turned the gain back up at 19:35 UTC while we are out of observing for SR3 heater tests to see if they come back.The interferometer unlocked shortly after I increased the gain again.
The gain was set to 0.02 for the lock segment from 20:30 UTC to 22:44 UTC, when I set it back to 0.05.
The SR3 heater crew continued to change this gain for different observing segments, and it does seem that lower OMC ASC gain reduces the low frequency scattering. I've lined up some screenshots in the attachment to show this. I will go ahead and put this into the guardian for now. It might be that we could further improve the situation by moving the OMC alignment back to the dithers, we were planning to try a test of that soon.
