Digging into the HAM6 ISI and shutter issues, I found that the "high" isolation filters had very low phase margin in DTT open loop measurements, something like ~10 degrees or less. These have not been changed since before O1, so either something has changed the plant or we've been living on the edge. The Matlab commisioning scripts say the installed loops should have had more like ~20 degrees.  I've done a quick adjustment on them, and now the DTT measurments show more like 20-30 degrees. First attached plot shows the X DOF DTT OL measurments. Blue is before, red is after UGF is now about 32 hz and phase margin is about 22 degrees.

I don't think this will have large effect on the performance of the ISI, the difference in gain at 1 hz is a factor of <10, but it went from 10000 to ~ 1000, the gain at 10 is different by less than a factor of 2. The attached pdf shows the matlab commissioning plots for X, Y and RZ, the first 3 pages are the old loops, last 3 pages are the new. Pages 1 and 4 are for the X dof, which should be compared to the first image. Still more digging to be done, I'm not sure why the commissioning scripts are off so much.

The shutter still saturates the actuators, but I've triggered the shutter a half dozen times this morning and the actuators get ~3000-5000 saturations each time it shuts. There are a few hundred more when the shutter resets, so we can probably ride out one cycle, before clearing the saturations, which I think the ISC_LOCK guardian does. I don't believe this fully avoids trips, yet, but re-isolating should be better in the event of a trip.