This alog has been edited.

I think that we have understood one of reason that we have had trouble with the stability of our DARM loop over the last few months, which I've tried to explain in a dcc note: T1900148.  The summary is that we are using length to angle decoupling, and because our spot positions are far off center we also are using gains of about 5 in the angle to length decoupling on the ETX PUM (a DARM actuator) to avoid having the angular drive show up in length.  Since the output of the length to angle decoupling doesn't go through the angle to length decoupling filter, the output of that filter couples to DARM through the spot mis centering, which has been large enough to cause instabilities and calibration problems.  

• There is a systematic error in the calibration model of the PUM actuator, 47979, which is not seen when the measurement is made bypassing the drivealign matrix (driving at test)
• The DARM loop has been unstable at 8-9 Hz with the L2P and L2Y decoupling filters off, 47824  This seems likely to be a crossover instability similar to what we might have seen when the PUM gain was lowered to 15 over the weekend: PUM gain lower model

There have been several problems with the stability of the DARM loop over the last few months that might be explained by this, including the 4.2 Hz instability, which was initially solved by adding a boost to the PUM lock filter (47164) but has come back in the last few weeks, There have also been several attempts to change the DARM loop in ways that should have been stable according to the model but weren't, this effect is probably contributing to that difficulty.

This morning Niko and Jenne have been struggling with 4.2 Hz locklosses again, so we took a guess that turning off the L2P and L2Y might help fix the problems at 4.2 Hz, and that turning off the boost in the PUM stage would give us more phase margin for the crossover around 8 Hz and might help us avoid the locklosses we've had with the L2A off.  So far it seems that this is working.  If we can run like this it would make the calibration more accurate and make understanding our DARM loop easier. 

The two most obvious ways to get rid of this parallel DARM actuation path are to set the A2L gains to zero, and turning off the L2A decoupling.  We don't want to set the A2L gains to zero because we do not want the spot to be centered on the optic (power recycling gain, point absorbers). We would expect that turning off the L2A decoupling will increase the size of the ASC control signals.  We have never had accurate measurements for the length to angle decoupling above 2.5 Hz, these filters are rolled off at around 10 Hz. 

Some other conclusions from thinking about these cross couplings more: (updated)

• Making the L2A filters more accurate to higher frequencies doesn't get rid of the cross coupling to length. Rolling the filters off faster (even if that makes them less accurate) would help.  
• If the output of the length to angle filters are summed into the angle drives at the input to the drivealign matrix, this would reduce the unintended length actuation at frequencies where the angle to length decoupling is accurate.  This becomes more important the further our spots are from the center of the opitc. 
• We could copy the L2L filter into the L2A banks, so that it can just be updated if the L2L filter is changed with out revisiting the decoupling.  (Not doing this might be the reason we have had difficulty any time that we have tried to make changes in the L2L filters).
• If we want to do frequency dependent angle to length decoupling, and continue to be able to move the spots around, we could use two parallel filters in the place where P2L and Y2L are currently.  (some modeling might be helpful to decide how important this is).