[Jenne, Anamaria, Sheila]

Sheila reminded us that back in 2015 (alog 19691) they adjusted the P2L coefficient for the OMC ASC, which helped improve scatter from the OMC.  We tried to do that again today, and think we were sucessful with P2L.  We then thought we should give Y2L a try, and that has us pretty confused. 

We drove the OMC SUS LOCK P filter bank at 0.2 Hz with 4500 counts.  I then checked the value in the P2L filter bank to ensure that the fringe wrapping was minimized.  The old P2L value from 2015 was 6.0.  I tried both sides of that (5ish and 7ish), and found that I could increase the scattering shelves by some amount at 5.0, and by the same amount with 7.7.  I'm leaving the value at 6.35, which is halfway between those values.  This is indistinguishable from the 6.0 value, but is halfway between the 2 matching values I found.  Since we're not actually sending our OMC ASC signals through the LOCK filter banks and the drivealign matrix, in order to propagate this value of P2L decoupling, we put values in the OMC ASC output matrix.  So, the pos and ang Y (Y is y-axis here, not yaw...) outputs get fed to the SUS_P as you'd expect for P2P, and then we also feed those outputs to SUS_L with matrix elements 6.35 times the P2P matrix elements.  The PosY -> SUS_L element used to be -670, and it is now -711.  The AngY -> SUS_L element used to be 310 and is now 327. 

We then switched to driving the OMC SUS LOCK Y filter bank at 0.2 Hz with 1000 counts.  With a Y2L value of 0 (we have never used any Y2L decoupling for the OMC suspension), I saw giant scattering shelves.  I then changed the Y2L gain to minimize the scattering shelves, and saw a huge improvement, which you can see in the attached figure.  The Y2L value I need to minimize those shelves is 35, which seems awfully high, but I was able to convince myself that maybe it needed to be large since the input beam to the OMC breadboard is not near the geometric center of the breadboard.  We then started stepping up the OMC ASC output matrix values for PosX and AngX to OMC L, to move them toward being 35 times the PosX and AngX matrix values.  However, we started to see (not nearly as big as the injection scattering) scattering.  This was repeatable, in that I took away the 'decoupling' matrix elements and the scatter went away, put them back, the scatter comes back.

Anamaria and I checked that there aren't any DC gains that are different in the OMC ASC path versus the LOCK filter banks, so it's not clear to me why the Y2L decoupling value that was good for the Lock Yaw drive isn't good for the OMC ASC Yaw drive. For now, we are leaving the Y2L elements of the OMC ASC output matrix at their nominal values of 0 until we understand more what is going on.  One supposition is that it matters what frequency we are moving the OMC at, and that we should be doing this at some other frequency.