[Vaishali, JimW, Jenne]
During maintenance, while no one needed the IMC, we tried out our new L2A decoupling filter for MC2. Recall that Jim found that it would be helpful in alog 35330, and we measured and fit TFs for it in alog 35397.
We found that (a) the pre-existing L2A decoupling was in fact worse than doing nothing at all, and (b) that our new filter works well.
In the attached screenshot, the upper left corner is the Foton Bode plot of our decoupling filter. In the other 3 DTT panels we have the coherence and transfer functions of 3 different MC2 L2A configurations. For these measurements, the IMC was unlocked, but MC2 still aligned. The red traces in all panels are with no L2A decoupling at all. The blue traces are with the old L2A "decoupling" gain. That was a flat gain, and clearly was making things worse than doing nothing at all, over a pretty broad frequency range. The green traces are with our new L2A decoupling filter on, and we win almost 20dB of isolation below a few hundred mHz.
We have accepted the new L2A settings for MC2 in the safe and observe SDF files, so they should always be on now.
I've looked at the coherence between MC2 M1 length drive and the two pitch witnesses (MC2 Trans qpd and MC2 M1 Pit) during locks before and after the install of the new L2P feedforward, and things look better. Attached plot shows the coherence between MC2 M1 L and MC2Trans (top plot, blue is before, red after) and MC2 M1 L and MC2 M1 P (bottom plot, blue is before, red after). Below .1 hz the coherence is much better, which should help for wind and earthquakes, which is what I was looking at originally anyways. Above .1hz the coherence is pretty much unchanged, not sure why, maybe the motion is dominated by the table motion at those frequencies?
Always useful to actually post promised plots...
