I've spent some time doing tilt-decoupling measurements on the BSCs in the last couple days, trying to figure out how to properly do the gain matching for the ST1 Z drive from RZ T240 subtraction.For these measurements: I put both stages in high blends, with both stages fully isolated, turned off all sensor corrections, used a 4th order butterworth bandpassed white noise excitation in awggui to drive at the error point of the isolation loops. In this configuration, I would expect the St2 motion to be the same as the St1 motion up to about 1hz

While doing this I looked at the Stage 2 motion,and there is some funny business there. First attached plot shows the transfer function from St1 Z T240 to the St1 X/Y T240 & St2 X/Y GS13s. At low frequency, a Z to RX/RY coupling would show up in X/Y as a g/w^2 slope. This might be what is happening on ITMY in the first image, below 100mhz. Above 100 mhz, something else is going on, but at least the St2 motion generally agrees with the St1 motion.

In the second image, ITMX below 100mhz looks more like 1/f slope, and the above 100mhz stuff is even worse. For some reason St2 seems to be moving a lot more that St1. 

For both plots, Z-X tfs are red, Z-Y tfs are blue, solid lines are St1 drive to St2 sensors, dashed lines are St1 drive to St1 sensors. 

It's possible that the Z/RX-RY tilt decoupling is bad, so I'll try to remeasure tomorrow. I think this might also be me rediscovering a problem RichM found a while ago in the SEI log and that I didn't understand when we were actively talking about it. Probably also related to an issue that Camillo and Arnaud were looking at for Camillo's BSC model.

I'm assuming the peak at .55 hz in the St2 dofs is some interaction with the quad.