For the past few months we have been suffering from an unwanted stage 1 Z to stage 1 rZ coupling (the attached plot Z to Z Subtraction shows an example).  It seems to have two component which show up in the T240 rZ signal.

   There is a 1/f^2 component (in Force to displacement units, the plot is in force to velocity) and a flat component (again in Force to displacement)

   The 1/f^2 part does not seem to represent center of mass motion of the platform, as shown by the optical levers (the black line in the Z to OpLev Yaw plot), it is not clear

for the flat component yet.

   Speculation for the 1/f^2 has centered on magnetic coupling or local deformation of stage 1, current measurements and FEA on these two effects are off by about the same amount.

   Since we believe that the signal is not CoM motion we are going to subtract it from the T240 rZ signal.  The plot Z to Z Subtraction shows the predicted amount of subtraction based on the fitted filter.

where the red line is (TF - FIlter)/TF so we are hoping to get a factor of~ 50 subtraction.

    The plot Z to OpLev Yaw  shows some results.

  The BLUE lines shows the transfer function from ISI stage 1 Z drive to the optical lever (solid line is yaw, dotted it pitch) while the ISI is in high blend with no T240 in the control path. So we expect to see

any mechanical coupling. 

   The PURPLE lines shows the same transfer function with the stage 1 of the ISI in low blend (~40mHz) and the T240s in the loop. Whatever extra pickup exists in the rZ T240 signal will be imposed onto the motion of the ISI

and show up in the optical lever signal (pitch as well as yaw because the suspension is not at the center of the ISI table and there is a translation to pitch coupling).

   The Black lines are the same transfer functions with the subtraction path enabled (i'm not even going to try and explain how we got 3 orders of magnitude of suppression)

   One disclaimer I calculated a filter gain of 3 and ended up using a gain of 3.24