Summary of the problem:
We have been struggling for several months with what we believe are direct (magnetic) couplings between the BSC-ISI actuators and seismometers . We have tried to measure and subtract the main coupling (Z to RZ). We have obtained substantial improvement in the T240 response, but no improvement in the optics low frequency motion as seen by the optical levers. Our conclusion at his point are, that we need to identify and subtract the couplings in other directions and/or there is noise injection somewhere between Stage 1 and the bottom mass of the quad.
Low frequency couplings analysis:
The two BSC-ISI units locked at LHO (ITMX, ITMY) give us a great opportunity to study the low frequency direct coupling from the actuators to the sensors. This document compares transfer functions of the BSC-ISI when the two stages are floating (unlocked/free) and when the two stages are locked. We use two series of measurements performed on ITMY. The goal is to figure out whether the low frequency features in the inertial sensors response are due to motion (translation or tilt coupling) or due to direct pickup (magnetic fields).
Comments are embedded in the document.
The last slide carries the most important information.
Following up on this open issue, here is a comparison of ITMX and ITMY. Both units were locked during these measurements to identify direct couplings. The first page shows the CPS transfer functions, from the V1 actuators to all local CPS. We can see some differences between ITMX and ITMY cross couplings. The second page shows the T240 response. The results of the two units are very similar.