Last night we saw ETMX had large 0.43 and 0.5 Hz modes in the oplev, despite the damping. These are long-pitch modes.

To investigate I made some top mass measurements. See the osem centering image. The M0 F1 (top left speed dial) is far from center, with the large alignment offsets. It might be beyond the linear range. I measured some TFs on the top mass. See the second image. Black and read are undamped, without and with the offsets. The these should match, but they don't at the first two modes. Blue and cyan are the same, but with damping. They should match, but they don't. It might be that the first two modes are dipping into the non-linear range. This could limit the damping, and permit these modes to ring up.