Last night, we tried Z HEPI sensor correction on the BS. It did not go well. When I look at the data this morning, I didn't see the usual reduction in Z motion on the ISI seismometers, with low frequency re-injection. Instead, all we got was bad low frequency reinjection. The attached PDF shows performance from last night on the first 2 pages, with perfromance from the previous night on the last 2. The attached png shows the coherence between the ground STS and the ISI's St1 T240's. The solid lines are with sensor correction on, dashed are with sensor correction off. For some reason turning on Z sensor correction reduces coherence in the Y direction. Jeff and Fabrice have both suggested that maybe the op-lev damping is interfering with the sensor correction. The ground STS that was used for the sensor correction also is having issues, which Hugh has noted before. We'll keep digging.