The alignment of the arms required a relevant vertical shift of PR mirror, with respect to the latest nominal position: about 400 um. There is nothing in the suspension behaviour which can suggest that the setting of the vertical sensors should have been changed in order to recover the same vertical position, so one would conclude that the position of PR is actually different (but we cannot be sure). If this is true, one possible explanation is that the beam coming out from SIB1 has changed the vertical inclination by about 50 urad.

This is not a concern at all regarding PR suspension control, or PR centering accuracy. It is just to ask if the intervention done in order to recover BPC could have determined or not such a beam rotation.