This morning we had some trouble in the recovery of north arm alignment. The reason was a large misalignment of NI, in a single step occurred at about 10:30 UTC (fig 1). The interpretation is that a fake offset was added to MAR signals, then the controls moved the payload in a different position, as seen by MIR optical lever (out of loop). The event is visible also in the optical lever power; it is very fast and it is coincident with some TCS CO2 turning on (fig 2).
In principle there is nothing warrying in the fact that we have a new offset in the optical lever, but after the event a line at around 5 Hz appeared in the signals. We are not aware of a mechanical resonance at that frequency. Moreover, the line is persistent, has several large harmonics and is not stable in frequency, therefore it could be due to a disturbance entering the sensor. It is worth to identify the source of the step and the disturbance.