Observing B7_DC and B8_DC all along three days, it was evident an anomalous fluctuation of B8_DC (fig 1). A possible explanation was that the fluctuation was not really due to an unstable power in west arm, but to a not perfect alignment of SWEB with respect to the beam.

Profiting of the unlock, a scan of SWEB position in TX and TY has been performed, trying to maximise B8_DC (fig 2). Finally the reference position has been changed from 100 to 120 for TX and from 1790 to 1785 for TY.

It is worth to remark that the window of TY values allowing a stable power on the PD looks quite narrow: not more than 30 urad. If I remeber well, it was at least the double.