Lorenzo noticed that during the OMC scan of 11h10-11h20 utc the OMC did not lock on the TEM00 resonance. This is due to the fact that the threshold of 1500 set on the B1x_DC_DARM_norm_prod signal was not reached. During the next attempt it happened again (Fig.1). Therefore I decreased the threshold from 1500 to 1000 and in this way the OMC could be locked (Fig.2). When the reaching DC readout the computed optical gain was about 0.9, which means that the OMC alignment seems to be good. Therefore the problem is rather related to a too low DARM line or to a too low DARM offset.

I think that the reduction of the B1x_DC_DARM_norm_prod can only be a temporary patch, but it is not a good solution as there are other modes that may be close to reaching this threshold (see for example the mode reaching 800 on Fig.1). I noticed that the power on B1_PD3 is now very small (order of 1uW). Therefore I would suggest to increase the DARM offset used to acquire the OMC lock. Once the OMC is locked the DARM offset can be reduced to its current value to reach DC readout.