Yesterday, it was planned o relock the north arm in order to send the reference beam reflected by the NI mirror on the squeezing bench for the fine alignment of the squeezer beam.

The ISC team called us since they were not able to find the beam neither at NE nor the B2 one on SIB2. They noticed the scattered light coming from the beam passing through the meniscus lens was not centered anymore on it.

First, we looked at the IMC and RFC cavity signals and everything looked as usual. The IB bench seemed to be in a position similar to the one we had last week when we installed the FI on the SDB1 bench. Then, we moved the PR  mirror quite a lot in order to see if we were able to find the B2 beam (without success).

We started to check the beam position on the optics of SIB1 bench. The beam is passing properly through the Faraday isolator. It seems to pass properly inside the IPC.

It looks properly reflected by SIB1_M3 and M4 and it seems well centered on SIB1_M1 and M2 parabolic mirrors. Then the beam looks miscentered by several cms on the meniscus lens.

Paolo, Valerio and Ettore checked that the PR mirror was fine.

Franco checked that there were no accidental movement of SIB1 actuators: no displacemeent of the motors has been asked since the last commissioning period.

We also asked to Antonio to open and close the IB t PR cryolink valves without noticing any significant change.

Up to now we have no clue about the origin of the problem. The beam was fine last friday (since it was used on the Detection system for OMC related works).

Between Friday and yesterday afternoon something wrong happened.

In consequence, it has been decided to anticipate the opening of IB tower. We will enter the tower tomorrow morning to check what could be wrong on the bench.

We plan to block the bench in a position close to the suspended one to check the beam path on SIB1.

The beam is now blocked on the laser bench.