In the last days we dismantled and cleaned the last polarizer installed on the third Faraday Isolator. it is the one without the cover since it is used to extract the beam for the two cameras. We can confirm that quite some dust was on both the surfaces of the polarizer. We tried to remove it using the first contact but, then, we found also sone traces (maybe left by the first cotnact itself). Therefore, we made also few acetone bath and, finally, the polarizer was fine.

In order to take a reference, we used the BAB. We installed a mirror at 0 degree and a quarter waveplate to backreflect the BAB and have it on the two cameras. At our best, we were able to reinstall the polarizer recovering the BAB position on the cameras with a difference of 100-150 um wrt to the reference. The alignment of the BAB/SQZ beam on the OMC should be rechecked, also considering the wokrs don on SDB1 this week.

We also took the occasion to install a plexiglass plate above the setup used to extract the beam sent to the cameras in order to better protect the polarizer from dust (see attached pictures). This first protection could be improved in the future.