After the morning shift we have re-checked the alignment signals, focusing on the reason why they are worse than in the past week.

As the error signals which were not working were all coming from B5, the first idea was to check the behaviour of this quadrant. I took a look to the data of last week and we saw that the H/V DC signals were saturating, while the DC reaching the QPD was as usual (Figure 1). This means that the beam was reaching only one sector of the quadrants, which would spoil any RF error signal.

The origin of this problem is that as we use the DC for the COMMp, we almost turn off the galvo loop by decreasing its gain. So if the beam gets misaligned on the QPD, the galvo is unable to center it. As for the moment the COMMp is open, I temporarily removedc the offsets set by hand and turn the galvo loop back on. At this point we recovered the vertical signal of the PR.

However, we it was necessary to align by hand the horizontal position of the PR in order to recover the horizontal one. At this point we were able to relock the PR loop in full bandwidth and the BS one in drift control. We re-tuned the other offsets later on, getting a pretty stable ITF. In this configuration the TCS crew will repeat the measurements of the morning, to check if this problem had an impact.