We decided to check the situation of the REFL MC beam path.
- No resolution of the weird MC REFL DC increase
- The reflection from the PD was adjusted to hit the beam dump
- The MC WFS paths were aligned again
We found that the reflected beam from the PD was hitting the mount of the beam dump.
So the entire MC REFL path was steered down such that the last steering mirror does not neet to steer the beam.
When the alignment was adjusted so that the reflection from PD hit the beam dump, the beam spot on the small mirror right before the PD
got a bit marginal but it seemed still OK after some tweak.
Then we looked at the reflection value. It is still about 6.5. No change.
As we messed up the entire MC REFL path, we aligned the MC WFS paths again.
This was done with the unlocked MC REFL beam. Once the cavity was locked,
it turned out that it was enough for the WFS too keep the MC locked.