Yesterday when we tried to align the cavity, we found out that the BS mirror both pitch and yaw had very small range can move, with this range we are not able to get the reflection beam back to the bench.
We did a few checks.
Firstly, we checked all the magnets, they all response when we inject noise.
Then, we put the pitch and yaw back to zero pretty precisely.
According to Eleonora, this problem could be related to the laser noise, so we turned off the laser for a few hours and turned it on again, nothing changed. Since the chamber is closed and it is very hard to see the red beam through the window, it is too risky to move or change the laser, so we did not do that yet.
We tried to get the transfer function when inject noise from yaw and pitch, and also the open loop one, compared it with the Eleonora's entry 481, they look exactly the same. Also in this entry, Eleonora mentioned that BS works better when only sent voltage to two upper coils, so from the begging, the driving matrix only have value for two magnets.
During the check of all the local controls, we found out that in the PR chamber, the red laser came out from the chamber was very weak. So we looked into the chamber and found out, on the last aluminum mirror, the beam was too low, so it missed the mirror and hit the mount. We tried to increase the beam height from the laser side by moving the screw of the mirror, in order to receive the beam on the PSD, we also increased the height of the PSD and mirror before it. After doing this, it seems the PR mirror has larger moving range. When we reached the limit of the PR mirror, the beam reflected from the backside of the BS mirror is almost reach the reference.
But still we cannot align the cavity under this 'better' situation, we turned off all the local control laser, to see if let if off for longer time, the situation will change or not. If not, our idea for the next step now is to move the picomotor.
The red mode cleaner has been assembled and tested, Marc will put the details later.