Marc and Yuhang

Before recover BAB to filter cavity, we checked again M0 to M3, we tried to make BAB roughly centered on mirrors M0 to M3.

Then we tried to recover reference points on PR chamber. As attached photo 1, we noticed the reference points IR1 can be either weak or bright depending on the tilt of incident beam. Especially, when a weak IR light hits on IR1, the light on IR2 can be closer to the reference point.

Therefore, we think we should make a weak light hit on IR1 while a brighter light hit on IR2, which should be the good reference.