Hugh, Evan, Kiwamu,

Since Keita established a new good SR3 alignment (alog 15020), we tried moving the oplev QPD to center the beam on it. However it turned out that the beam seems to be largely clipped on its way to hit the QPD. We could not identify what clips the beam.

We decided to steer SR3 back to the old angle such that we can reliably engage the oplev damping loop for tonight. This should be fixed tomorrow morning.

(some details)

When SR3 was on the new alignment values, the sum of the oplev signals was about 200 counts. We thought this was due to the beam mostly falling off from the QPD, but it turned out that the beam was actually still on the QPD but with the most of it clipped somewhere. We also made sure that we did not catch a ghost or some kind of fake beams by moving the QPD stage around. We steered SR3 back to the old angle and saw the sum going back to 30000 counts or so with pitch and yaw going back to 30 urad or so from 0 urad in the oplev screen. We are concluding that the oplev beam is clipped at some point between the SR3 optic and the QPD when SR3 is in the new angle. We need to open the receiver box and see what is going on. For tonight, we decided to set SR3 back to the old angle in order for us to be able to engage the oplev damping loop which was a key for stable SRC.