[Evan, Gabriele, Louis, Georgia]
We struggled to properly align the IFO. We saw low flashes in the y arm during ALS locking and had to move the beamsplitter to increase the flashes. Then, after completion of initial alignment, the ALS Y arm alignment would be poor again. After some trial and error and fighting with the "increase flashes" guardian state, Evan saw that PR3 has drifted in yaw 0.4 microradians. When he moved PR3, it significantly improved the Y arm flashes and the input alignment. We don't know why PR3 drifted and it had so much effect on the Y arm but not necessarily the X arm. Our third initial alignment succeeded and locking is proceeding well.
Attached are some trends of PR3 witness sensors.
The hepi and ISI position sensors say that those are not drifting. The oplev yaw was restored by the move described above; the top mass osem doesn't see any drift except for the move of the slider to restore the optical lever.