[Sheila Lisa Jeff Arnaud]
Sheila and Lisa were having alignment issues in the X arm this afternoon, in the sense that some fine alignment was always needed after recovering from a lock loss. A good example is shown in the attached screenshot. In that case, diff lost lock, and kicked both etmx and etmy suspensions (top right plot showing the diff error signal in purple). At that point the green transmitted power in the X arm (bottom left plot) dropped to 0, and came back to 0.5 after the test mass settled down (instead of expected 1). This was due to the alignment : comparing the pitch of the test mass with the oplev before and after the event, we see that etmx pitched down by ~1.5urad (left middle plot in green). Neither ISIs HEPIs SUS were tripped during those times, the Ry DC values (corresponding to sus pitch) from ST1 ST2 ISI and HEPI were identical, etmx yaw was identical, itmx pitch and yaw were similar.
Sheila then used itmx (bottom right plot in yellow) to realign the cavity, pitching it by ~1.5urad which was just enough to bring the transmitted power back to 1 (bottom left plot in red).
It would be interesting to know if this is what happens all the time.
Wrong login, this was me.
Can you check if the ETMX keeps pitching down from lock to lock, or if it just snaps back? We are actuating to the top mass, so it is conceivable that a lock loss changes the alignmement through the relief of the length actuation. The problem is that the new alignment scheme feeds back to the PZT which will not snap back, but would follow an alignment drift over time when locked.
2 hour graph of X-arm locking with WFS off. Power drops to 0.5 in about an hour due to length to angle coupling. The tidal motion is about 60µm/h. The effect is clearly seen in ETMX YAW and to a lesser degree in PIT. ITMX yaw is flat. What's going on with ITMX PIT? No external drive was sent to ITMX.
