Keita, Sheila

At 11 am pacific time (19:00 UTC Nov 22nd), there was a sudden shift in alignment in the SRC that caused bad noise in DARM. The first attached screenshot shows the noise in DARM changing.

At this time, there is a shift in the alingment of SRM and SR2 in both pit and yaw, and other interferometer optics don't move significantly.  The OMs and OMC suspensions move as well, but that appears to be a result of the feedback. There is also an increase in POP90, a small drop in AS_A DC, and a small increase in AS90.  The signals on all 4 quadrants of ASA36 I change (the in loop signal, where we are using a funny matrix to take care of an offset).  On AS36 B, we see pitch signals, but not much signal in yaw. Also, the RF levels on all quadrants of AS36 WFS change by 5 to 8dB, which could be the result of the alignment shift.  

We looked at conlog for any changes at the time, there were only PI settings changing, we don't think there was anyone in the LVEA or CER at the time.  

The most obvious sign for an operator that this has happened would be that POP90 jumps suddenly in the middle of a lock stretch.  If this happens again, people could (go out of observe) open the SRC1 ASC loops by ramping the gains to zero, then try moving SRM to bring POP90 back to a normal level.