Today we noticed that the EX beam diverter has been open since June 18th. We don't normally switch this so it is not guardian controlled but we "usually" leave it closed. It is beckhoff controlled so it is not monitored in SDF. Today TJ added all the beam diverters we don't normally change in guardian to SYS_DIAG to prevent this from happening again (it was already checking the others).
We have now locked with the beam diverter closed, and see that the QPD spectra look the same between EX and EY. (20418 and 21767 have some history.) The peak in DARM at 78 Hz is also not here. We will see if it stays gone, and check soon to see if the blinear coupling of ETMX ISI motion to DARM is gone with the beam diverter closed.
Spectra attached. The performances of the X and Y QPDs are now comparable.
Glad it was found. Attached are before/after spectra from good science times at 9UTC yesterday and today. The first two show the differences in the strain channel. The biggest feature removed is a nasty wide bump around 78Hz. The last three show the differences in the QPD signals. Very different. For detchar folks that might be wondering about what the beam diverter does, see the intro in T1100252.
Finally, the last three plots shows another difference I spotted. There is a nasty feature at 640Hz that is gone today. I'm not sure if this is associated with the diverter and will keep an eye on it.
Stan pointed out that a line around 1278Hz also disappeared. See spectrum below. The spectrogram of how this line varies is quite similar to the 640Hz line, and lo and behold 640*2 = 1280.
I still don't know if the 640 and 1278 are for sure from the diverter. If the diverter was switched off during lock, and I knew the exact time, I could tell. But I haven't been able to find the time or the diverter switch channel to read in my MEDM hunting, please advise.
Josh -
The beam diverter was closed in between lock stretches. Because it is mounted on the suspended TMS, moving the diverter moves the TMS, which requires realignment of the interferometer.
It was open through the lock stretch that ended ~14:00 UTC on 6 Oct 2015, and was closed before the beginning of the lock stretch that started ~00:30 UTC on 7 Oct 2015.
The beam diverter readback channels are:
H1:SYS-MOTION_X_BDIV_A_POSITION (similar for Y) 1 is closed.
Thanks Sheila and Jenne, the 78, 640, and 1278Hz stuff was all there at the end of the last lock when the X BDIV was zero (open). They were all gone on the first lock when X BDIV was 1 (closed).
Just to note, I got a chance to make a few excitations on ETMX ISI last night (WP5528 again) and it seems that there is still a bilinear coupling from ETMX ISI motion to DARM which is much larger than the linear coupling from ETMY ISI motion; that this coupling at 75 Hz hasn't changed much with the beam divereter closed; and that noise from this is not neglibigle in the DARM noise budget over a broad range of frequencies from about 50-90 Hz.