Upon entering the control room, I noticed that we have a low frequency oscillation in the PRC gain, POP DC, etc. on our front wall striptool. Turns out, this is the same as what Jim documented last night in alog 34999, and likely comes from Sheila's changing of the oplev damping cutoff in alog 34988.
I would like to go back to the old 10 Hz cutoff that we used to have in the oplevs, to see if that fixes this. However, Sheila noted to me that the old cutoff is zero history, no ramp, so we can't just go out of Observe and switch filters. I'll need to change the filters to always on, few sec ramp, load the filters, then we can switch back.
[Jenne, Sheila]
Hmmm. Reverting the cutoff frequency filters did not fix the problem. We're leaving them with the old 10Hz filters for now.
We also tried lowering the oplev gains by a factor of 8 on all test masses at the same time (in factor of 2 steps), but that caused CSOFT to start being unhappy, so we put them back to nominal before we lost lock.
We're going to go back to Observe since we don't have another idea right now of how to fix this.
As a note, it looks like this is upconverting with the 36Hz Cal lines to broaden them significantly, which is probably part of the reason we're at a reduced range right now.
Still no clever ideas, but it certainly seems like ITMY's oplev or something in the suspension is suspicious, and perhaps imposing noise through the ASC on the other optics. All 4 test masses see the 0.43 Hz oscillation in pitch, but ITMY seems to have many harmonics and other "hair" in the oplev spectrum. Not plotted, but ITMY yaw also sees the 0.43 Hz motion, but the other optics don't seem to see yaw problems.
EDIT to add spot position plot - spots on the test masses aren't moving very much over the last month, so it doesn't seem to be related to that. (Except ETMX yaw - still no idea why that's so scattered)
Beverly, Andy
On 22 Mar, a line of glitches at about 3.3 kHz appeared in apparent coincidence with these ASC problems. A known, always present line at the same frequency shows glitching at about 0.43 Hz (see attached spectrogram). A previous alog (27488) referred to this line as an SRM mode at 3335 Hz. In fact, the SRM spectrum (see attached spectrum) developed a feature at 0.43 Hz on 22 Mar that was not present on 21 Mar.
We haven't checked out anything related to Beverly's suggestion yet since the 0.43 Hz hasn't seemed to be a problem in the last ~day, and we've had other things to worry about. But, Sheila's 8Hz oplev cutoffs were in the guardian, so other than the lock that I had changed them back to 10Hz, they've been on the 8Hz cutoffs.
The DARM spectrum is noticeably better below about 25Hz, so it's good that Sheila's new oplev cutoffs aren't causing any problems, and are just helping.