[Jeff, Robert, Jenne]
Since yesterday's move of the LSC FF actuation from ETMX test mass to ETMY PUM (alog 73420), it has seemed like our 20-30 Hz noise is 'breathing' more than it had been.
Jeff pointed out that a major change here is, of course, that we are now actuating on a different test mass (which did not previously have any length actuation in low noise), and so there could be scattered light between the test mass and other items in that chamber. Then, we looked at ETMY's R0, and it looks like it's DAC output is off, which would mean that the R0 length tracking can't be working, which could easily be a source of excess scattered light. In the attachment, I show that it looks like the ETMY R0 DAC output changed from green to red about 16 days ago. Again, we've probably never noticed, since we hadn't been using ETMY for any length, so didn't have a need to track anything before today. Although, it may also mean that the ETMY R0 damping is also not working as expected - I'm not sure how or if we'd expect to see that.
We went out of Observe for a moment and I tried changing the Enable coil / test out value from 1, to 0, then back to 1, but the light on ETMY R0 is still red. Hopefully we can look into this further on Monday / Tuesday, and we can see if there's something that needs fixing.
Also, Robert notes that perhaps the breathing that we're seeing in that freq range is just more pronouced now that we've improved the LSC FF, so it may or may not be an issue with a lack of EMTY R0 tracking.