It's pretty breezy out, and the microseism is creeping up above our 90th percentile. As a result, we have pretty terrible range. Sheila pointed out that a simpler way to test whether the scattering that Robert postulates (in alog 54298) does indeed come from scatter between the end test masses and reaction masses is to change the angle of the end reaction masses. If we can affect the scatter by changing the angle between the etm and reaction mass, then it points to Robert being correct that this relative displacement is a source of scatter.
Keita okay-ed this work to happen during Observation while the environment is bad, as long as the ramp times for moving the reaction chains are long. In order to do this in Observe, Niko unmonitored R0 offset and tramp channels for the optic align sliders for both ETMs.
I started moving both the ETM reaction chains at the same time, only in pitch, up to about 8 urad from their nominal positions. I was getting some pretty bad glitches (which I think were due to increasing wind), so just in case, I moved them both back to their nominal positions. About 80 seconds after those ramps finished, we lost lock. I attach a screenshot of my ndscope, but mostly as a reminder to myself what channels I was looking at. I don't think I had moved enough yet to see anything difinitive. I had been hoping to move a few tens of urad, both in pitch and in yaw, and then do a series of ~10 min new positions, 10 min old positions, 10 min new, etc. to see if the angle really makes a change.
I wonder however if I was moving the ETMs by moving the reaction chain, and since the glitches are loud enough to drown out the ADS lines, the ETMs weren't really following. Something to try when we get relocked will be to turn up the ADS line amplitudes to get better signals, and make sure that the SOFT angular degrees of freedom are doing what they need to do.
L1 doesn't see a change either for angular offsets: https://alog.ligo-la.caltech.edu/aLOG/index.php?callRep=50811
I redid these tests while we were locked later this afternoon, and it seems suggestive that perhaps going 200 urad (almost as much as I can without saturating) on the ETM R0s might help a bit with scattering. But, it could also just be that the wind is varying at nearly the same timescale as my on/off tests.
In the attached figure, the top plot is the alignment offsets of the R0s in urad, the second plot is 2 of the DARM BLRMS (20-30 Hz in purple and 30-60 Hz in yellow), the 3rd plot is POP DC, 4th is the range in Mpc, and 5th is the wind in MPH (yellow is corner station), and the final plot is the seismic blrms in the useism band. It seems that when the alignment offsets are at the -200 urad values (both pit and yaw, both EX and EY), the BLRMS see fewer glitches than when the offsets are at the old nominal position of 0. I tried for a while +200 urad, but didn't think that was as good as -200 urad.
We're supposed to get even worse wind tonight (probably part of what caused our lockloss a few min ago), so I'm leaving the ETM R0 offsets at -200 urad for yaw, and -300 urad for pitch. If locking is a problem and the wind has come back down to the levels we've had in the last 2 hours, then Jim (on Eves) or the Owl operator could revert those to 0 urad for all 4 channels. I'd like to ask one of the Fellows to check in the morning to see if the scattering was a little less egregious tonight than it has been. Perhaps we'll find, as LLO did, that it's not a really big difference, and we need to offload to the R0 or SEI stages to get a bigger improvement.
