We got some no squeeze quiet time almost an hour into lock this morning and our low frequency noise looks worse than yesterday - so impacting our range.
Sensor correction is on (confirmed by Corey).
Not sure what has changed.
Template is saved as /ligo/home/jennifer.wright/Documents/Noise_DARM/20240403_DARM_comp_sqz_no_sqz2.xml
I compared this with NO SQZ quiet times from yesterday, 17th March, and December 2023 during O4a. These other times were all after thermlisation.
https://ldas-jobs.ligo-wa.caltech.edu/~gabriele.vajente/bruco_1396204818_GDS_CALIB_CLEAN/
Low frequency coherence with CHARD_P and HAM1
PRCL is high, but we've seen something at this level even before
No smoking gun for 30-40 Hz
The spectrogram attached here (whitened to the median of the "good" DARM spectrum from yesterday) shows hints that the excess noise is not very stationary. BruCo and coherence might not give us much info, but I'm running it nevertheless
OMC suspension MASTER_OUT signals look unchanged after Jeff's modification
More corroborative evidence that changes to the OMC ASC control signal path through the OMC SUS is not causing the excess noise in DARM -- LHO:76929.
It does seem possble that we are clipping on the LSC pop diode. The attached screenshot shows that compared to the lock April 2nd we have very smiilar arm circulating powers, but 0.3% less power on LSC POP.
We've previously seen clipping on POP cause low frequency nosie like this: 74641 69931 The second screenshot shows what these trends looked like at that time, it was much more clear that we had a clipping problem then.
