With the updated AS A WFS yaw offset, the most significant ASC coherence to DARM comes from CHARD Y, see Gabriele's bruco.
To try to improve this, I revisited the ITMY Y2L gain. I chose to inject broadband noise from CHARD Y from 10-40 Hz and adjust the Y2L gain to reduce the coupling. I was able to acheive a 20 dB reduction of the CHARD_Y/DARM transfer function at 20 Hz, and a 10 dB improvement in the DHARD_Y/DARM at 30 Hz. The CHARD Y coupling to DARM may be a few dB worse from 10-15 Hz.
The good news is that it appears that with the new WFS offset, the CHARD and DHARD Y coupling improve together over the same frequency band. The bad news is that there doesn't seem to be much appreciable change in the noise, suggesting that despite the high CHARD Y coherence with DARM, we are likely limited by other noise there.
Here is a summary of the steps I took and the notes:
| Y2L gain | Notes |
| -1.65 | Nominal gain, DHARD Y and CHARD Y references measured |
| -1.5 | CHARD Y coupling worse than -1.65 |
| -1.4 | CHARD Y coupling worse than -1.5 |
| -1.7 | No significant change of CHARD Y coupling relative to -1.65 |
| -1.8 | Improvement of CHARD Y relative to -1.65, some loss of coherence in the injection |
| -1.9 | Improvement of CHARD Y relative to -1.8, had to increase injection strength 60% |
| -2.0 | CHARD Y coupling worse relative to -1.9, sign flip |
Once I determined that a gain of -1.9 was better than nominal, I remeasured the DHARD Y coupling and saw that it was also better, although not the the same degree as the CHARD Y coupling.
First picture shows the transfer function measurement of CHARD_Y_OUT/DARM with different Y2L gains, second picture shows DHARD_Y_OUT/DARM with the initial and final Y2L gains.
I then measured DARM with no calibration lines to check if there was any improvement. I observed no significant change in the noise up to 100 Hz.
Since this Y2L gain appears better, I updated lscparams.py to change the gain from -1.65 to -1.9 (and loaded the guardian, but I did not do any svn commit). I also took SDF to safe and updated. I could not see any SDF diff in observe for the ITMY Y2L gain.
Although the DHARD/CHARD P coherence to DARM is less significant than the Y coherence, it is probably worth checking the P2L gains again to see if we can do any better.
