Sheila, Kiwamu
Since last wensday, we have had several sudden changes in SRC alingment that started in the middle of the week and have continued (31865 31849 31844 31823) We don't know what causes this, but it only shows up in the alignment of SRM and SR2, (as well as the OMs and OMC) and it seems to be a pitch problem.
One likely suspect is the AS36I sensing, allthough we don't have any reason to suspect it changed mid week. We decided to try running a dither loop on SRM pitch for a few days as a test. We changed the demodulated signal to SRCL control, which has a higher SNR than any of the build ups. We reduce the amplitude of the dither line from 600 counts on the clock gain to 100 counts, use a gain of -100 and no 60dB filter (FM1). We set the lock point of this loop by adjusting the P2L gain in SRM M3, which we set to -1.3 for now to get the best build ups.
We are hoping to see if problems continue in the next few days with this on, or if there are jumps in the AS36 I signals. We have added it to guardian but this is not tested yet.
Is it possible that the excess motion in ITMY pitch, that was fixed last week, improved AS36I sensing in the same way that the pitch dither on SRM ASC is this week?
Last week the excess motion in ITMY pitch was identified, alog 31675 , and the filter causing it was turned off, alog 31701.
In searching the alog, I happened to find my own alog from June 2015, alog 18930, where I noticed the ASC ITMY pitch signal was about 3 times larger than ITMX's, at 500pp and 150pp (counts?) respectively. This may have changed since that alog, but if that was still true before the ITMY fix last week, it gives a scale of how much bigger ITMY was vs ITMX, which might be helpful in comparing to what the SRM ASC needs now.