We left the IFO on a DC lock last night. Attached is the spectrum. Two nights were notable: 1) The hump around 800Hz (apparently known as "twin peaks") was bigger than ever. Interestingly it was NOT there at all during earlier locks during the day. The only thing we know we did between the locks was a complete initial alignment. Also, over the period of about 1h, the hump completely disappeared. 2) The low-frequency noise (20Hz to 100Hz) is significantly less stationary now. (This did not improve over time.)
ASC showing up in displacement / roll mode ringup a) The first plot, middle graph, shows coherence between ASC loops and DARM. CHARD shows up very prominently - it's narrow stop-band get imprinted on the coherence. The same noise is also seen in IMP1_P, but it is likely just a witness, since it shares sensors with CHARD. Bottom lime: CHARD needs a proper low-pass. b) The first plot, bottom graph shows some coherence between the 20Hz-100Hz excess and ASC_MICH_P. ASC_SRC2_P also shows it, but might again just be a witness (it uses AS_C as error signal). That excess BTW is all glitches. c) The roll mode was seen to ring up over the 2h lock. Interestingly both ETMX and ETMY modes seem to be ringing. Since DARM feedback only goes to ETMX, but CHARD goes to both, I again suspect CHARD. Again: CHARD needs a proper low-pass.
The bounce and roll mode ringups were due to human intervention -- Chris and I stayed a bit later last night working on the violin mode damping, and we couldn't resist changing the gains on the feedback loops, enough that things became unstable. At 09:10 UTC when Stefan took his DTT spectrum, the gains on the feedback were nonzero, which impresses bounce & roll mode noise on the M0 stage OSEMs. When the feedback gain is zero you can't see these modes in the top stage OSEMs -- this makes identifying the optic that's bouncing or rolling a challenge. It may have been only one ETM was rolling, or an ITM (although I suspect both ETMs thanks to the ASC loops, as Stefan says.)
The attached images from the summary pages show the roll mode increasing around 09:40 UTC, when we realized our feedback was hurting more than helping. After this I zeroed the gain on the DARM --> ETM M0 feedback and the mode stayed the same height for the rest of the night. (When the noise got very bad after 11:00 UTC the bounce mode stayed pretty much the same.)
That said I would not argue against some roll-off filters for CHARD!
The attached screencaps show the roll mode damping setup that has worked well for us, tonight. (ETMX gain 10000/ETMY gain -600)
