Last year in June we found a 11.35Hz mystery peak on ETMX (see LHO alog 63766 and 63775 for details) which was thought to be because of some suspension rubbing (and couldn't be directly verified using top to top TF for both main chain and reaction chain, however ASD of raw osems also showed these unwanted peak). I am also attaching a screenshot from June2022 which shows this 11Hz issue on ETMX top stage and ISI stage 1 and stage 2 channels. To solve this issue Georgia had put in large offsets in the M0_Test filters (T, V and R dof) in ETMX as shown in this screenshot.

This week we decided to revisit this 11Hz issue (after Betsy found that Robert has been doing PEM injections at EndX at 11Hz comb and not seeing any major coupling).

This morning when the IFO lost lock I took this opportunity (after talking to the Operator) to re-take the ETMX spectra after switching off all three M0_Test offsets. The offsets were switched ON after my tests were complete.

The results of the spectra for ETMX (6 osem channels for the top stage) and ISI (stage 1 and 2) is shown here and they look all clear and 11.35Hz peaks seems to be gone (when compared against this plot from June2022).

I am also tempted to take another ASD of raw osem signals (damped and undamped) for sanity check (maybe during late evening or over the weekend when IFO is down).

If this 11.35Hz mystery peak is truly gone then I am tempted to switch OFF the offsets in M0_Test filters of ETMX and check if we are able to lock the IFO without any issues.