J. Kissel, R. McCarthy, B. Weaver, D. Sigg
FRS Ticket 9683

Furthering the investigation of H1SUSITMY's R0 noise problems revealed by vertical transfer functions at the top mass, I grabbed an amplitude spectral density of all of the top mass OSEMs on the QUAD. Attached are the results. 
The first two, for comparison, are all of the top mass OSEMs. While all other OSEM channels look to be as expected, the R0 RT OSEM shows a dramatically elevated noise floor with many sets of interacting frequency combs. The remainder of the attachments are just various levels of zoom around various parts of the spectrum to show details of the various combs.
Note these spectra were taken with damping loops ON.

Remember, while Fil was snooping around last week, he found that OSEM's LED power return had a short to ground -- but ruled out everything ex-vacuo -- so it's inside the chamber (see LHO aLOG 40099).

These three chunks of evidence (the electronics checks, the transfer functions, and the raw OSEM ASDs) are indicative that we certainly have *a* problem we need to fix in chamber, WHENVENT. I'll be updating the FRS Ticket to reflect this. 
We have agreed that there is no need to rush vent and fix this now -- we have other priorities with the IFO -- but we *may* couple a quick-fix vent with say, the 70W laser install, or before / during the ITM discharge cycling. To be determined.

However, given our past history with oscillating opamps in UK satellite amplifiers, I'm not convinced this short is the *only* problem. So, at a "medium priority" speed, Richard and the CDS crew will temporarily swap out the M0 (LF RT) / R0 (LF RT) sat amp, and/or move cables around to see if the problem goes away or follows the amp or the cable (since the problem is so easily identifiable from the ASD). I'll also grab some ASDs with the damping loops OFF, in order to rule out drive chain problems and confirm it's a sensor only issue.

Stay tuned!