During relocking effort, for no apparent reason ISS went crazy several times (attached left), oscillating at about 1.6kHz (attached right). This is not unlike 2.5kHz oscillation reported in alog 26666, but the frequency is different.
This is not FSS though FSS is affected by this, as the refcav was once knocked out of lock during the oscillation but the oscillation in ISS continued. It seems that FSS is affected because there's simply too much intensity noise (attached right top cyan and right bottom shows the refcav transmission DC).
HPO output itself seems to be fine (right top, brown trace), so this really sounds like something that happens in ISS.
Anyway, when this happens, do the following.
- Open PSL_ISS MEDM.
- Press AUTOLOCK OFF.
- Press Manual Mode. Manual mode MEDM screen will open.
- Press red OFF button in Manual mode MEDM.
- If Refcav is unlocked, open the PSL_FSS MEDM, press AUTOLOCK OFF, then AUTOLOCK ON, and wait for the FSS to acquire lock.
- Go back to Manual mode MEDM screen and press green ON button.
- At first the red average trace in "DIFFRACTED POWER in %" on the ISS screen will be off the screen. Wait until the red trace becomes visible.
- Press green ON+INT button.
Another FYI.
Left is when ISS was NOT oscillating. Bumpy structures at about 1k and 4k seem to be from HPO (left top, brown). Lower than 1kHz, out-of-loop sensor (PDA) is not much coherent with second loop 1-4 SUM. This shows nothing new, but there's a lot of sensing noise in first loop sensors. Noise eater was at its usual (28 or so).
Middle is when it was oscillating and it's not that interesting. The oscillation was causing so much noise in the intensity that all sensors agree with each other. Again noise eater was as usual.
Right is when it was oscillating. The oscillation shows up in many things, and the channel that showed the highest coherence in non-intensity channels was PMC mixer. This could just be that the PMC lock point was thrown off in one direction because of huge intensity noise, causing a large intensity-to-PDH coupling, or this could be that the PMC is locked a bit off to the shoulder.
