A short bit ago, Jim locked HEPI on BSC1 and BSC3. Somewhat simultaneously, Kyle and Gerardo closed the gate valves near by. Sometime during this work, both ITM oplevs witnessed yaw pointing shifts. The ITMy witnessed a somewhat negligable shift of ~5uRad, however the ITMx saw a ~45uRad shift.
A trend of the gate valves show that the GV7 (x arm) closure lines up well with the ITMx oplev shift. The y- arm gate valve does not correlate as well, but the shift is so small it may (or may not) have been due to the HEPI locking or other pier associated work going on near that chamber at the time. So, it seems that the oplev jumped due to the mechanics of the gate valve closure. Quick glances at trends of HEPI locking signals could show much correlation to the shift times.
Sorry Gerardo and Kyle, I'm eating my words - you were right.
I think we'll still be able to get good information out of the repeatability of the optical levers once we've done some careful analysis and kept track of these gross changes due to necessary activity. So we'll just have to keep a running count of the different offsets as they accrue.
If shutting the gate vavle messes with the optical lever, we could try to alleviate the symptoms by softening the bellow connection between the viewport and the OL enclosures.
Yes, we noticed that one of the bellows on an ITMx oplev pier was compressed alot and therefore must be pretty stiff.