J. Kissel (after being asked by M. Landry and K. Izumi and contacting S. Biscans, B. Lantz, H. Radkins, G. Grabeel, because V. Lhuillier is on vacation)

Kiwamu was looking to re-align beams from the ALS Table into BSC6, and asked me to restore the chamber to an H2OAT-kind of alignment. I took this as "untrip all the watchdogs, and get damping loops and alignment offsets running." The suspensions were easy, the ISI was not-so-bad, but HPI would not restore -- the process of turning on the isolation loops (which are unfortunately necessary in order to apply the large RZ offset needed to align the ETMY yaw) would trip the HPI WD, with actuators as the trigger, indicating that the error signal was too large.

After some digging, and some question asking, I found the problem arose from the inductive position sensors (IPSs). A trend of the last 30 days (see attached) shows that on Feb 25th, at ~21:30 (1:30p Local) something jolted HPI, most predominantly in the Corner 3, 
V3 = 4500 [ct] * 0.001/655 [in/ct] = 0.0069 [inches] ~ 6.9 [thou] DOWN 
H3 = 3600 [ct] * 0.001/655 [in/ct] = 0.0054 [inches] = 5.4 [thou] -RZ 

with V2 and V4 moving roughly 2 [thou] DOWN, and it has stayed offset as such since. (See G1000125 for indication of location and pointing of BSC6 HPI sensors)

LHO aLOG 5576 indicates that both Apollo (chamber cleaning) and tours (for our future Indian colleagues) were in the end station that day.

A similar trend of the ISI ST1 capacitive position sensors (CPS) shows a very small (~100 [ct] * 0.001/800 [in/ct] 0.00013 [in] = ~ few micron) jolt at the same time, but otherwise continued on unaffected; certainly no large, several [thou] DC shift.

This implies that somehow the IPS were bumped / moved / mis-centered somehow, and that there wasn't really a permanent gross misalignment.

Greg and I went down to the end station and confirmed there was no obvious sources of rubbing or fixed forces (say from the work platform that surrounds it). We found nothing obvious. Unfortunately, though there were dial indicators on Corners 3 and 4, comparison with previous positions yield nonsense, so they were probably moved or bumped into uselessness (with is easy to do). Noteably, we forgot to look at the stops inside the boot, but that should be a quick check when we head back again.

Other large-scale activity has gone on since the signal drop on the 25th, e.g. the clean room move and staging for door removal on 2013-03-01, and is visible but does not result in any such large shift in DC signal offset. 

Since Kiwamu's alignment into the chamber has been delayed slightly waiting for PZT mirror assembly parts, we've (Greg, Hugh, Kiwamu and I) resolved to check the alignment of HEPI with an optical level over the next day or so (with priority given to tomorrow's cartridge install, naturally). If that reveals no such change in alignment, we'll recenter the IPSs and move on.