Earlier this morning we thought things were looking pretty good. IPPOS, IPANG, and the AS and REFL spots looked like they hadn't moved too much over the weekend. Our plan was to do a quick check of things, check clearances, etc., tweak up the oplevs, and then close up. This is when I made the ill-fated decisions to check the table levelling.
The BS table was slightly off so I moved one of the thick disk weights off of the other disk weight that it was sitting on, and on to the table next to it. This seemed to improve things enough so I left it there. ITMY didn't need any adjustment, and I move a couple smaller weights around on ITMX. Meanwhile Jenne was adjusting the output PSL power back into it's nominal range (<100mW), and re-tweaking up the mode cleaner.
When we then looked at the vertex situation again it was far off in yaw. This was clearly evident on PZT2, where the beam was no longer centered on the PZT2 mirror and was near the edge. This was causing us to clip at the BS aperture.
We took some deep breaths and tried to figure out what we did that could have messed things up.
Jenne noticed that we had moved slightly on the PSL QPDs, so she adjusted the PSL output pointing to re-aquire the previous pointing, and realigned the MC. This had a very small positive affect, but not nearly enough to compensate for whatever happened.
We spent some more time trying to track down what might have changed, but were unable to come up with anything conclusive. We then decided to see if we could recover things by just adjusting the PZT input steering mirrors. We couldn't; recentering at PRM, BS, ITMY, and ETMY was moving us off of PR3.
Jenne suggested we look at the spot positions on the MMT mirrors. I had checked MMT1 and it looked ok, but we hadn't looked at MMT2. When we checked MMT2 we noticed that we were also off in yaw. This made us consider the possibility that the BS table had twisted, most likely when I was securing the moved mass. Sure enough, when I manually twisted BS table, by grabbing it with my hand, very little force would cause the input beam to walk much of the way across PZT2, more than accounting for the offset. The effect was also very clearly hysteretic as well; I could twist the table a little and it would stay in the new position.
At this point we had fucked things up enough that we realized that we're basically going to have to walk through the whole alignment procedure again, for the third time this vent. We were able to recover the PRM retro-reflection a bit, but the tip-tilts have drifted in pitch (likely again because of the table levelling). So we're going to have to walk through the whole procedure systematically again.
Lessons learned: Many things are MUCH more sensitive than I had been assuming. The tip-tilts are of course ridiculous, in that lightly touching the top or bottom of the mirror mount will move it by quite a lot in pitch. The tables are also much more sensitive than I had realized. In particular, tightening screws can twist the table hystereticly by milliradians, which can of course completely loose the pointing. We need to be a lot more careful.
Assuming the table hasn't moved too much we should be able to recover the alignment by just adjusting the PZTs and tweaking the pitch of the tip-tilts. At least that's the hope. No more touching the table. No more leveling. Hopefully we can get this mostly done tomorrow morning.