eeek. I've been running around all day, so this is an incomplete elog. I'll fill in more stuff in the next hour or so, but just to let people know what's going on:
Valera noticed that lots of things in and around the PSL table are drifting with temperature. This is why he and Steve installed a temp sensor on the table earlier today.
Since the alignment into the PMC, and also the alignment downstream of the PMC have been drifting in angle, we supposed that it might be the PMC itself which is changing somehow with temperature. We don't have a good idea of how exactly it is sensitive to temperature, but we're working on figuring it out.
Round 1 of testing: We put a foil hat over the PMC to shield it from the HEPA air blowing directly down on top of it. I made sure that the foil is also covering the PZT and the metal ring at the end of the PMC, because this could potentially be the problem (metal is usually more temperature sensitive than glass, or the PZT itself could be changing, either of which could make the end mirror twist, and change the alignment of the PMC). We'll see later if this did anything useful or not.
I have photos of the aluminum foil setup, which I will post later when I get back to the lab after teaching.