Kokeyama, Akutsu, (and Ushiba remotely)
Summary: we have no idea!!! 
What we did: As reported at 11802, a strange coupling between "pitch" and" yaw", which are actually "vertical" and "horizontal" was observed. So today we came to see what was going on at the TMSX IR QPDs. During our check working, either ITMX or ETMX was (mainly ITMX, as it seemed larger signals at the QPDs) shaken at 0.1 Hz as like before. ITMX: 40cnt, ETMX: 400cnt, the difference is due to the high/low power of the coil driver.
- We partly opened the aluminum-foil shroud (wind sheild) for the IR path on the optical table to see if there were any suspicious ghost beams into the QPDs; but not found.
- Then, we touched aounrd the aluminum foil by expecting such touching would vary the coupling, but nothing changed.
- We also checked if the IR beam spot was clipped by optics but it seemed no such apparent clipping.
Additonal facts:
- There is a filter to cut the green beam from the IR beam before the QPDs; once we removed it but the situation did not change, so we got it back.
- On top of that, we can do centering of the IR beams on the QPDs by moving the micrometers on the QPD stages. So the QPDs seemed ok.
- Suspecting some electronics, we swapped the cables at the QPD driver between IR QPD2 and Gr QPD2 but nothing strange happend; the response of this swap was just as expected. So we (surely) get the cable connection back.
Discussion: so far, we have no idea. I (Akutsu) personally think of opening the GV beween the TMSX chamber and the ETMX chamber (EXC chamber), as the window on the GV is also clipping the light beam. Moreover, as observed at 11802, the green beams at the TMSX did not show this coupling. This was also confirmed as well today.
Hmm...
As we detemined to keep opening the aluminum-foil shroud, alternatively, we turned off the room light on the TMSX optical table. On the switch of the room light, we found a tape saying "Always turning on!" (see Fig), but we turned off at 18:55 after consulting with Miyoki-san.
