[Akutsu, Kokeyama]

Continued investigation of the mysterious P and Y coupling in the TMSX QPD1 and QPD2. We think we got some clue.

Condition: X arm locked, ITMX MN LOCK P/Y excited at 0.1 Hz.
Below test did not make any change:
- All the tin foil around the TMSX IR QPDs were removed, an we saw no change.
- We opened the gate valve between EXC and TMSX chambers, and saw no change.

Finally, we changed the one of the two QPDs position upstream. There are two gouy phase telescope lenses, and one QPD was placed after the first lens. In this configuration, P/Y couplings disappeared! See the attachement. We still don't know why, but something after the second lens may be somewhat wrong.

* We left the lights off at EXC.