[Hasegawa, Miyo, Kambara, Furuhata, Akutsu]

The DC PD in the BS chamber was outputting a large DC offset, so we have investigated the reasons. After removing the aluminum foil covering the AS port side of the BS chamber, we played around the PD to investigate the main cause of the DC offset; hiding the front of the PD, back of the PD, X transmission light from BS, Y of it, and many. Then We realized that the main light of the offset would be the reflected light from the gate valve (GV) between the BS chamber and the IXC chamber; the light beam was hitting obliquely upward of one of the viewport windows on the GV.

Our first plan was to locate a screen agianst the Y-direction beam from the BS. But, because the viewport window has AR coated surfaces on both sides, we could expect the same effect of the screen when we input the beam into the window. So we move the picomotors on the BS suspension so that the Y-direction light beam could enter the window. Then the DC PD's offset was decreased and the level is almost the same as the one when the screen was set, so we remove the screen finally.