[Kokeyama, Akutsu]

At the IFI chamber, a viewport window was installed in the backside of IMMT2 so that oplev beam can reach and come back from IMMT2, and so that a transmission beam of IMMT2 (namely, IMMT2T) can be extracted from the viewport window (via some pick-off mirrors in the IFI chamber). See this cartoon.

First, we attached a "VN7879" sealing flange to the chamber. Note that the viewport welded to the chamber is IFC203, while the window flange to be installed is ICF152. In addition, the oplev beams won't pass through around the center of the flange, rather tend to slightly shift in the +X direction. So, a specially designed sealing flange "VN7879", which is an ICF 203 flange having a 5-mm off-centered (in the -X direction) ICF152 flange, was installed.
Note that KAGRA made a rule to stop using larger viewport windows than ICF152, because the chance of glass break could increase as the glass size increases. (For some places, ICF203 windows are installed, though.) Then we attached an ICF152 oplev viewport window (S/N#001).

During the work we realized some of the counterbores for M5 screws to hold a glass seem not well manufactured... need to ask the company!

Note to the future: use small size M8 washers to fix the ICF152 oplev viewport windows so that the washers are not to overlap the lid of the most-inner annular part.  So far we used regular size washers, though.

Fig.1 shows the window, and Fig.2 is a sight seen from the window port.