[Chris W., Sheila D., Kiwamu I.]
This is a report from yesterday.

We completed the alignment works which were associated with the TMSY. We accomplished three missions : (1) balancing of the TMSY optical table, (2) alignment of the infrared QPD path on TMSY and (3) alignment of the infrared beam extraction path through a viewport.

TMSY optical table balancing
The optical table (the bottom mass) of the TMSY was re-balanced by shifting the position of an existing small weight, which was attached on the bottom side of the table. A picture of the weight is attached below. The motivation of this mission was to correct a big pitch misalignment which was found after Betsy released tension of the cables [1]. Since the misalignment was largely in pitch according to our observation on the last Thursday, we shifted the existing weight by 2 inch from the original point away from ETMY. This adjustment nicely recovered the alignment. The static pitch offset of the BOSEMs to obtain retro-reflection from ETMY is within a reasonable actuation range. Thus we conclude that we succeeded in re-obtaining the table balance.




Infrared QPD path alignment
This time the issue of the highly transmissive BS at 532 nm [2] was mitigated by introducing a half wave plate to rotate the polarization of the green light such that the reflectivity of the HR surface increases somewhat. Thanks to the half wave plate, which was placed on the launching path on the EY in-air table, the HR beam became barely visible (although Sheila wasn't able to see the spot while I was somehow.) The infrared path on the QPD sled had been about 1.5 mm off in horizontal translation, so we tweaked two steering mirrors which are the ones right before the sled. The path is adjusted with a precision of less than 1 mm by simply looking at the beam spot on the QPDs. A difficulty was to identify the right beam spot as there are lots of ghost beams in the path because the optics are not intended for green light.

Infrared extraction path alignment
This adjustment was much easier than the last alignment missions because the green light was very visible. After we confirmed that the beam hit the center of every optic, we steered the last launching mirror to let the beam go through the bottom middle viewport. The beam was adjusted such that it goes through the center of the viewport glass with a precision of about 5 mm.

Note :
We increased the laser power to make the beam more visible. This needs to be decreased to the point where it used to be.
The half wave plate is still in the path. This needs to be removed.

[1] LHO alog 5810 "Another Round of TMSY De-rubbing"
[2] LHO alog 5774 "50-50 IR beam splitter is a good AR for green"