Tomaru, Ushiba, Sato, Takahashi R, Izumi,
We managed to obtain the return beam from ETMY to the center area. See the attached.
Even though the torsion mode was excited and moving by ~200 urad pk-pk, we managed to initialize the optical lever QPD at a few urad precision.
[The process]
Now that we are able to steer the yaw mode of the ETMY type-A suspension (3860), we tried to let the beam return off to the center area. As noted in 3848, we repeated the great Miyoki technique; the slit technique with the slit set in the EYC chamber. The alignment was actually good from the beginning. Though we did small adjustment both in pitch and yaw by using the moving mass on the marionett and the pico motor on the BF stage for pitch and yaw, respectively. Then we removed the whole slit setup and moved it to the EYA chamber.
The return beam in the EYA chamber was found immediately and we subsequently did fine adjustment of pitch and yaw. Removing the slit, we immediately found the beam at the photo detector signal in the BS chamber (3845).
Throughout the process, the yaw mode was excited presumably due to the combination of us working nearby the heat links and the opened top chamber. The yaw motion was moving by ~200 urad pk-pk and the pitch was about 20-30 urad pk-pk. We then initialized the optical lever QPD so that the highest peak is obtained when the oplev signal goes across (0,0) at a precision of something like 5 urad. This should be good enough to reproduce the alignment on a daily basis later.
Note that the BF stage has been locked throughout the process today.
[The amount of light]
When we had the PD at EYA, the maximum we observed was about 14,000 counts with a certain gain setting which I forgot. Since we are using physically the same PD at the BS, we expect the amount of the signal to be half of that we obtained at EYA. Indeed, the highest we got today out of the BS PD was 6,000 counts with the same gain setting and this is consistent with our expectation. We didn't check the beam size or shape at the BS chamber yet.