[Jenne, Gabriele, Daniel, Keita]
Once the gate valve was opened, we started work on aligning the Yarm to the green beam. Short story: green now locks nicely, we have beam coming to ISCT1, we moved BS to get MICH flashes (with the new ITMY alignment and the ITMX that we've had for a while now), which got us some IR flashes in the Yarm. Team Hartmann (TVo, Craig, Georgia) are now setting SR3 to get the green beam out onto the TCS table.
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Next up for main IFO locking is to move SR2 to get the beam back to the AS port (after the TCS team has set SR3 to get beam out to their table).
Final attachment is our current slider values.
Methodological comment on MICH alignment
After realigning ITMY to lock the Y arm on green, the Michelson alignment was changed. We were sure about the alignment of ITMY, since it was based on the arm, and decided to keep the alignment of ITMX also as a reference. Therefore the only free degree of freedom left was the BS.
The AS port was completely misaligned, so we had to use POP to look for MICH fringes. We could not see any at first, and we did not have any aligned camera on that beam either.
By misaligning either IMTX or ITMY we could see that both reflected beams were hitting the photodiode: with one mirror only aligned the power on POP was ~2 a.u., while with both beams it was ~4 a.u. Since we could not see fringes we decided to use the following procedure, which we report here for future reference
The attached pic shows the full camera frame. The beam circled in green is the ITM HR surface scattering green light back to the camera. The spots circled in red are ghost beams from the rear surface and the two surfaces of the compensation plate. The brightest spot just left of the green circle doesn't move as function of CP alignment where the others do. So, it is most likely the reflection of the ITM AR surface. It is possible to align all red-circled spots on top of each other, at which point we can see a clear etalon effect. One explanation for this picture is that the ghost beams actually hit the camera lens.
The ITM has a vertical wedge with the thick side down. According to D080657 the wedge is 0.07°(+0.03°-0.00°). Taking the largest angle, doubling it and multiplying by the refractive index, we get a ghost beam that is angled down by about 5 mrad. Even after 30m propagation this amounts to only about 15 cm separation. This doesn't seem enough to hit any camera on the Y manifold flange.
PDH locking: From alog 42384 we see that the cavity pole moved from 1.2 kHz to 280 Hz. This is close to the original expectation of 200 Hz. In the common mode board the first boost stage was used to account for the green coating error, whereas the second stage acted as a servo boost. Both are 100Hz/1kHz pole/zero pairs. We locked by disabling all boost stages and increasing the gain by 3dB. The first boost stage now acts as the servo boost. This seems to work fine, but needs a open loop measurements to confirm.
Another pic taken by the "red" camera (mounted on VP6) after it had been resurrected. As it turns out, it doesn't have the green filter installed and is also sensitive to green. None of the ghosts are visible.
Some pics from the high resolution camera taken by Jeff B. Redish stuff is due to the oplev.