[Keita, Elenna]
Sheila and I struggled to lock SRY last Friday, 91360. Keita and I revisited today.
To start, I ran the usual SR2 alignment, which centers the beam on AS_C, and the AS centering which centers OM1,2 on AS A and B.
Then, we could see a large beam at the center of the AS AIR camera, and a strange fringing beam at the top edge of the camera. We swept SRM in both pitch and yaw to both edges and saw the beam disappear. This convinced us that we were seeing the two beams we needed to overlap in SRY, however, there was not an SRM alignment that could overlap the two beams.
Keita first moved ITMY 10 urad in pitch, and was able to overlap the two beams. He then undid that alignment and moved the beamsplitter instead. We were able to achieve the SRY fringing and then lock SRY with the guardian. The resulting beam ended up on the top edge of th AS AIR camera view.
Using that beamsplitter alignment, we then relocked MICH dark, which required a large movement of ITMX, since we had decided to maintain the beamsplitter and ITMY alignment. ITMX is now at P: -135 and Y: 113.1 on the sliders. This means it has been moved relative to where it should be according to a previous arm alignment we achieved.
We can still see the MICH fringes on the ISCT1 refl camera with this alignment.
We can also lock PRX, with a movement of PRM. We still see no beam on POP A or B, so Keita is scanning the input alignment.
The attached screenshot shows the alignment sliders after we locked SRY, MICH, and PRX.
I'm not sure if we are happy with this alignment overall, since we're not sure if this is where we want the ITMs.
Attached is a not-to-scale, super-simplified cartoon so people understand what happened. As of now, ITMY is unchanged (because I reverted the changes temporarily made to ITMY), BS is angled in PIT, ITMX was angled mostly in PIT to follow the BS motion.
Initial state is in the top cartoon. The symptom that the two beams (single bounce and reflection of SRM reflected back by ITMY) didn't interfere well only meant that the beam was not retro-reflecting on ITMY.
This could be remedied by many different ways. At first I moved ITMY just to confirm that something like this was going on, and we did confirm that (because we were able to make the SRY fringe deeper).
Once we confirmed this was actually the case, I reverted the ITMY back, moved BS, and refined SRM using SRY fringe. The bottom cartoon represents this final state.
Though it's not captured in the cartoon, this of course made the MICH fringe wrong, so we moved ITMX, too.
Using BS or ITMY (or a combination of them) is not the only way though, you can change the angle of the beam from PR3 hitting the BS and refine SRM alignment to recover a good SRY fringe depth. As long as the beam is still on ITMs it will work. I haven't done that bacause it was late and I wanted to see the beam on POP_A or B, but we should.
Note that, ultimately, the good alignment of the ITMs, BS and PR3 won't be known until we open the arms. There's no magic in the current alignment we have (or even after using PR3 instead of BS to regain SRY fringes).
The main purpose of this excercise is just to confirm that nothing terrible is going on in the SRC chain. Because of the things described in Elenna's alog above, we already know that nothing terrible is going on. The rest is just an effort to gradually, maybe, approach the good alignment (on a belief that we already know the good angles for ITMs).
After discussion this morning, I put the ITMX sliders back to the O4 alignment position: pitch = -113 and yaw = 104.6