I went to HAM3 to see that the MC2 beam position wasn't crazy. See the first photo, the beam is to the left (+Y direction) relative to the baffle on the HR side but the baffle itself is offset in -Y direction relative to the cage. Green lines are extension of the EQ stop screws to guide your eyes.
I also opened the ISCT1 and moved the mirror in front of the REFL BBPD out of the way and directed the beam to IFO REFL camera (because I couldn't see the beam at all without moting the mirror).
While manually aligning the IMC, we found that somehow things are in such a bad state that the MC2 TRANS SUM decreases when IM4 TRANS SUM increases, and vice versa. Improving the IMC alignment using IM4 TRANS as well as IFO REFL camera made the flashes stronger to the point that we can see 00 mode once in a while. I was also able to see the beam in the ISS path. But MC2 TRANS was nowhere near centered. Attempts to resolve this by incremental changes failed.
We wondered if something behind MC2 was bumped and changed the alignment into MC2 trans. I looked at the path and didn't see anything obvious. The beam transmitted through MC2 was visible using a card and a viewer, it was not clipped by the baffle behind MC2 nor the BS for the beam dump. I could not see the transmission of the BS, though, it was too weak, so I cannot confirm if the steering mirror in front of the QPD was bumped or not.
Elenna started making big changes for JM3 in PIT (to make big YAW changes in the beam injected into IMC, remember that YAW and PIT are flipped between JAC and IMC, IMC WFS takes care of this but that won't help when you're manually aligning JM3) and moving MC2 and MC1 so that the IMC follows the input beam. Repeating this in YAW anShe successfully centered the beam on MC2 trans.
At this point I looked at the MC2 beam position again, see the second picture. Apparently the beam moved in YAW by a few mm to the right (i.e. -Y direction).
Elenna will post which optic was moved by how much in which direction.
Jenne is now trying to put IM4_TRANS and ISS QPD beam position back where they used to be using IM2 and 3.
We found that people opening BSC door cover(?) somehow disturbs IMC whether or not HAM3 and HAM2 door covers are on. The IMC fringe becomes super fast and it almost becomes impossible to align anything. Purge air seems to go to strange places to do strange things.
OTOH, when people are out of BSC, we had to put 0.2Hz 150cts excitation to MC2 M1 drive align L2L so the IMC goes across the entire FSR.
At first, I only moved some combination of MC1, 2 and 3 because we believed that the pointing into the IMC was fine. However, as Keita summarizes above, this was a futile process and veru confusing because it sometimes seemed as if the camera, MC2 trans QPD and IM4 trans QPD all gave differing directions.
However, although Keita said that the beam hit MC2 in a good place, he did clarify this was within mm, so this gave us room to move around JM3 by many microradians. Then, we had this whole realization that JM3 pitch and yaw are flipped relative to IMC pitch and yaw, so some of our other confusion about what we were walking (and why it wasn't really working) started to make sense.
In the end, this is the process that worked: move JM3 a large amount, follow up with MC2 move and some MC1 move in opposite dof (so JM3 pitch goes with MC2/1 yaw). At first, I relied only on IM4 trans, but then the flashes on MC2 trans started to improve, so this became a much more useful signal to follow.
Below, I compare the OSEM readbacks of each suspension from before we started moving to now at the end of the day:
JM3 pitch (IMC yaw): -177 urad
JM3 yaw (IMC pitch): -93 urad
MC1 pitch: -186 urad
MC1 yaw: +83 urad
MC2 pitch: +58 urad
MC2 yaw: -62 urad
MC3 pitch: + 12 urad
MC3 yaw: +21 urad
We should probably put some note next to the JM3 sliders that the pit/yaw dofs are flipped compared to IMC pit/yaw, or I predict that we will recommit this mistake many times over!
Attached is a screenshot of the IMC aligned in air with associated signals (and dog)
Once Elenna had the IMC nicely aligned, we moved on to setting the pointing of the beam headed to the IFO. We need this to be roughly correct, so that we can use it to align the ISS array.
Back in March when the IMC was locked, Elenna found the locations of the beam on IM4 Trans and ISS QPD:
- IM4 Trans Pit (March 2026): 0.22
- IM4 Trans Yaw (March 2026): -0.06
- ISS QPD Pit (March 2026): -0.92
- ISS QPD Yaw (March 2026): 0.94
We then worked to move IM2 to get to the right spot on IM4 trans, and then IM3 to get to the spot on the ISS QPD. The tricky thing is that, since we can't lock the IMC (IOT2 is away from the chamber, so no IMC REFL PD, so no IMC locking), we're just looking at flashes. So, the spot on the QPDs has to be calculated by looking at peak heights when we get a flash, and doing the matrix math to go from segments to pit and yaw.
.....After 23 different iterations of setting IM2 and IM3 based on an educated guess of where they should go, calculating the QPD spots, finding that we weren't quite right, and then tweaking again, we're leaving the IMs such that we're back to the March location on IM4 trans, but we're less on the edge for the ISS QPD. Current spots (calculated from the peaks of IMC flashes):
- IM4 Trans Pit (May 2026): 0.2
- IM4 Trans Yaw (May 2026): 0.05
- ISS QPD Pit (May 2026): -0.53
- ISS QPD Yaw (May 2026): 0.85
This helps assure us that we've got a pretty reasonable beam headed toward the ISS array, and that even if we didn't get the IMC alignment quite right earlier, we should be in a pretty reasonable place and we can use this beam to replace the ISS array.
One final thing we could do as a last check is to calculate the spots on POP A and POP B QPDs (or, at least the one that is used for initial alignment and acquisition), and make sure that we can move IM4 to get to that spot. That would mean that IM4 trans QPD and POP QPDs are both correct, which sets the pointing of the beam into the PRM and into the IFO, so if that line is correct in air and we use it to align the ISS array, then we will certainly be in a good place when we pump down. Again, this would just be a check that the IM1+IM2+IM3 position that we've got right now to give us good pointing to the ISS array is compatible with some IM4 pointing to the POP QPD. The individual segments aren't _DQed, so we'll have to check this tomorrow when the light pipe is open again.
Keita closed the light pipe for the night.