sheila.dwyer@LIGO.ORG - posted 22:03, Tuesday 23 July 2024 - last comment - 10:18, Thursday 01 August 2024(79291)
HAM 5 alignment laser day 2
Sheila, Jason, Jennie, Keita, Jenne, Camilla, Naoki. Following 79274.'
Summary: We still need more work on the alignment laser tomorow. Today we believe that we understood the right process for setting it up, but we will need to iterate a few more times to get it set.
- This morning when we went to the chamber we saw the reflection from one of the ITMs coming back near SRM, we were confused by this for a minute and misaligned the ITMs.
- Jennie found we needed to move SRM Yaw to 136 on the osems 79272, we did this (could see the beam slightly move but barely) and then touched the AUX laser BS to center the retrofection on the iris right after the laser.
- Beam was on AS_C (170uW vs 100uW blocked) but not centered (-0.3 pit; +0.2 yaw)
- Jennie found we were off 10urad in Pitch and moved SRM to 659.9 uradians on osems. This was not visible on the iris but we again touched the AUX laser BS to center the retrofection on the iris right after the laser (wasn't centered this morning <1mm off but was centered last night).
- Sheila entered the chamber and walked to SR2, the beam was well centered in yaw but somewhat less than an inch too low. Jenne moved SRM by about 350urad in pitch to bring the beam closer to center.
- Jenne put SRM back half way to the starting place (so that we were half way between the alignment that gave us a good centering on SR2, and the one that we think should be close to what we had in lock. Then Jason, Jenne, and Keita walked the temporary steering mirror and beamsplitter. They moved the beam splitter to bring the beam lower on OM1, since they noticed that it was slightly high of center, and then walked the mirrors to restore the retroreflection on the iris right after the collimator.
- The combination of these moves meant that the beam was very low on SR2, clipping on the scrapper baffle and hitting the optic right at the bottom, around the level of the earthquake stops.
- We took a lunch break, and installed an iris in front of OM1 to mark that position, and set SRM back to the alignment that we think that we were using for locking. Jenne Jason and Naoki tried to move the beam the restore the retroreflection while keeping the beam on the OM1 iris, they found that in order to do that they had to move the beam visibly low on the Faraday aperture, on the temporary beam splitter (which Betsy set to 8.3", the SRM center height.). We then realized that this might be because the beam has been high of center on OM1 while we have been locking, consistent with the fast shutter bouncing.
- We decided to move the beam closer to center in the OFI aperture, and the temporary beamsplitter, and allow it to be above center on OM1. After this Jenne, Naoki and Jason realigned so that the beam was retroreflected with SRM aligned as we think it was for locking. I went into the chamber again to look at the beam position, and found it was between 2 and 3 inches to low (a bit more than the height of the thorlabs beam card), and about 1 cm off in the -X direction.
- When I left, the team was planning to reset the iris by OM1, and possibly make a move of SRM and walk the temporary mirror + BS to restore the retro reflection and spot position on OM1 iris. If this worked we will check in the morning where the beam is on SR2.
It's useful to know the geometry of this setup.
The SRM is a convex mirroor with R=5.678m ROC (and therefore a converging flat-convex lens with f=12.6m focal length).
All rays retro-reflected by the SRM HR surface converge at a single point. Because of the lensing effect of SRM, this point is NOT the center of the ROC but away from SRM by Rf/(R+f)=3.92m. OM1 is 3.65m away from SRM. These are all you need to know (plus a basic knowledge of thin lens), but I drew a cartoon for you (1st attached, top, but you can also see 2nd attachment for the math) where four different rays, all retro-reflected by SRM, are converging at one point.
Now, Sheila's 3rd bullet point from the bottom is like the green line in the cartoon. The green dot at the OM1 position marks the center of the OM1, however there's only one ray that is retroreflected and goes through the center of OM1 (green line), which happens to be VERY low on SRM.
Without touching SRM, we can walk the beam on SRM while keeping the retroreflection and bring up the position at SRM by deltax, and as a result the beam position goes up about 0.07*deltax at OM1 and about 3.7*deltax on SR2.
Note that a small pit offset on OM1 has a big effect on SRM and SR2, i.e. 1mm on OM1 will become 14mm at SRM and 54mm at SR2.
SR2 beam position when Sheila left was 1cm to +X, too low by 3"~76mm (i.e. the beam needed to move to -X by 1cm, up by 3"~76mm).
Now the task is to rotate the SRM, maintain the retroreflection while keeping the beam position on OM1. This is explained in the attached cartoon again. Anyway, conclusion is that if we want to move the beam up by deltay at SR2, we have to rotate SRM by deltay/215m, i.e. deltay/[SRM rotation] ~ 2mm/10urad.
(The calculation in the attached assumes that we're fixing the OM1 beam position, but in reality the iris is maybe 50cm or so closer to SRM, so L=3.15 instead of 3.65. This should have a large effect on one of the important numbers d-L=3.92m-L, it changes from 0.27 to 0.77. Mostly because of this, my calculation is severely overestimating the deltay/[SRM rotaion] maybe by a factor of 3, roughly.)
Anyway,we used 2mm/10urad number at the time, and moved SRM in PIT by negative 380urad (PIT slider 2121 -> 1741, YAW slider -3492 no change).
At this point we reset the position of the iris to the beam. Naoki, Jason and Jenne adjusted the steering mirror and the BS so the beam hits the center of the iris while keeping the retroreflection. We stopped the work there.
This is a picture of Sheila entering HAM4-HAM5 tube through the eye-shaped baffle.
Should also add that when restoring the aux laser alignment to OM1 position and the retroreflection iris in the afternoon, the alignment converged much quicker when using the beamsplitter to align the retroreflection and the steering mirror to align OM1 position. In the morning we were doing the opposite (beamsplitter to OM1, steering mirror to retroreflection), and while we were able to get the alignment to converge it was picky and had a tendency to diverge if we made large steps.
After working through the math Keita had said this might be the case. Our first run at restoring the aux laser alignment in the afternoon, still using beamsplitter to OM1 and steering mirror to retroreflection, was diverging when we were moving in the direction we clearly had to move, so we switched to beamsplitter to retroreflection and steering mirror to OM1; alignment went much smoother afterwards.
Tagging for EPO.