[Jamie, Jenne, Raji, with consultation from Nic, Ayaka and Manasa]
We went back and re-looked at the input alignment, and now we're "satisfied for the moment" (quote from Jamie) with the PRC alignment. Also, by adjusting the PR folding mirrors, we are almost perfectly aligned to the Yarm.
What we did:
Set PRM DC biases to 0 for both pitch and yaw.
Aperture was attached to PRM cage, double aperture was attached to BS cage, free-standing aperture was placed in front of PR2.
Adjusted PZT1, PZT2 such that we were centered on PZT2, and through apertures at PRM and PR2. This was mainly for setting beam height in PRC.
Checked centering on PZT1, MMT1, MMT2, PZT2.
Adjusted PRM pitch bias and PZT2 yaw such that REFL beam was retro-reflected from PRM.
Checked that REFL beam came nicely out of Faraday.
Checked that beam was still going through center of PRM aperture, and pitch height at PR2 was good.
Moved PR2 sideways until beam hit center in yaw of PR2.
Twisted PR2 such that beam was hitting center of PR3.
Moved and twisted PR3 (many times) so that beam went through BS input and output apertures, and through center of ITMY aperture.
Found that beam was just getting through black glass aperture at ETMY, top left corner, if looking at the face of ETM from ITM.
Locked down dog clamps on PR2.
This required some re-adjustment of PR3. Re-did making sure going through BS apertures and ITMY aperture, locked down PR3 dog clamps.
Found that we are centered in yaw at ETMY, a little high in pitch on ETMY.
Replaced all of the light doors, to take a break. 4 hours in bunny suits seemed like enough that we earned a break.
This all sounds more straighforward than it was. There was a lot of iteration, but we finally got to a state that we were relatively happy with.
What we will do:
Tweak PZT2 a *tiny* bit in pitch, ~0.5 mrad, so that the beam goes through the ETMY aperture.
See if we can align EMTY and ITMY to get multiple bounces through the Yarm.
Remove ETMX heavy door, steer BS such that we're getting through the center of an aperture at ETMX.
Align ETMX and ITMX such that we get multiple bounces through the Xarm.
Check SRM, AS path alignment.
Check REFL out of vac alignment.
Check other pickoffs.
Check all oplevs.
We have a open-sided 2" mirror mount that we are considering using for the POY pick-off mirror. This might help us get a little more clearance in the Y-arm of the Michelson. Problem is the mount is not steerable, so we need to determine if that's doable or not.