[Sheila, Louis, with input from others in CR]

It was suggested that it would be good to check 1.) the beam spot position on PR2 and 2.) determine if we are still at the O4 PR2 beam spot and if we are not, how far off we've moved due to all the recent input alignment changes.

Yesterday, we tried to measure the beam spot position on PR2 using the typical A2L dither approach. We did the same exercise recently on MC2 (91420). I placed an 8.125 Hz dither in PR2's M3 DITHER bank in pit and yaw and adjusted the corresponding A2L gains to minimize PRCL_OUT / DITHER_IN. We have some doubts about the validity of these measurements as we had some issues with the TF magnitude drifting around and taking a while to stabilize. We also had some issues with reproducibility during the same session. This may need to be redone with higher injection amplitudes. Another confusing feature is that, while both the PIT and YAW measurements experienced a sign flip as we moved through the gain steps, some TF measurements had phases that were not fully flipped by 180 deg. Not sure if this is an SNR issue. 

O4 A2L gains: I confirmed that the PR2 A2L DRIVEALIGN gains in place at the start of the test are the O4 gains. They've been unchanged since mid-O4. These gains are: 

P2L: -0.36
Y2L: +0.15

Based on the calculations by Koji referenced in 42600 and 42601, the O4 beam spot position on PR2 is offset from the center of the optic by -0.675 mm in PIT and +0.281 in YAW (pit dtt, yaw dtt, interp). I updated Jenne's CalcSpotPos.py script (77443) to use the corrected coil force imbalance factor (alpha) to mm relationship. This script calculates the beam offset position in mm based on the a2l gains using Koji's calculations. The script lives at: /opt/rtcds/userapps/trunk/isc/common/scripts/decoup/BeamPosition/CalcSpotPos.py.

Yesterday's measurements give:

P2L: -0.60
Y2L: +0.52

which translates to -1.125 mm in PIT and +0.975 mm in YAW from the center. The fact that we were able to find another position on PR2 that minimizes A2L coupling suggests that we are not at the O4 beam spot position on PR2. The estimated delta from the O4 position on PR2 to now is: -0.24 mm in PIT and +0.37 mm in YAW.


The dtt templates I used are in my home directory under Templates/pr2_a2l_pit.xml and Templates/pr2_a2l_yaw.xml. The 8.125 Hz injection was placed via awggui.