After our clipping saga and moving PR3 to un-clip, we wondered where are we on PR2. The spot on PR3 should always be in the same position lock-to-lock, because it is in roughly the same Gouy phase as the ITMs, and we servo to the spot position on the ITMs. However, the pointing of PR3 is free to drift, which would make the spot on PR2 (and PRM I suppose) be free to be different. The pointing of PR2, PRM, and IM4 would follow any such PR3 drift to ensure that the cavity axis is normal to the PRM, since that's a requirement for resonance. This indicates that the spot position on PR2 could be a witness of sorts for PR3 pointing, since we seem to not be able to trust either the top mass PR3 OSEMs or the PR3 oplev over long periods.
I found that Gabriele and Elenna had most recently checked the spot position on PR2 in April 2023 (alog 69025) and minimized the lines in PRCL_IN1. The PR2 A2L gains have not changed since that time (until I temporarily changed them today). I'll note that, just to make it easy to re-find the link, around that time there was also some PR3 dithering done in alog 69026.
I dithered PR2 in yaw at 7.9 Hz, and looked at the peaks in both PRCL_IN1 and DARM_IN1. Just in case it matters, this first measurement was taken *before* Sheila changed the DARM offloading, but with all LSC FF off, and also quite early in the lock (so not fully thermalized). Somewhat confusingly, the yaw line heights seem to do *different things* in DARM vs PRCL when I change the Y2L gain. In the first attached screenshot, I've changed the Y2L gain from nominal -7.4 (blue trace in DTT) to further-from-center -7.7 (red trace in DTT), and the peak in PRCL (top panel of DTT) has increased while the peak in DARM (lower panel of DTT) has gone down. I would think that PRCL is the thing to trust, since that sould be a more direct measure of A2L coupling. But, overall, we probably want to be minimizing the effect in DARM (although, we don't really care in DARM at this low a frequency). Perhaps this is somewhat similar to the phenomenon we've seen where A2L gains for the test masses have to be set for DARM around 20 Hz to be effective, since the A2L values one would chose by looking at DARM below 10 Hz aren't good.
In the second attachment, I instead make a Y2L change when dithering PR2 at 24.1 Hz, and (moving PR2's Y2L gain closer to zero) both PRCL and DARM agree that moving closer to center of the optic makes the peak go down slightly. Also notable though is that we already seemed to be fairly close to the minimum peak height I could get with making Y2L step sizes of 0.1 for PR2. This means that the best place for the Y2L gain of PR2 has only changed by about 0.2 from April 2023 to Dec 2023. I'll have to look up what that means in mm, but it's going to be small. We don't have a measurement of where it was at any time between those times though, and in particular we don't know what value of A2L would minimize the coupling during the time when we had clipping issues.
In the third attachment, I had put the Y2L gain back to its nominal value of -7.4, and instead change the P2L value. Even though this measurement is at about 25 Hz, PRCL and DARM's responses are back to being opposite of each other. There was a dramatic lowering of the peak height in PRCL, but the peak got a bit bigger in DARM. Again, overall not a big change though.
Next steps: re-find the conversion between A2L units and mm for triples like PR2. Potentially could try small move of PR3, to see what is the effect on optimal A2L values, to see if PR2's A2L values can be used even as a rough reference for PR3 pointing.
