There could be POP clipping outside of the PRC, probably in HAM1. This is just based on the two YAW spot move attempts:
- Louis changed PR3 angle in PRMI with PR2_SPOTMOVE such that the POP_AIR moved in +X direction by ~1.7mm and POP_X_SUM dropped: alog 91702
- (Skip this if you're in hurry: Louis moved PR3 by +2.1urad. The beam position at HAM1 POP viewport should have moved by about 1.7mm in +X direction (the change from black to red path in POP_beam_path.png, beam position is on the left side of POP_beam_spot_pos_VS_PR3.png), DC power in POP_X_SUM dropped to nothing.)
- (Skip this if you're in hurry: This only moved the beam position at in HAM3 by 100um or less. The beam radius after 3" fixed mirror is pretty much 2mm. Hard to imagine that this causes a large clipping.)
- (Skip this if you're in hurry: In PR2_SPOTMOVE state, you move PR3, the guardian moves PR2, PRM and IM4 such that PRC alignment is still intact without changing ITM-PR3 line and without changing IM3-IM4 line. )
- I changed PR3 angle alone in single bounce without any automation such that the POP_AIR moved in -X direction by a HUGE amount (~14mm) and POP_X_SUM only mildly increased by about 14%: alog 91647
- (Skip this if you're in hurry: I rotated PR3 by -8.8urad. I didn't use PR2_SPOTMOVE so the calculation details are different but this was a huge change in the opposite direction (14mm -X direction on the HAM1 POP_AIR viewport).)
- (Skip this if you're in hurry: Even then, the beam motion in HAM3 was 1mm or less.)
- (Skip this if you're in hurry: POP_X_SUM only saw a mild increase. See the right side of POP_beam_spot_pos_VS_PR3.png.
One hypothesis is that the POP path in HAM1 is too low as of now.
When we set up the HAM1 POP path in air in April, the green beam was too high and it was missing the top periscope mirror in HAM1. We used pico in HAM3 to bring the path down (alog 89745). It never made sense and we didn't know why that happened, but we believed that the PR3 angle we were using at the time was good and stuck to that angle.
However, IF PR3 angle in PIT at the time had been wrong, and IF we were now somehow closer to the good PR3 angle in PIT, the entire path would be too low as a result of the April pico adjustment.
In this scenario, the HAM1 top/bottom periscope mirror and the dichroic might look like shown in the bottom of beamPos.png. In this cartoon, the beam size, beam separations and the mirror size are to scale, though we don't know exactly where the beams are. There could be other places where IR is clipped at the bottom, but HAM1 periscope mirrors are most likely because of 45 deg PIT angle.
As for intra-cavity loss we've been worried about (alog 91696), Sheila wondered if good numbers in the past were measured with the arms kept off-resonant by ALS and some of the bad numbers now were without ALS. If that was really the case, that makes about a factor of 2.2 difference.
Power buildup without ALS is proportional to ~(t_PRM/(1-r_PRM*r_ITM))**2 ~ 60 where t_PRM**2~0.03, r_PRM**2~0.97 and r_ITM**2~0.985.
If the arms are anti-resonant, the reflectivity of the arm is almost 1, so the buildup becomes (t_PRM/(1-r_PRM))**2~130.
Comparing PR3 the day we did this alignment, 1st April 2026 23:30 UTC plot, to last week, plot. PR3 is <25urad different which is less than 1mm on the POP periscope. Edit: Keita notes there is a curved mirror after PR2 so 11urad could be a big change.
PM1 (RM3) is moving a lot while in chamber in April but settled with DAMP INMONs around P +460, P +1170. Last week we did not spend any time higher than 500urad in the PM1_M1_DAMP_Y.
You can use the left panel of POP_beam_spot_pos_VS_PR3.png for PR3 PIT motion too, though the sign might look confusing (for PIT, positive displacement on the plot means lower). -22urad of PR3 in PIT means that the beams are now ~14mm down on the HAM2-HAM3 septum if we believe M3_DAMP.
(OTOH if we believe oplev the spot position change is too small to matter, if we believe M1_DAMP it's 7mm higher on the septum so it's the wrong sign. Note that PR3 angle is not the only thing that changes the spot position in HAM1, for example the beam could have been too low on the ITMs.)
The only fact we know for sure is that the beams were found to be too high in HAM1 back then and we used pico for "fix" it. It's possible that the real issue, whatever that was, is gone now and our beams are too low in HAM1.