When aligning the X arm I thought that the green beam is too small on ITMX and ETMX, but when I actually looked at the data, the beam size on ITMX is not bad.
| Beam radius at ITMX [mm] | |
| Theoretical (using nominal ROC of ETM and ITM) | 37.8 |
| Measured (using straight shot beam on baffle PD) | 33 |
I adjusted the TMSX sliders to maximize one of the baffle PDs, moved TMSX in yaw such that the power on the PD drops, and assessed the beam size assuming that the beam is sufficiently smaller than the beam radius.
Attached plot shows that 4.4urad TMSX slider change in YAW makes one of the baffle PDs to drop from the peak by about half (.125/.240 = 0.521 to be more precise). From alog 9126 we know that the slider calibration for YAW is 1.07 [urad/urad], so the beam displacement is x=1.07E-6*4.4urad*4E3m=18.8mm.
exp(-2x^2/w^2) = 0.125/0.240= 0.521, x=18.8mm, therefore:
w=sqrt(-2/ln(0.521))*x=32.9mm.
I don't claim that the measurement has a mm accuracy, though, because I don't know the baffle PD size.
