Nic, Kiwamu

We made a brief measurement for the ITMX spot position to study the alignment situation.

According to the result, the beam is off from the center roughly by 2.3 cm in yaw toward the Y arm. But, we are not confident with this number as we had some concerns in this measurement. We did not measure the off-centering in pitch yet.

(Method)

We shook the ITMX L2 stage at 2 Hz and monitored the oplev output and ASAIR_A_RF45_Q signal with the Michelson locked on a dark fringe. This is a type of the standard angle-to-length coupling measurement. By taking a transfer function at this excitation frequency from the oplev output to the length signal, we estimated the amout of the off-centering.

(Results)

According to the measurement, the angle motion in yaw was about 0.17 urad at the bottom stage and length coupling was about 0.21 nm both in rms. I used the MICH optical gain calibration that was obtained yesterday (see alog 13664), 5.8 x 10⁹ counts / meters. Also, at 2 Hz, the MICH open loop has a gain of about 20, so the actual longitudinal motion would be 4.14 nm in rms. Dividing the two numbers, we obtained a off-centering of 2.4 cm. The orientation of the off-centering was then indendently checked by introducing some inbalance in the yaw actuation just like the usual spot position measurement (for example, see LLO alog 5010). Note that we assumed that the bottom stage moves at 180 deg off-phase compared with that of the exciation at the L2 stage.

(Some caveat and concerns)

• Oplev has not been accurately calibrated yet. This introduces some uncertainty in the measurement.
• We saw a weird phase discrepancy between the oplev signal and the longitudinal signal. The signals are off by 90 degrees from each other, which seemed very strange.
• We tried to move the beam spot on ITMX by introducing beam translation/angle by moving PR2 and/or PR3, but this did not seem to change the position. We need to think about the ABCD ray trace to understand what we are doing.