I tried to repeat Masayuki's measurement in this alog to calibrate the JM3, MC1, MC2 and MC3 angular motion into uradians using the IMC cavity transmission, see this alog #91487.

This injects a line in turn on each steering mirror in pitch and yaw and looks for the height of the second harmonic of this line in the IMC transmitted power, as this should depend only on the size of the dither we apply to the alignment and not any existing static mis-alignment.

First I paused the IMC lock guardian and then turned off the IMC WFS (take gain to 0).

I then injected a line on JM3 pitch, then JM3 yaw. There were both at 30 counts into H1:SUS-JM3_M1_TEST_P_EXC and H1:SUS-JM3_M1_TEST_Y_EXC respectively.

The 2f line is viisble at 22 Hz as seen in the transmitted power ASD for the pitch excitation and the yaw excitation.

I then tried to do the same for MC1. I used 17Hz as my dither frequency and checked this didn't overlap with any modes of the MC1 mirror by checking the DAMP filter bank.

I could not get good measurement coherence at 34Hz with a 30 count excitation, when I went up to 60 counts the cavity unlocked, so I turned off the excitation, unpaused the guardian and brought the mode cleaner to down.

This plot shows the ASD during the 30 count injection with the red lines marking the 1f and 2f lines.

Looking back at the ndscope for the time when I thought the IMC unlocked (the mc trans camera spot flashed), it looks like the cavity did not unlock completely but started oscillating at 1Hz.

The first cursor is when the cavity lock started oscillating and the transmitted signal on IMC-TRANS PD went from 11000 counts to 6700 counts. When I turned off the excitation it went back up to the locked level again.

Second cursor is where I purposely brought the mode cleaner to down and the IMC TRANS PD went to 4000 counts.

Summary: I can use a dither on JM3 to measure the calibration of this mirror but to measure the MC mirrors I need to tune my excitation more precisely.