We calibrated POY error signal(C1:LSC-POY11_I_ERR). It was 1.4e12 counts/m.
Modeling of Y arm lock:
Let's say H is transfer function from Y arm length displacement to POY error signal. This is what we want to measure.
F is the servo filter (filter module C1:LSC-YARM).
A is the actuator TF using ITMY. According to Kiwamu's calibration using MICH (see elog #5583),
A_ITMY = 4.832e-09 Hz^2*counts/m / freq^2
We used ITMY to lock Y arm because ITMY is already calibrated.
What we did:
1. Measured openloop transfer function of Y arm lock using POY error signal using ITMY (G=HFA). We noticed some discrepancy in phase with our model. If we include 1800 usec delay, phase fits well with the measurement. I think this is too big.
2. Measured a transfer function between actuator to POY error signal during lock. This should give us HA/(1+G).
4. Calculated H using measurements above. Assuming there's no frequency dependance in H, we got
H = 1.4e12 counts/m
For sanity check; Peak to peak of the POY error signal when crossing the IR resonance is about 800 counts. FWHM is about 1 nm, so our measurement is not so crazy.