We have calibrated the overall actuators of each suspension independent of the optical levers. So, we know how much we are
moving the optic in POS in real units as a result of the dither we inject for the lockin measurement. The amount the oplev beam
appears to move if there is only POS motion is
where theta is the oplev's angle of incidence and d is the distance the optic has moved in POS. None of the of the steering mirrors in the
oplev path matter.
I propose that I will add an option in the lockin path to subtract away the apparent angle from the oplev output just before the signal
goes into the lockin module. Then we will be balancing the actuators based on only the actual angular motion.
The success of this technique depends on how well we know our actuator calibration and the oplev angle of incidence. This also
assumes that the oplev beam is centered on the optic, so we don't have beam displacement from A2L of the oplev beam, which then
makes another apparent angular motion. I suspect that we are close enough that we won't have to worry about this effect.