[Craig, Georgia]

Tonight we checked on the OMC ASC coupling to DARM. We re-ran a template which Sheila used in 2017 (alog 35664), injecting broadband noise into each degree of freedom (POS_X, POS_Y, ANG_X, ANG_Y), and measuring DARM. In previous measurements an offset was added to the ASC loops, linearising the coupling. We did not add an offset tonight, instead just comparing our excitation to previous data.

The first attachment shows DARM with no excitation in yellow, DARM with an excitation in each degree of freedom in mint green, and the a reference from 2017 of DARM with the same level of excitation in dark green. For 3 out of 4 degrees of freedom the coupling was similar to this measurement in 2017. POS_Y stands out as showing additional coupling to DARM, perhaps suggesting some offset in the loop.

Concerned about POS_Y, we did a quick linear projection to DARM using the control signal with and without the excitation. Craig did some quick calibration magic to make the control signal with the excitation on line up with DARM. He scaled the control signal (OMC_ASC_POS_Y_OUT_DQ) by a factor of 3e-12 and added a pair of poles at 0.1 Hz to simulate the control loop, resulting in the dark purple trace in the second attachment. If we then apply the same scaling to the ambient control signal we see that it is well below DARM (light purple trace).