We were requested to measure the transfer function from the ASC actuation channels to the motion of the optic measured by the diagonalized oplev.

We want to do the measurement in a situation which mimics as close as possible the configuration we will have during observation. In the case of the payload this means the IM DC control system should be off and the IM pass-band comb damping should be on. However, because we still haven't implemented the band-pass filters we use the usual broadband damping filters. The suspension was arranged as follows:

1.   When the suspension is in ALIGNED state I took note of the actuation applied in DC to the IM by the DC control loop (pitch: 96 counts for a 62 µrad, yaw: 56 counts for -13 µrad ).
2. I turned off the IM DC control manually.
3. Using the OPTIC ALIGN block I appied a DC offset equal to the previously measured values.
4. In order to actuate directly on the IM from the  K1:ASC-BS_PIT_EXC channel I set to off the switch after the ISC INPUT FILTERS block to off in the payload screen.

Because the aim is to quantify the cross-coupling between different degrees of freedom upon actuation from the ASC channels the following transfer functons are relevant

• ASC PITCH EXC →OPLEV L
• ASC PITCH EXC→OPLEV P
• ASC PITCH EXC →OPLE Y
• ASC YAW EXC →OPLEV L
• ASC YAW EXC→OPLEV P
• ASC YAW EXC →OPLE Y

The transfer functions above provide couplig constants in units of displacement over amount of actuation (µrad/cnt or µm/cnt). Nevertheless, using the diagonalized oplev it's also possible to quantify the coupling in units of displacement  over displacement (µrad/µrad or µm/µrad) by measuring the following TFs at the same time as the ones above:

• For ASC PITCH actuation.
  • OPLEV P →OPLEV L
  • OPLEV P→OPLEV P
  • OPLEV P →OPLE Y
• For ASC YAW actuation
  • OPLEV Y →OPLEV L
  • OPLEV Y→OPLEV P
  • OPLEV Y →OPLE Y

The following results and observatios apply :

• The values in the table are good at low frequencies (~100 mHz and/or below).
• The coupling ratios in units of displacement over displacement were calculated from oplev measurements.
• The coupling ratio  from ASC PITCH EXC to TM-Y is large (10%) likey because the coupling at the IM level is also large (IM-P to IM-Y: 9%, where there's coherence at low frequencies). I don't know yet why this coupling ratio is so large.

Directory:

/kagra/Dropbox/Subsystems/VIS/TypeBData/BS/TF/Measurements/20191016/

Files:

ASC_P_EXC_PAYLOAD_20191016.xml

ASC_Y_EXC_PAYLOAD_20191016.xml