Additionally, I checked the amount of coupling between pitch and yaw using the oplev as readout  in DC. This quantity has already been measured down to 5 mHz and the physical reason for a possible difference with a DC measurement is not yet clear. Still, doing the DC measurement is very quick so I measured it.

The result is the same as before: the coupling ratio TM-Y/TM-P is 9.7 %, where %=100*µrad/µrad when applying an offset of 82 µrad in pitch with 300 counts.