[Joe B, Louis D, Vlad B]
In an attempt to understand some of the strange low frequency sensing measurements made at LHO, we've been looking at alternative paths between sensing (meters of motion at the mirror) and actuation back on the mirror that might not go through the typical DARM filter bank.

While looking, we found that there is clear line coupling (i.e. lines above a certain height in DARM) to the DHARD_Y (and DHARD_P) ASC input. Interestingly, the PCALY transmitter channel (so the power on the PCAL laser before it bounces off the optic) is clearly seen by the DHARD_Y loop, shown in attached image.  The PCAL is a separate actuator from what the ASC loops use to actuate, and thus knows nothing about angle and length coupling.  One can also see all the other calibration lines also showing up very clearly in DHARD_Y as well.  If there is imperfect diagonalization of the ASC outputs such that they couple back into length, this effectively becomes a parasitic path parallel to DARM.