I've been trying to understand the origin of the noise we see in DHARD_Y (and other ASC signals) below 10 Hz, as a side quest to the udnerstanding of the 2.6 Hz peak. 

I've stated many times that DHARD_Y is coherent at those low frequencies with many other signals: OMC ASC, AS DC centering loops, DARM, etc.

I recently noticed that there is coherence with the M0 Y damping of all test masses: ITMX, ITMY, ETMX, ETMY, althouhg the coherence is not close to one.

The interesitng observation is that, althouhg DHARD_Y is coherent with each of the test mass damping signals at levels of 0.2-0.3, the test mass damping signals are not coherent with each other (with the exception of the 2.6 Hz peak). See first plot. This provides some evidence that the test mass damping signals are not seeing the same signal.

I therefore made the assumptions

1. the test mass Y damping noise is coupling to DHARD_Y
2. each test mass damping signal is incoherent with the others (not true at 2.6 Hz, but pretty good everywhere else)

I can then measure the transfer function from each M0 Y damping signal to DHARD_Y, and use it to project the contribution into DHARD_Y (summed incoherently).

The incoherent sum of the four test masses Y damping loop lines up with the measured DHARD_Y noise almost everywhere below ~8 Hz. 

This shold be confirmed by doing some test mass M0 damping noise injections.