The peak at 78 Hz seems to have come from a different noise coupling than the bilinear coupling of ISI motion. Yesterday we noticed that the beam diverter at EX has been open since June.  When we closed this the excess noise in the TMS QPDs disappeared, as well as the 78 Hz peak in DARM.  alog 22286

In the course of investigating the 78 Hz peak, we saw a bilinear coupling from ETMX ISI to DARM (where there is only a linear coupling from ETMY), which we had thought could be related to the niose in DARM although we could see immediately that the motion seen by the GS13s was not enough to explain any noise in DARM.  alogs 22107 and 22159 21767  

In the course of investigating this we learned a few things:

• We checked that motion of the ITMX ISI does not induce this kind of bilinear coupling.
• The noise introduced by driving depends on ISI motion not TMS motion (driving TMS produces a peak, but also moves the ISI.  Driving the ISI to create the same amount of ISI motion but orders of magnitude less TMS motion creates a similarly sized peak in DARM.)  
• Exciting the ISI in all DOFs by 1-2 orders of magnitude above the ambient motion does not produce any noise in DARM.  

Here I've attached a few plots from my last two measurements, which I made just before noticing the beam diverter was open and then repeated last night with the beam diverter closed for completeness.  The results are the same with the beam diverter open and closed, which makes sense since we now that TMS motion is not involved in this noise coupling. 

The first plot is a ratio of DARM noise to GS13 motion in nm.  You can compare this to the B&K measurements that Betsy compiled alog 22169.  It seems that the shape of this coupling is very similar to the quad cage resonances, so scattered light off of some part of the cage seems like a plausible mechanism.  

The second plot shows a projection of this noise into DARM, which is a factor 20 below DARM around 70 Hz.