This is a complementary alog to 47078.  In that alog, I was exciting DHARD pitch, and looking at how that coupled to DARM and other angular degrees of freedom.  Here, I excite DARM (at DARM1_EXC, so going to both stages of ETMX that are in use at the time) while in LOWNOISE_ASC.  This state is before we transition the analog coil drivers, and before we transition to 3 stages of actuation (ITMX ESD + 2 stages of ETMY).  So, I will redo this measurement again at a time when we are locked on 3 stages of actuation just before we attempt to transition back to ETMX, and drive ETMX directly.  But, here is a look at how DARM length couples to angular motion while at 30W in LOWNOISE_ASC. 

In the first attachment, I show the various coupling transfer functions from DARM to several angular degrees of freedom.  Notably, the SRC2 coherence with DARM actuation is very high, although the DC3 and DC4 coherences are quite low.  DHARD yaw's coherence and coupling is a bit higher than DHARD pitch, which was also somewhat surprising to me. 

In the other attachments, I plot DARM's contribution to the in-loop noise of various angular degrees of freedom.  None of them seem dominated by DARM noise, particularly at our suspicious 4.5 Hz, so I'm not sure that (at LOWNOISE_ASC) length to angle is causing us trouble.

When we have the IFO back, I'll retake these measurements driving ETMX when it's set up for NomLowNoise, but we're locked on IX and EY.