Evan, Jim, Jenne, Sheila

We had a look at the noise in DARM from 77-80 Hz, which we know from BRUCO is coherent with X transmon QPDs. 

The TRX A SUM is lower than the other QPDs, 

The first attached plot shows the coherence with DARM, the second attached plot shows that the 2 QPD sums are coherent with each other at End X but not END Y.  The ground motion measured by the STSs is a little higher at EX, but by a factor of 5 at most.  The spectra of the QPDs at END X is noiser.  If this noise is coherent with DARM at these frequencies, it seems probable that it is not far below DARM at nearby frequencies.

One possible theory would be that we could have clipping somewhere on TMSX that is common to both QPDs, and the source of the clipping also causes some scatter back into DARM.  We could try opening the beam diverter to see if we have the same coherence with the PD on the in air table, or driving TMS to see the excitation in DARM.