I'm posting an early version of this alog so that people can see it, but plan to edit again with the results of the second test. 

Yesterday I took a few minutes to follow up on the meausrements in alog 21869.  This time in addition to driving TMS I drove the ISI in the beam direction to reproduce the motion caused by the backreaction to TMS motion. We also breifly had a chance to move the TMSX angle while exciting L. 

The main conclusions are:

• The motion that produces the noise in DARM while driving TMSX seems more likely to be due to motion of the ISI than motion of TMS.  The smaller, linear coupling from TMSY does seem to be due to transmon motion.
• Driving the ETMX ISI produces a broad peak in DARM, however based on the normal level of GS13 noise in the X direction you would not predict that this noise should not normally dominate DARM at 75 Hz. The same drive at ETMY does not produce any noise in DARM.  To see if this coupling can explain the noise from 77-80 Hz we should make injections at those frequencies (and with white noise) on the ISI in multiple directions to allow comparisons to the ambient motion of the ISI and let us make a projection of how much of our unexplained noise is due to this blinear coupling.
• driving the ETMX ISI in X produces a yaw QPD signal, but no pitch signal.  This seems counter intituive to me, and I wonder if it is a clue about the scattering/clipping or just something I don't understand about how TMS reacts to ISI motion.
• There is a ghost beam near X TRB which has about 0.5% as much power as the main beam.
• The coupling produced by the driven peak at 75 Hz is not very sensitive to changes in the TMS position or angle, although the sidebands on the peak do seem to change as TMS is moved.

Comparison of ISI drive to TMS drive for X and Y

The attached screenshot shows the main results of the first test (driving ISIs and TMSs).  In the top right plot you can see that I got the same amount of ISI motion for 3 cases (driving ETMX ISI, TMSX, ETMY ISI) and that driving TMSY with the same amplitude as TMSX resulted in a 50% smaller motion of the ISI.  Shaking the TMS in the L direction induces a larger motion measured by the GS13s in the direction perpendicular to the beam, than in the beam direction, which was not what I expected.  I chose the drive strength to get the same motion in the beam direction, so I have not reproduced the largest motion of the ISI with this test. If there is a chance it would be interesting to also repeat this measurement reproducing the backreaction in the direction perpendicular to the beam.  

The middle panels of the first screnshot show the motion measured by OSEMs. TMS osems see about a factor of 10 more motion when the TMS is driven than when the ISI is driven.  The signal is also visible in the quad top mass osems, but not lower down the chain.  For the X end, the longitudnal motion seen by the top mass is about a factor of 2 higher when the TMS is excited than when the ISI is excited (middle left panel), which could be because I have not reproduced the full backreaction of the ISI to the TMS motion.  However, it is strange that for ETMY the top mass osem signal produced by driving TMS is almost 2 orders of magnitude larger than the motion produced by moving the ISI. It seems more likely that this is a problem of cross coupling between the osems than real mechanical coupling. The ETMY top mass osems are noisier than ETMX, as andy lundgren pointed out (20675).  It would be interesting to see a transfer function between TMS and the quad top mass to see if this is real mechanical coupling or just cross talk. 

In the bottom left panel of the first screenshot, you can compare the TMS QPD B yaw signals.  The TMS drive produces larger QPD signals than the ISI drive, as you would expect for both end stations.  My first gues would be that driving the ISI in the beam direction could cause TMS pitch, but shouldn't cause as much yaw motion of the TMS.  However, we see the ETMX ISI drive in the yaw QPDs, but not pitch.  The Y ISI drive does not show up in the QPDs at all.  

Lastly, the first plot in the first screenshot shows that the level of noise in DARM produced by driving the ETMX ISI is nearly the same as what is produced by driving TMSX.  Since the TMS motion (seen by TMS osems) is about ten times higher when driving TMS, we can conclude that this coupling is not through TMS motion but the motion of something else that is attached to the ISI. Driving ETMY ISI produces nothing in DARM but driving TMSY produces a narrow peak in DARM. 

For future reference:

I drove ETMX-ISI_ST2_ISO_X_EXC with an amplitude of 0.0283 cnts at 75 Hz from 20:07:47 to 20:10:00UTC sept 29th

I drove 2000 cnts in TMSX test L from 20:10:30 to 20:13:30UTC 

I drove ETMY-ISI_ST2_ISO_Y_EXC with an amplitude of 0.0612 cnts at 75 Hz from 20:13:40 to 20:16:30UTC 

I drove 2000 cnts in TMSY test L from 20:17:10 to 20:20:10UTC

Driving TMSX L while rastering TMS position and angle

I put a 2000 cnt drive on TMSX L from about 2:09 UTC September 30th to 2:37 when I broke the lock. We found a ghost beam that hits QPD B when TMS is misaligned by 100 urad in the positive pitch direction.  There is about 0.5% as much power in this beam as in the main beam (not accounting for the dark offset).  I got another chance to do this this afternoon, and was able to move the beam completely off of the QPDs, which did not make the noise coupling go away or reduce it much.  We can conclude then scatter off of the QPDs is not the main problem.  There were changes in the shape of the peak in DARM as TMS moved, and changes in the noise at 78 Hz (which is normally non stationary) Plots will be added tomorrow.

Speculation

There is a feature in the ETMX top mass osems (especially P and T) around 78 Hz that is vaugely in the right place to be related to the excess noise in the QPDs and DARM. Also, Jeff showed us some B&K measurements from Arnaud (7762) that might hint at a Quad cage resonance at around 78 Hz, although the measured Q looks a little low to explain the spectrum of the TMSX QPDs or the feature in DARM. One could spectulate that the motion driving the noise at 78Hz  is the quad cage resonance, but this is not very solid.  Robert and Anamaria have data from their PEM injections that might be able to shed some light on this.