Stefan, Evan
We spent some time trying to see if some of the unexplained noise in DARM is caused by jitter (or other nonlinear pointing noise) in HAM6.
First, some things that didn't work:
- I tried shaking the table directly from 9 to 300 Hz by exciting the ISI isolation loops in X, Y, and Z (times to look at are 00:52:30–00:57:30 for Z, 01:01:00–01:06:00 for X, 01:11:10–01:21:50 for Y, all 2015-09-12 Z). I drove until the GS13s registered more than 10 times the quiescent table motion, but I did not see any effect in DARM.
- I tried shaking the table from 700 to 900 Hz by exciting the ISI isolation loops in all six dofs. [Robert had previously shaken HAM6 and found that motion in this frequency band was showing up in DARM from 50 to 200 Hz. Additionally, his assessment was that this was probably coupling in through the table.] I drove until the GS13s registered 100× the quiescent table motion, but I did not see any effect in DARM. Of course, since May we've installed an OMC shroud and moved the OMC angular dither line frequencies, so it's possible that one of these things could have eliminated that coupling. (Or is there some subtlely about using an external shaker versus shaking the ISI?)
We were able to make jitter noise by driving OM3 (I guess it would be surprising if we weren't able to do this). The first attachment shows some example bilinear coupling made by driving OM3 in yaw at 94.3 Hz.
Then we opened the OMC REFL beam diverter and tried some bandlimited injection between 35 and 45 Hz. The result is attached. We saw no coherence between OMCR and DARM (consistent with the notion that the coupling is all bilinear). Also, pitch and yaw of OMC REFL are highly cross-coupled. [Sorry for choosing a polluted band—I wanted a place where the DARM noise was low but where the OMC REFL QPDs are still plausibly seeing displacement noise.]
Finally, we made a broadband injection from 10 Hz to 300 Hz, in pitch and yaw. The times were 01:44:40–01:51:40 for yaw and 01:54:30–01:59:30 for pitch, both 2015-09-13 Z. The result for yaw is attached.
