With a small break in commissioning, I thought it would be a good time to try to optimize the ETMX optical lever since we're really close to hitting the noise floor at low frequency (f<2Hz). Previously, we had two levels of 1:10 whitening in analog land and it was good enough for a while until we fully commissioned the ISI.  With TBetter blend settings, the suspension resonances seen in the low frequency regime are getting closer and closer to not being visible at all, which will be a problem if we want to use the optical lever for active damping.

This is the premise for changing the gain settings in order to amplify the suspension mode signals at low frequency using the whitening chassis gains we also wanted to add more laser power to match the other optical levers on site and at LLO:

Old:
2 levels of 1:10 whitening
9,000 counts on the QPD

New:
1 level of 1:10 whitening
1 level of 1.414 gain
1 level of 2.0 gain
15,000 counts on the QPD


We've also had their respective anti-whitening and anti-gain stages in software in:
H1:SUS-ETMX_L3_OPLEV_SEG1_Name2
H1:SUS-ETMX_L3_OPLEV_SEG2_Name2
H1:SUS-ETMX_L3_OPLEV_SEG3_Name2
H1:SUS-ETMX_L3_OPLEV_SEG4_Name2

I've attached the pitch, yaw, and the counts on each individual segments to show if the gain switching is working.  The REFs are the old configuration.  You can see that it really doesn't help the situation at all when it comes to the low frequency stuff which means we might have to be more clever on how to make the optical levers more useful in the future.  In fact, one could argue that Yaw got worse, but it's hard to compare since the data sets differ by about 12 hours in time.  Jeff Kissel suggested looking at the laser intensity noise as well as the electronics chain to see if that could be a limiting factor in our spectra, this means that the adding extra laser power could be the cause of the increase in noise floor.