[Jenne, Alicia, Joan-Rene, with suggestions from Sheila]

Today we tested using ETMY to transition the ETMX ESD, rather than the usual locking procedure of ITMX.  I had checked on the guardian code for this last week, alog 87997.  It worked!  The first time we tried transitionining back to ETMX, we lost lock, but the second time that also worked.  We are currently locked at NLN and observing, having gotten here using ETMY and not any stages of ITMX for the ESD transition. This means that we're ready to try grounding the ITM ESDs, to see if that helps reduce the combs that Alicia and Joan-Rene have been investigating. 

• The first time we tried locking, we went through the state LOWNOISE_ESD_ETMY by hand, so that we could transition ETMX -> ETMY slowly and measure the DARM loop along the way.  
  • We measured the DARM open loop gain before any transitioning, then after 25%, after 50%, after 75%, and after we were fully on ETMY.  See attached plot with the measurements labeled on the right side with percent of actuation on ETMY.
  • When we tried to transition back ETMY -> ETMX 25% of the way, (doing the state LOWNOISE_ESD_ETMX by hand), we lost lock.
    • After looking through the settings, it could be that the ETMX L1 Lock L filter bank wasn't set correctly (FM2 not on), even though I see that in my guardian shell I told it to turn on. 
    • Or, as Sheila points out to us, it could be that this transition back to ETMX is rather delicate, and we're going from using 2 stages of the temporary (ETMY) suspension to 3 stages on low noise ETMX, and we just can't do that part way.  
• The second time we tried locking, we let the guardian go through LOWNOISE_ESD_ETMY, which was successful.  We also checked that the ETMX L1 LOCK L filter bank was set up correctly. Since we were worried about going partially back to ETMX being delicate, we just ran the LOWNOISE_ESD_ETMX state to ramp all the way back to ETMX at once, and that was also successful. 
  • It turns out that I had forgotten that we'd need to disable the ETMY L3 Length actuation after we were done using it, since ETMY is used as part of our LSC FF actuation path, and that signal is only supposed to go to L2.  We had a funny bump in the DARM spectrum, and also some SDF diffs on ETMY that helped us find that easily.  
  • I have added (but not tested) a few lines in LOWNOISE_ESD_ETMX to reset the ETMY L3 settings to match the Observe.snap SDF, which should also fix the lump from improper feedforwarding. 

As of now, the guardian is set to acquire lock still using the nominal ITMX ESD.  So, before we ground the ITM ESDs we'll need to modify the weights in the guardian path, to make it so that the nominal path uses the ETMY and not ITMX.