[Jenne, Gabriele, with thoughts and ideas from Elenna and Sheila]

The last few days, our sensitivity has been degrading a small amount, and Gabriele noted from a bruco that we're seeing increased MICH and SRCL coherence.  It hasn't even been a full 2 weeks since Gabriele and Elenna last tuned the MICH FF, so this is disappointing. Elenna has made the point in alog 72598 that the effectiveness of the MICH FF seems to be related to the actuation strength of the ETMX ESD.  We certainly see that the Veff of ETMX has been marching in a monatonic line for the last few months in Ibrahim's alog 72849. After roughly confirming that this makes sense, Gabriele and I took measurements in preparation for soon switching the LSC FF to use the ITMY PUM, just like LLO does, in hopes that makes us more immune to these gain changes.

Today, at Sheila's suggestion, I tried modifying the ETMX L3 DriveAlign L gain to counteract this actuation strength change.  (Fear not, I reverted the gain before commissioning ended, so our Observe segments do not have any change to any calibration.)  To check the effect of changing that drivealign vlaue, I looked both at the DARM open loop gain, as well as the coupling between MICH and DARM. 

• As-found, DARM OLG at 60 Hz was gain of 1.14 and 30.1 deg, when EX L3 drivealign L was at the nominal value of 184.65.  Corresponds to blue trace in first attachment.
• I adjusted the EX L3 drivealign L gain to get the DARM OLG back closer to its nominal value of 60 Hz.  After adjustment, the DARM OLG at 60 Hz is gain of 1.02, 29.8 deg, with EX drivealign 166.8. The MICHFF is quite bad though.  Ran a bb calib meas to check the calibration, although this is before Jeff reminded me that it would be useful to have the CAL-CDS_DARM_FE mimic of that drivealign gain match the actual gain: /ligo/groups/cal/H1/measurements/PCALY2DARM_BB/PCALY2DARM_BB_20230913T195421Z.xml, but H1:CAL-CS_DARM_FE_ETMX_L3_DRIVEALIGN_L2L_GAIN had not been fixed to match my updated drivealign gain.  Corresponds to green trace in first attachment.
• I modified the calibration mimic of the ETMX L3 drivealign gain, ran another broadband measurement, and indeed saw that this made the calibration better.  This indicated that much of the loss in sensitivity was due to the poor LSC FF.  The bb measurement for this is at /ligo/groups/cal/H1/measurements/PCALY2DARM_BB/PCALY2DARM_BB_20230913T200136Z.xml . 
• I realized that probably we didn't want the DARM OLG to be the 60 Hz value it was when we started O4, but rather we wanted it to match what it was on the day the LSC FF was last tuned (which is closer to 65 Hz or so, from this plot from alog 72422 from Louis).  So, I switched to using drivealign to minimize MICH-> DARM coupling, and got a drivealign gain of 182.5.  This puts DARM 60 Hz values at mag 1.14 and phase of 30.3 deg.  (So, basically the same as earlier today....Hmmm. Likely some measurement uncertainty there.)  Had a small amount of quiet time when H1:CAL-CS_DARM_FE_ETMX_L3_DRIVEALIGN_L2L_GAIN (the Calib matching value) was also 182.5, so that's the channel to check for a quiet time.  Corresponds to red in first attachment. 
• Reverted ETMX L3 drivealign L gain back to 184.65 (original value, so we haven't altered our calibration).

This all seemed to jive with the MICH FF effectiveness being related to ETMX ESD actuation strength.  So, rather than try to track that, we decided to work on changing over to use the ITMY PUM like LLO does.  I note that our Transition_from_ETMX guardian state uses ITMX, not ITMY, so it should be safe to have made changes to the ITMY settings.

• Used -1 on ITMY in LSCFF output matrix, for both MICH and SRCL.  I'm not sure why LLO uses a -2 there.  Made sure signal goes through L3 filter banks but not to the ITMY ESD, and that signal does not go up to L1 or M0 (so, we'll only be actuating on L2).
• Gabriele and I copied over the f^2-ish filter that is in the LLO ITMY L2 LOCK filter bank, which is used to increase the drive at high frequencies since we're going to be driving up one stage.
• Our first try at measuring coupling functions rang up some violin modes, so Gabriele copied over a violin bandstop from the ITMY angular L2 dofs.  Our nasty doublet of 508.35 Hz violin modes is on ITMY, so perhaps that's why they got rung up, even though LLO does not use a bandstop and we didn't have one for ETMX.
• Gabriele took measurements for updating both MICHFF and SRCLFF.
• After this, we reverted everything back to using our nominal ETMX LSC FF actuation, since we didn't have time for fitting and testing the new filters today.
• Gabriele is working on fitting the new filters for the ITM, and we will come back soon to trying them.  I am hopeful that this will alleviate much of our need to retune these regularly!
• We did save the new ITMY settings in the Observe SDF (There were 2 LOCK_L gains, and then two filters engaged in ITMY L2 LOCK L).  I did not think to save these in the safe.snap file, so they will likely get reverted, and cause an SDF diff for our next lock :/ .