Lisa, Evan
We repeated the DARM-to-OMC coupling test (25852) at full power.
Tentatively, the change in the violin mode heights with the OMC locked and unlocked suggests only a 75 % coupling into the OMC (i.e., a 25 % loss).
We are not sure why the violin mode at 502.8 Hz is fatter with the OMC locked, so we would like to repeat this test with a calibration line.
This 502.8Hz mode seems one of the violin mode for ITMX, accoding to the wiki https://awiki.ligo-wa.caltech.edu/aLIGO/H1%20Violin%20Mode
If that's true, just to be sure, you should use one of the ETM violin modes, or ETM cal line as you mentioned.
When I was doing a beacon scan using some of the violin modes, I found that the 45MHz modulation sidebands also had finite response to the violin modes.
Assuming this came from the motion of the ITMs, I learned that I needed to use the ETM motions in order to correctly figure out the signal mode matching.