Today we saw in CAL-DELTAL_EXTERNAL_DQ some kinda weird hump in the DARM spectrum around 100 Hz.  
It made us curious about the calibration, so we measured the DARM OLG and sensing functions (PDF one).

1. The nominal DARM open loop gain unity gain frequency is around 80 Hz.  The loop suppression has a factor of 3 gain around 100 Hz.  This is higher than I recall, seems like we're over the phase bubble for sure.  
We lowered LSC-DARM1_GAIN from 400 to 285 to make DARM sit at the top of its phase bubble at 60 Hz.  This affects the way that CAL-DELTAL_EXTERNAL_DQ looks, which is expected, but is overall not impacting our GDS sensitivity significantly (PNG one and two).

2. The DARM sensing function seems to exhibit significant detuning, with an anti-spring kicking in lower than 12 Hz. 

3. We have 10^10 W/m DARM sensitivity.  That's super high compared to one year ago, increased by the increased amount of OMC DCPD light.  Seems to indicate the current OMC is pretty healthy.