I was trying to figure out which things are in the intersection of 1) could give a flat noise in DARM and 2) has been changed during the vent. One suihc thing is OMC lenght noise. This got me down the rabbit hole of understanding the OMC lock.

OMC dither lenght control

The 4.1 kHz we inject on PZT1 for the lock shows an amplitude modulation: this is normal since 4.1 kHz is high enough in frequency and close to 16384 Hz, so that the line produced by the DAC is a bit undersampled.

The OMC length locking error signal H1:OMC-LSC_I_OUT shows now a large 4 Hz bump that was not there in O4a: we think this is due to the fact that the I/Q demodulation phase of the OMC dither line is mistuned: we are coupling line amplitude modulation into I. So hopefully tuning the demodulation phase will help with this problem.

We retuned the OMC lenght demodulation phase while at 2W in PREP_DC_READOUT. The old value was 35 degrees, and gave us almost equal amplitude in I and Q of a 11 Hz PZT line I injected. The optimal phase that moves this line all in I and none in Q is 56 degrees. Decoupling is better than a factor 100. We'll see when we lock to NLN if this improved the 4 Hz bump in the OMC lock and DCPD, and if this mistuning has any effect on the OMC locking point.

I don't know where this phase is set (probably SDF), so we need to check that and update this parameter where it is set

Still to do: measure OMC lock open-loop gain and check the UGF. Lock loss at 13:42 LT caused by the OMC UGF template, the excitation was probably too large