Tonight, following the OMC alignment work, Michal locked the first OMC and we transitioned DARM to DC readout using the light reflected off the 2nd OMC.  In this state, the detection PD (B1s2 in this case) is getting about 10x less light than we expect the B1 PDs to receive in nominal low-noise, and we're not getting the benefit of the filtering from the 2nd OMC.  But, by reducing dither lines and tuning the DARM offset, we were able to have B1s2 be free of saturations.  This gave us a much better noise curve compared to last week.  Compared to that curve (with the saturating B1 PDs), the lock of the OMC was more robust and the blend of the B1s2 DC and Audio was free of glitching.

In order to maintain the alignment, we turned off the DIFFp TY loop, and also the PR alignment loops in dark fringe.  Just before transitioning to DC readout we also turned off all of the dither lines on the mirrors (so really only the BS was controlled, with the quadrants).  This isn't a great way to fly, but should be enough to stay locked for a few minutes and measure the noise.

Our best lock, in the lowest noise configuration, was from 19:44:45 UTC to 19:46:40.

First plot attached is a comparison of DARM on RF (purple) to DARM on DC (blue) - there's about three orders of magnitude improvement above 1kHz.  DC readout works!  In this state our strain sensitivity at 1kHz is probably better than 10^-21, we'll see what the calibrators say.

Second plot attached are some spectra and coherences.  The SSFS boost was on during durng this lock, and the DARM noise is perfectly coherent with MICH between 10-100Hz.  Notice the notches in the DARM spectrum at 219 and 237Hz, these are notches in the MICH control filter to stop the MICH->PR feedforward from ringing up the PR violin modes.

We need to fix the MICH->DARM noise before we can hand off to the real detection PDs, the current noise level will saturate the B1 photodiodes.