This is more like a preview.

Top red is DELTAL_EXTERNAL (not anti-whitened) multiplied by 2E6 to make it easier to compare the features with DBB QPDs measuring the HPO beam. Bottom is the coherence between DARM, jitter, and intensity noise measured by DBB QPDs.

As was expected a huge coherence peak is at 1kHz, but the coherence between DARM and 1QY (QPD1, PIT) is pretty high from 200 to 1kHz and at around 4.2kHz.

QPD signals look somewhat different from usual DBB scan plots (e.g. this one) in that 10-1000Hz slope is steeper in DBB scans.

It would be useful to measure coherence between DBB and various QPDs and ASC sensors (and it would be good if we can do it without going to the floor).

There's a caveat.

Jason and Peter taught me that DBB shutters could be manually opened using DBB crate front panel switches (it cannot be done via MEDM), so I went to the rack and opened the HPO DBB shutter while the IFO was locked in DC at 46W but not yet in nominal low noise (still lownoise ASC). That was a good part.

However, with manual mode, I didn't know if I can lock DBB PMC or keep it off-resonance and how, so all that was available to me was the QPD signals from the beam reflected by the DBB PMC which might or might not have been totally off-resonant.

I think it's a high priority to make DBB fully operatable.