Summary

Varying the DARM offset by a factor of 2 has no effect on the unexplained noise above 70 Hz.

Details

Nominally we run with 20 mA of DCPD sum current, which (with an optical gain of 3.2 mA/pm) corresponds to a DARM offset of 13 pm. I took quiet data (15 minutes at a time) with sum currents varying from 10 to 40 mA, which corresponds to DARM offsets from 9 to 18 pm.

Thanks to the automatic gain scaling that Dan and Stefan installed before O1, no manual gain adjustment is needed either for the DARM loop or the calibrated freerunning channels. At each offset, I remeasured the DARM OLTF just to be sure.

I did not adjust the SRCL feedforward during this test, and indeed the coherence below 70 Hz between DARM and the SRCL control signal is increased at DARM offsets other than 13 pm. At 40 mA sum current, the coherence is >0.3 from 25 to 40 Hz. Therefore, the excess noise below 70 Hz during this test is due almost certainly to the SRCL feedforward becoming mistuned.