[Sheila, Jenne]
We did a few tests of changing the DARM offset, to see if there are any notable differences in the noise. In particular, we're looking to see if there are noise terms like RIN from 9MHz that scale with the DARM offset. Because of this test, the front end calibration is not correct during this time, and so the range reported in the control room is not correct. The offline calibrated data takes into account the calibration lines, and will correct for these changes, so range on the summary pages should be fine to trust.
Attached is a screenshot of an ndscope showing the DARM offset changes, and the resulting online (incorrectly calibrated) range calculation.
Analysis of the data is forthcoming, I just wanted to let people who wander into the control room (or our screenshots) what is going on.
We may need to repeat this measurement with the squeezer off, since changing the DARM offset seems to affect the squeezing that we see.
In the attached screenshot, the top plot is whitened DARM, with the time dependent correction applied, and the bottom is SRCL coherence with DARM. Green is when we had a smaller-than-usual DARM offset, and red is when we had a larger-than-usual DARM offset. I checked that the CFTD time dependence correction is working by checking that the calibration lines are at the same height in both DARM traces (they aren't the same height if you look at the pre-correction CAL-DELTAL that we plot on the front wall). So, the differences in the 2 DARM traces are real.
The high frequency classical noise shouldn't have been affected by changing the DARM offset. Probably the squeezing angle shouldn't have been either, if there were no weird effects. But, perhaps changing the DARM offset is changing the SRCL detuning, which would then change the optimal injected squeezing phase angle.
At low frequency, the extra noise in the green trace is likely mostly due to the excess SRCL control noise in DARM, as seen by the increased coherence. since the SRCL FF is tuned only for our usual DARM offset.
All that said, since squeezing shouldn't be affecting the noise between 10-30 Hz, it is still probably worth while to subtract away that SRCL noise to see if there is any evidence of 9 MHz noise change (or other noise change) when we changed the DARM offset.
