Because we were curious, we looked at the residual DARM motion when locked in low-noise, and looked at the effect of the PUM boost filter that was added earlier this week (47164). I used H1:CAL-DELTAL_RESIDUAL_DBL_DQ, but had to remove the effect of the detuned SRC optical spring that was being applied in the CAL model. The attached PDF shows the calibrated residual spectra for 3 cases:
- Before the PUM boost was added (data from a week ago, 23 Feb 2019, 10:00:00 UTC). The residual in the band DC-10 Hz band is 7.1e-16 m-rms.
- After the PUM boost (data from today). Residual in the DC-10 Hz band = 2.9e-16 m-rms.
- With the 'resG' filter engaged (FM3 in the DARM1 bank). Residual = 2.6e-16 m-rms
'resG' was an existing filter, but wasn't being used until now. It contains a couple of resonant gain filters (0.45 Hz, 1 Hz, 0.15 Hz), and an integrator below 0.5 Hz (which we just added). It also had a bandstop filter at 332 Hz (PCal line), but we removed that because it is already notched in another filter. Engagement of 'resG' has been added to the guardian.
There is no obvious benefit to the reduction in DARM residual from the new filters, but they just seem like a good idea. The residual from the test mass violin modes is not captured in these spectra, but looking at the time series we can see that their amplitude is about the same level as this DC-10 Hz stuff. The nominal DARM offset is 11 pm, so the fractional residual is about 5e-16/1e-10 = 5 ppm.
Calibrated DARM offset in READOUT_X0_OFFSET
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Peter, Stefan
Measuring the transfer function from OMC_DCPD_SUM_DQ to DELTA_RESIDUAL_DBL_DQ we can cast SUM mA into DARM m.
Above the whole radiation compensation business, and below the cavity pole (i.e. at 80Hz) that gain is m=2.73e-13meter/mA.
The set point for the OMC is d=20mA.
Thus, the DC offset is given by 2*d*m = 10.92pm.
We updated the fringe length H1:OMC-READOUT_XF_OFFSET from 14 to 10.32. This guarantees that H1:OMC-READOUT_X0_OFFSET is now calibrated in pm.
The guardian and SDF were updated to reflect that change.