This alog is a continuation of previous efforts to correctly calibrate and compare the LSC couplings between Hanford and Livingston. Getting these calibrations right has been a real pain, and there's a chance that there could still be an error in these results.

These couplings are measured by taking the transfer function between DARM and the LSC CAL CS CTRL signal. All couplings were measured without feedforward. The Hanford measurements were taken during the March 2024 commissioning break. The Livingston MICH measurement is from Aug 29 2023 and the PRCL measurement from June 23 2023.

As an additional note, during the Hanford MICH measurement, both MICH and SRCL feedforward were off. However, for the Livingston MICH measurement, SRCL feedforward was on. For the PRCL measurements at both sites, both MICH and SRCL feedforward were engaged.

Results:

The first plot attached shows a calibrated comparison between the MICH, PRCL and SRCL couplings at LHO.

The second plot shows a calibrated comparison between the Hanford and Livingston MICH couplings. I also included a line indicating 1/280, an estimated coupling level for MICH based on an arm cavity finesse of 440. Both sites have flat coupling between about 20-60 Hz. There is a shallow rise in the coupling above 60 Hz. I am not sure if that's real, or some artifact of incorrect calibration. The Hanford coupling below 20 Hz has steeper response, which appears like some cross coupling between SRCL perhaps (it looks about 1/f^2 to me). Maybe this is present because SRCL feedforward was off.

The third plot shows a calibrated comparison between the Hanford and Livingston PRCL couplings. I have no sense of what this coupling should look like. If the calibration here is correct, this indicates that the PRCL coupling at Hanford is about an order of magnitude higher than Livingston. Whatever coupling is present has a different response between both sites, so I don't really know what to make of this.

Calibration notes:

The Hanford measurements used H1:CAL-DELTAL_EXTERNAL_DQ and the darm calibration from March 2024 (/ligo/groups/cal/H1/reports/20240311T214031Z/deltal_external_calib_dtt.txt)

The Livingston measurement used L1:OAF-CAL_DARM_DQ and a darm calibration that Dana Jones used in her previous work (74787, saved in /ligo/home/dana.jones/Documents/cal_MICH_to_DARM/L1_DARM_calibration_to_meters.txt)

LHO MICH calibration: I updated the CAL CS filters to correctly match the current drive filters. However, I made the measurement in March 11 before catching some errors in the filters. I incorrectly applied a 200:1 filter, and multiplied by sqrt(1/2) when I should have multipled by sqrt(2) (76261). Therefore, my calibration includes a 1:200 filter and a factor of 2 to appropriately compensate for these mistakes. Additionally, my calibration includes a 1e-6 gain to convert from um to m, and an inverted whitening filter [100, 100:1, 1]. This is all saved in a DTT template: /ligo/home/elenna.capote/LSC_calibration/MICH_DARM_cal.xml

LLO MICH calibration: I started with Dana Jones' template (74787), and copied it over into my directory: /ligo/home/elenna.capote/LSC_calibration/LLO_MICH.xml. I inverted the whitening filter using [100,100,100,100,100:1,1,1,1,1] and applied a gain of 1e-6 to convert um to m.

LHO PRCL calibration: I inverted the whitening using [100,100:1,1] and converted from um to m with 1e-6.

LLO PRCL calibration: I inverted the whitening using [10,10,10:1,1,1] and converted from um to m with 1e-6.

I exported the calibrated traces to plot myself. Plotting code and plots saved in /ligo/home/elenna.capote/LSC_calibration