J. Kissel

Given that we now see evidence for an even lower optical gain (via the calibration lines) with the IFO's new global alignment position, and we haven't been able to take a sensing function measurement since Jul 17th, I "snuck" in a sensing function measurement suite this morning (thankfully just before the giant chilean EQ that knocked us out of lock).

The processed data is attached, and I've added it to the collection of data we've been gathering to help inform us of for what we'll eventually need to correct for in offline analysis and recalibration of h(t).

The message: in this new global alignment position, 
    - the optical gain is indeed lower (3.029e+06 ct/m), and consistent with the reported \kappa_C of 0.93 (given the reference model optical gain is 3.25e+06 ct/m)
    - the cavity pole remains at 410 Hz (though this number has been bouncing around quite a bit in O3).
    - the low frequency response (whether it be from parasitic L2A2L coupling or SRC cavity detuning), appears to now align with the physical model of a pro-spring, with f_s = 7.103 Hz (well, an f_s^2 = -50.453 Hz^2), and a Q_s of 11.67.

In order to confirm that this is an optical spring (now) as opposed to parasitic L2A2L coupling, we'd have to run the gamut of tests that we've run in order to decide that *previously* the low-frequency response was dominated by parasitic L2A2L (see a summary in G1901353). 

I've at least run the now normal PCALX vs. PCALY comparison, to confirm that the low frequency response is still not a PCAL spot-position torque issue. (the fact that the magnitude is ~0.5% different between the two has been brought to the PCAL team's attention and is now under investigation.)

I also attach the now standard plot which shows the MCMC fit values for the sensing function parameters over the entire run. We still have yet to compare these against the calibration-line-determined TDCFs. If we're going to be sticking with this global alignment, today's lock stretch will likely define thestart of a new epoch (and we already know that 2019-Jun-11 defined the start of the 2nd epoch in O3 when we powered up from 35W to 37 W.).

Raw data files live here:
/ligo/svncommon/CalSVN/aligocalibration/trunk/Runs/O3/H1/Measurements/FullIFOSensingTFs
    2019-08-01_H1_DARM_OLGTF_LF_SS_5to1100Hz_15min.xml
    2019-08-01_H1_PCALY2DARMTF_LF_SS_5t1100Hz_10min.xml
    2019-08-01_H1_PCALX2DARMTF_LF_SS_5t1100Hz_10min.xml

    2019-08-01_H1_PCALY2DARMTF_BB.xml                     2019-08-01 18:10:22
    2019-08-01_H1_PCALX2DARMTF_BB.xml                     2019-08-01 18:12:40

    2019-08-01_H1_OMCDCPDSUM_to_DARMIN1.xml

Scripts to produce the attached plots:
/ligo/svncommon/CalSVN/aligocalibration/trunk/trunk/Common/pyDARM/src/sensing.py    rev 8127

/ligo/svncommon/CalSVN/aligocalibration/trunk/Runs/O3/H1/Scripts/FullIFOSensingTFs/
    process_sensingmeas_20190801.py
    plotMCMC_vs_GDSTDCFs.py
    process_sensingmeas_manydates_wPCALX.py