J. Kissel

This is a follow up on updating the sensing function part of calibration more carefully and completely than last Friday's "you could just" suggestion in LHO aLOG 58629.

Again, the exciting new results that among other things replacing ITMY has improved the DARM loop's sensing function by
    - increasing the cavity pole frequency,
    - increasing the optical gain
    - little sign of SRC detuning (though this statement is not strongly supported by data yet, just hopeful)

I'm starting to run through the T1800469 checklist of things to do in order to update the calibration accordingly. Thus far, I'm only considering an update to the sensing function. I've not processed, nor do I plan to update just yet, any stages of the actuation function.

I've created a new (py)DARM model parameter file:
    ^/trunk/Runs/O3/H1/params/modelparams_H1_20210417.py

It's got the following changes and why:
    (1) New ITM = higher optical gain and cavity pole frequency, and new foton export file
    (2) No evidence yet for detuning = zeroed out the detuned spring frequency
    (3) The power normalization is different for DC readout = New DARM_ERR/OMC_DCPD_SUM transfer coefficient
    (4) higher cavity pole = new file for export of foton representation of inverse sensing function
    (5) A long careful study of the OMC whitening chassis frequency response = new high frequency pole frequencies for OMC whitening chassis

Other than the following "small" levels of sensing function or actuation function systematic error, 
    (a) potentially new actuator strengths (they've not been updated since 2019-04-03)
    (b) that we *want* ETMX PUM coil drivers to be in state 3, but we are oscillating between being able to use State 1, and being stuck in State 2
    (c) we're still having trouble with violin modes, so we're using the "1st stage whitening and 2nd stage lowpass ON" state of the OMC whitening chassis, instead of either of the two configurations we used of that chassis in O3.
I don't know of any other changes (say, e.g. to the control filters). This model should be much better that the late-O3 model we have now, with only the above 5 changes.

So, after creating the model, I 
Re-ran the MCMC with 
    ^/trunk/Runs/O3/H1/Scripts/FullIFOSensingTFs/process_sensingmeas_MCMC_model20210417_meas20210417.py

to get 
    Optical gain, H_c (ct/m)                 | 3.473e+06 (+3598,-3641) or (+0.1036%,-0.1049%)
    Cavity pole, f_cc (Hz)                   | 450.8 (+1.526,-1.502) or (+0.3386%,-0.3333%)
    Detuned SRC spring frequency, f_s (Hz)   | 4.277 (+0.4536,-0.5162) or (+10.61%,-12.07%)
    Detuned SRC spring quality factor, Q_s   | 31.47 (+74.72,-86.12) or (+42.11%,-36.54%)
    Residual time delay, tau_c (usec)        | 7.98 (+0.7229,-0.7048) or (+9.059%,-8.831%)
and self-consistent posteriors of uncertainty, 
    ^/trunk/Runs/O3/H1/Results/Uncertainty/
    2021-04-19_H1_20210417Model_fmin30Hz_PCALSYSERR_None_meas2021-04-17_sensingFunction_MCMC_posteriors.hdf5
in case we need them to create a 68% CI on the response function systematic error (doubt it, but, just in case).

Updated and loaded the following in to the CS-DARM_ERR filter bank:
    FM4: "NewITMY_NoD2N": zpk([450.8],[7000],1,"n")
    FM5: "NewITMY_Gain": gain(2.879e-7)                        == 1/3.473e6
(and copied the gain over to the CS-DARM_CFTD_ERR bank, in case we have time to ever resurrect that.)
and loaded coefficients -- so these are now available for use in FM4 and FM5.

The, though I did not *write* them, I used
    ^/trunk/Runs/O3/H1/Scripts/CALCS_FE/createEPICS_for_20210417.py
to create the file for the "EPICs records" (the realized values of various transfer functions and TF ratios from the DARM model at calibration line frequencies)
    ^/trunk/Runs/O3/H1/Results/CALCS_FE/epicsrecords_model-H1_20210417_created-20210419.txt

Also, I checked if, with this model update, whether we still have the right the relative delay between the front-end sensing and actuation path. Did so with the script
    ^/trunk/Runs/O3/H1/Scripts/CALCS_FE/compute_relativedelay_AvsC_20210417.py
    
which indicates that the delay at the DARM UGF is -7.589 clock cycles. But -- remember -- the front end delay block doesn't change between 7 and 8 (see LHO aLOG 48620). So, maybe it's time that we figure out why Keita's thiran filter design (see LHO aLOG 48008) didn't work, and/or at least fix the bug in the RING_BUFFER.c code that limits the delay cycles to 7.

And finally, for the eventual (if we do) update GDS, I exported the "NewITMY_NoD2N" filter
    ^/trunk/Runs/O3/H1/Measurements/Foton/2021-04-17_H1CALCS_InverseSensingFunction_Foton_NewITMY_NoD2N_tf.txt

So -- now I think we're ready for an update to the calibration.
To do so, I'll need to:
    - Have a fully functional, running IFO, in at least a good a noise state (and in the same switchable filter configurations) it was on Friday 4/17
    - Measure a broadband PCAL to DARM TF for my "before change," then
    - Switch over the CS-DARM_ERR filters from having only FM2&3 to having only FM4&5 on,
    - use the "pushtoepics" input on createEPICS_for_20210417.py to push the update the EPICs records in the front-end
    - Measure a broadband PCAL to DARM TF for my "after change"
    - Look at sensing function TDCFs to make sure they make sense.
and if folks are feeling ambitious (or we need to use GDS-CALIB_STRAIN instead of DELTAL_EXTERNAL), then we can