J. Kissel, S. Dwyer, E. Goetz, L. Sun

Lilli Shiela and I were able to successfully update the calibration of DELTAL_EXTERNAL today. The residual systematic error is no worse that 15 % and 7 degrees (likely due to a slightly wrong coupled cavity pole, and slightly wrong actuator strengths, and slightly wrong sensing vs actuation relative delay). Recall that this calibration is informed by quick, limited frequency band, measurements, so I'm happy that the results are even this good.

The big gotcha that took us so long: the MEDM screens (recently upgraded by Jeff) do not correctly reflect what's happening in the front-end in each actuator replica's ESD path. Namely, with the replica ESD linearization bypassed, the replica BIAS bank has no influence on the ESD path (this was not Jeff's impression of how it should be, thus he didn't update the MEDM screen that way). Thus, when we had installed the correct replica bias -- and it just happened to be negative -- it did not affect the replica signal path. Thus -- unless we change the model in the future to be more reflective of real life -- we need to apply the bias voltage *sign* and only the sign somewhere in the replica control path. Combined with several other sign flips between the O2 configuration of interferometer control and the current interferometer control (a different arm, a different output matrix, a different bias sign), this was very difficult to find (which is why we needed Sheila's new set of eyes to help figure out the problem).

I've accepted all of the settings in the CAL-CS model as they stand now, so we should have this good(ish) calibration permanently.

I've confirmed the updated calibration and it's systematic error with a PCAL to DARM sweep and an DARM Open Loop Gain, which can be found here:
/ligo/svncommon/CalSVN/aligocalibration/trunk/Runs/O3/H1/Measurements/FullIFOSensingTFs/
    2018-10-23_H1DARM_OLGTF_23to800Hz_10min.xml
    2018-10-23_H1_PCAL2DARM_TF_23to800Hz_2min.xml
they've been exported for future processing by Evan and Lilli. These measurements and their exports have been committed to the above location in the repo.

The attached 2 screenshots show two things:
 - The PCAL to DELTAL_EXTERNAL transfer function (left two panels of first attachment), which quantifies the residual systematic error in the calibration
 - A calibrated (to the accuracy quantified above) amplitude spectral density of DELTAL EXTERNAL.

There are plenty of qualifiers on the noise of this ASD -- it's in the middle of tonight's round of commissioning. The biggest know problem is that they're actively working on tuning the LSC feed-forward, so there is no MICH, PRCL or SRCL feed-forward on during this ASD. Please focus on the fact that the PCAL lines (in green X's) at 35 and 330 Hz are matching the LIVE DELTAL_EXTERNAL (in red).