J. Kissel, E. Goetz

Here's a recap of where the calibration of DELTAL EXTERNAL stands at LHO. The story roughly follows "Steps towards calibrating aLIGO interferometers" T1800469, but the story and some details will be covered in order to reveal "what's taking so long," and the problem is that the details and problems are mostly in Step 0 of that checklist. Thank you all for your patience.

Actuation Function:
(0) During ER13 (Dec 2018) -- the last time I was reasonably (just barely) comfortable with the calibration -- we actuated DARM with a "split actuator" configuration EX L3 (TST), but EY L2 (PUM) and L1 (UIM). We switched the L3 (TST) stage from the O3 configuration of drive EY because after an unsuccessful attempt to fix it, the EY ESD remains with one quadrant non-functional. 

(1) On 2019-Jan-10, we began actuating DARM entirely with ETMX, motivated to switch because we found evidence for scattering shelves when driving in the split actuator configuration that were not present when driving entirely with EX (see LHO:46329).

(2) Around 2019-Jan-11, we (the calibration group) managed to update the reference model parameters in the time-dependent correction factor path such that they were accurately reproducing all the signs correctly for the full ETMX DARM actuator (see LHO:46349).

(3) Uncontrolled by the calibration group, during this time, the commissioning team have been adjusting the actuator strength to match the ~35 Hz PCAL line in order to get something "good enough." This "good enough" resulted in frequency-dependent flaws in the calibration (see PCAL-to-DELTAL sweep from LHO:46530) resulting in ~5 Mpc systematic error (ignoring any time-dependent response function changes, which we know will also cause 5-10% level systematic errors).

(4) On 2019-Jan-14 we (the calibration group) measured EX TST, EX PUM, EX UIM with the full interferometer (see data processed on 2019-Jan-23 in LHO:46605) finally, after long periods of unsuccessful locking.
This was *before* the PCAL team updated PCALY's calibration values on 2019-Jan-16 (see LHO:46449), but I was able to account for the change.
However, measurements revealed what we already knew: 
    (a) The EX TST driver's quadrant paths recently have had their low-pass filter changed from [z:p] = [2.2,2.2:50,50] to [2.2,2.2:50,50] (see ECR:E1800233, installed and only roughly compensated in Oct 2018 LHO:44756)
    (b) None of the EX drivers, ESD or Coil, on any stage have ever had their frequency response characterized well enough to update their compensation filters beyond "dead reckoned from circuit schematic."
    (d) The PUM L to TST L transfer function has out-dated violin mode frequencies.
    (c) The UIM L to TST L transfer function has high frequency dynamics for which we do not (yet!) account for in the dynamical model (LHO:38295)

All of the above means that the full-IFO measurements of the full ETMX actuator has lots of unmodeled frequency dependence  -- and thus our fit of a single scalar value for the actuation strength is flawed by as much as 20% for each stage.
That being said, the fit results of the measurements were roughly consistent (at the ~5% level) with what the commissioning team has concluded from (3) -- but there remains frequency dependence in the installed / running calibration because their "good enough" is only measured at one frequency. 

(5) On 2019-Jan-25, in order to begin to rectify (4) (c) and (d), I measured some details of the UIM stage. The data was good, but the IFO lost lock before I could complete the measurements of the PUM. 
    - I still am working on a fit to the UIM data and thus an update to the UIM L to TST L transfer function dynamical model -- there were problems getting the QUAD dynamical model functional after improvements have been made in 2018 (see G1800540). (This is a low priority, so I've focused my energy on the TST and PUM stages.)
    - Without updated PUM violin mode harmonic frequencies, the high frequency flaws in the PUM L to TST L transfer function will remain. Offline investigations continue (namely deciding whether the violin mode table, Borja's numbers 46597, or the current violin mode damping filters are correct), but we really need is IFO time to repeat driven measurements done by Borja and Evan in O2 (e.g. LHO:22959)

(6) In order to rectify (4) (a) and (b), I attempted to measure the EX ESD and coil drivers last Sunday 2019-Feb-03 LHO:46754. While we've gotten good results from the L3 / TST / ESD Driver, the efforts to simplify the measurement technique resulted in confusing problems with the PUM and UIM driver measurements (see LHO aLOG 46773). *Instead* of repeating the measurements physically in the field without simplifying the method, I plan to take the measurements in the control room using the coil driver monitors, as was done before O1 LHO:20846. Read: yet again, I need more IFO time.

(7) In the panic of Friday's supernovae alert, we tried our darnedest to use the "nominal" configuration EX L1, EX L2, and EX L3 and get to Nominal Low Noise -- but for some as-of-yet-unknown reason the transition would not work (see LHO:46751). Thus, since then, we've reverted to actuating DARM with the "split actuator" configuration, EX L3, EY L2, EY L1. This means that the roughly adjusted calibration from (3) is now wrong again. The commissioning team made their best effort to compensate, but the results were at best ~15% (LHO:46781). Note -- in that same aLOG -- Craig also suggests that he saw the same scattering shelves that drove us away from the "split actuator" configuration, so
Today the goals are squeezing, so we are *not* dedicated any energy to finding out why the EX L1 and L2 actuators don't work, and thus the calibration remains a complete mess. 

Steps to success:
    (i) Find out why ETMX L1 and L2 doesn't work, and fix it, such that we can go forward with full ETMX DARM actuation. (needs IFO time)
    (ii) Measure the PUM and UIM coil drivers, and update the compensation. (needs IFO time)
    (iii) Truly identify ETMX all 8 violin mode fundamentals and their harmonics to update the PUM dynamical model (needs IFO time), finish fitting the UIM transfer function data to update the UIM dynamical model (needs Kissel time)
    (iv) Remeasure all stages of ETMX with the full IFO (needs IFO time)
    (v) Update the front-end calibration (including reference model parameters at calibration line frequencies such that time-dependent correction factors are accurate), and 
    (vi) confirm success with the full IFO (needs IFO time).

Sensing Function:
    The story here is a lot less sordid history -- but just lacking in data to confirm success. We have a single measurement of the IFO in the current, 30W configuration from LHO:46530. From this data, we have an answer -- LHO:46632, that should be good enough to update the front-end calibration. In fact, we've already installed the proposed update (just not turned it on).
However, what currently running (again through uncontrolled adjustments from (3) comparing against single PCAL line), I believe is good enough to get by -- and the single measurement we have doesn't resolve the SRC detuning pole very well.
Further, always, when making a change to the calibration, we want to watch and verify our results with a further 10 minute measurement suite. And we just haven't had that IFO time either.

Steps to success:
    (i) Get another set of sensing function sweeps, with an updated frequency vector to better resolve the SRC detuning. (needs IFO time)
    (ii) Fit the results and update the front-end calibration (including reference model parameters at calibration line frequencies such that time-dependent correction factors are accurate), and 
    (iii) confirm success with the full IFO (needs IFO time).