As a summary of my previous alog, which detailed a number of issue I had whilst propagating UIM dynamics recorded in another alog, with the early indication of of their relevance being alluded to in yet another alog.

One of the main things I found is that when you have a sufficiently complicated transfer function, Matlab appears to start breaking down in ways that vary in each Matlab version (see last comment in previous alog). I have written a script:

/ligo/svncommon/CalSVN/aligocalibration/trunk/Common/pyDARM/matlab_scripts/

sus_ss2zpk.py

Which takes a Matlab SS model and converts it to ZPK. The comparison can be seen in pythzpk_vs_matlabss.png. Matlab appears to struggle to numerically resolve the TF after the 150Hz feature, giving rise to apparently large errors in the TF comparisons. In the version of the TF that didn't have the UIM dynamics in the range 45-150 Hz, this numerical error was happening at much higher frequencies. The main point is that the ss2zpk function in matlab is creating hugely spurious features at 0.5 Hz, and here we keep the error at that frequency at 1%. This may not be perfect, but it is a hell of a lot better than what Matlab is doing (Compare the magnitude error between this (matlab SS vs matlab zpk) and this (matlab SS vs python zpk)).

Having propagated this transfer function fully into pyDARM, with a new parameter model in:

/ligo/svncommon/CalSVN/aligocalibration/trunk/Runs/O3/H1/params/

modelparams_H1_20190909_UIMdyn.py

, using the fit (2019-11-20_H1SUSETMX_L1_LFMeas_vs_Fit.pdf), we can see the the effect on the DARM response: 2019-12-16_O3_H1_DARMLoopCritique_ContributionsToR_mag.pdf. In comparison to my first estimate on the impact of this update to the response (old alog), you can see the little blip created at 150Hz quite clearly in the 1/C curve.

The hope is that this will resolve the observed miscalibration at 150 Hz. I am working on regenerating that same plot to take into account this new dynamics factor to (hopefully) show that we can be correctly calibrated there as a result of this work.