In case you need to retune the filter used for subtracting the HPO jitter (DBB QPD) from DARM, here are the steps

1. take some data with the feed forward off (set H1:LSC-JITTERFF_GAIN to 0, the jitter feed forward MEDM screen can be accessed from the LSC menu in the sitemap)
2. use the script attached below to fit the transfer function. Some notes
   1. set the GPS time and duration at the beginning. Typically 10 minutes of data are enough.
   2. the script will shows the measured coherence between DBB Q1Y and DARM, and the measured transfer function. Typically points with coherence above 0.05 are good for the fit
   3. you might have to tweak the frequency range and the coherence threshold for the fit, in order to get a good result
   4. sometimes the fit algorithm add a pole/zero pair with very high Q close to the edges of the fit range. Normally they are fake and you can remove them
3. the script will output the list of poles and zeros, upload them to the LSC-JITTERFF filter bank. Note that there is a gain of 8e14 in the DBB_FF_QPD1Y filter bank. This is added such that the typical gain H1:LSC-JITTERFF_GAIN is close to 1.

Notes:

• the transfer function should be compensated for the DARM closed loop response. For the moment being I haven't done it, since the subtraction matters only above ~100 Hz and there the DARM loop does not have a significant effect, at least not at the level of accuracy needed for the subtraction as done so far.
• the filter which is loaded now (named 16_10_13) was measured after the IFO was locked for few hours. It might be enough to retune a bit the gain to achieve reasonable performances