Yesterday, I implemented length to angle decoupling filters for the three stages of SRM. The L1toY3 decoupling was tested and results show an improvement of ~10dB at low frequencies, and few dBs above .8Hz. Plot attached in screenshot shows the L1toY3 transfer function without (blue) and with (red) compensation. The 2nd pdf attached shows the measurement, and the fitting used for this particular filter design. The other pdfs show the design for the rest of them.

The method used was :

1. Measured cross coupling transfer functions from one stage to test mass using the script Matlab_tfs.m (started with LHO_Matlab_TFs under /ligo/svncommon/SusSVN/sus/trunk/Common/MatlabTools/SchroederPhaseTools/).
   Measurements were done with the suspension damped and the ISIs damped. The three stages of MC2 SRM and SR2 should be already measured (dates can be found in the spreadsheet attached in previous alog). Only SRM M3 tf needs to be retaken since a chunk of data was missing for the pitch drive of M3 stage.
2. Plotted the measurements to make sure they worked using plothsts_matlabtfs_cross_levels.m under /ligo/svncommon/SusSVN/sus/trunk/HSTS/Common/MatlabTools/
3. Fitted L2P/P2P for pitch decoupling and L2Y/Y2Y for yaw decoupling using Keita's awesome fitting script, and a wrapper called fitting_for_decoupling.m under /ligo/svncommon/SusSVN/sus/trunk/HSTS/Common/MatlabTools
4. Installed filters in foton with autoquack. The filters are installed in two filterbanks : FM1 for {L2P or L2Y} and FM2 for {1/P2P or 1/Y2Y}, and gain should be -1. When the number of zeros of the decoupling filter is higher than the number of poles, script will warn you that a low pass roll off should be added in foton. The new filters need to be saved with foton before being loaded.
5. Tested the filters taking a white noise tf with/without the filter. Template can be found for the L1toY3 coupling of SRM under  /ligo/svncommon/SusSVN/sus/trunk/HSTS/H1/SRM/SAGM1/Data/2014-08-22_SRM_decoupling_L2Y_test.xml

TO DO :

• Check stability/performances of other SRM filters using the template of step 6.
• Design and install filters on three stages of MC2 and top/bottom stage of SR2 (middle mass cross coupling is too high because of the non functioning osem actuator)
• Make a safe.snap when happy with a configuration
• Work on getting the coherence with the matlab tf script
• Take a safe.snap with the filters installed Jeff and I realized that the step response looked like the filter was unstable (we looked at L1toP3 and L1toY3 of SRM) even though the transfer function didn't show instability.