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Entry  Fri Nov 18 15:45:11 2011, Mirko, Update, IOO, Mode cleaner noise projection WholeMCNoiseProjection.pngWholeMCNoiseProjection.fig
    Reply  Fri Nov 18 19:57:19 2011, Mirko, Update, IOO, Mode cleaner noise projection MC_ELP.pngWithAndWithoutHPs.pdfSeismCoh.pdf
       Reply  Fri Nov 18 23:44:33 2011, rana, Update, IOO, Mode cleaner noise projection 
          Reply  Sat Nov 19 01:26:16 2011, Den, Update, IOO, Mode cleaner noise projection 
          Reply  Sat Nov 19 12:57:55 2011, Mirko, Update, IOO, Mode cleaner noise projection CouplingMClengthsToMCF.pdfNpModeCleaner.pdf
Message ID: 5960     Entry time: Sat Nov 19 12:57:55 2011     In reply to: 5953
Author: Mirko 
Type: Update 
Category: IOO 
Subject: Mode cleaner noise projection 

Quote:

Could use some more detail on how this measurement was done. It looks like you used the SUSPOS signal with the mirror moving, however, this is not what we want. Of course, the SUSPOS with the mirror moving will always show the mirror motion because the OSEMs are motion sensors.

Instead, what we want is to project how the actual OSEM noise in the presence of no signal shows up as MC length. For that we should use the old traces of the OSEM noise with no magnets and then inject that spectrum of noise into the SUSPOS filter bank with all the loops running. We can then use this TF to estimate the projection of OSEM noise into the MC length.

As far as improving the damping filter, the 2.5 LP is not so hot since it doesn't help at low frequencies. Instead, one can compute the optimal filter for the SUSPOS feedback given the correct cost function. To first order this turns out to be the usual velocity damping filter but with a resonant gain at the pendulum resonance. This allows us to maintain the same gain at the pendulum mode but ~3x lower gain at other frequencies.

In the past, we had some issues with this due to finite cross-coupling with the angular loops. It would be interesting to see if we can use the optimal damping feedback now that the SUS DOFs have been diagonalized with the new procedure.

 The measurement was done with the MC in lock and the OSEMS active.

1. I injected noise into MC1-3 SUSPOS_EXC at a level that domiated the SUSPOS output.
2. Then I calculated the coupling coefficients of the SUSPOS outputs to MC_F during the time the noise is injected.
3. Without noise injection I projected the SUSPOS outputs to MC_F by multiplying the coupling coefficients with the SUSPOS outputs.

All on 11-11-18. White noise inj. from 0.1Hz to 20Hz. Duration 4mins each.

DOF      Amplitude(counts)     Time(UTC)
MC1      200                           22:08
MC2      25                             22:25
MC3      25                             22:50

Some thoughts on this, bare with me:

As you say this does not show the dark / bright noise of the OSEMs. It shows the influence of the OSEMS output onto MC_F in normal operation of the MC. I would have expected that to be very low everywhere except at the pendulum resonance. Reason for that not to be true could either be the OSEMs having considerable gain off of the resonance, or noise intrinsic to the OSEMs knocking the mirrors around. Since we know the OSEM signal to MC_F TF we only need to compare the OSEM signal to OSEM noise to see the noise contribution to MC_F. We know from http://nodus.ligo.caltech.edu:8080/40m/5547 that the OSEM sensor bright noise is considerably below the OSEM signal above 0.1Hz in actual operation. We checked that the MC OSEM signals are above the noise in the reference above 0.1Hz by a factor 3-10.

We actually measured the cost function with the noise projection (valid to 10Hz). It's just the coupling coefficient, right?

CouplingMClengthsToMCF.pdf

 

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