Remember, we tried to separate the mode frequencies between test masses, and cluster a given test mass, to make them more easily identifiable (see E1700342, G1701332), but the reality of implementing such a system didn't work out as planned (an already minimal supply of fibers, few sets of fiber breakages, changes in frequency after welding and annealing, etc.). See some details in LHO aLOGs 41216, 40292, and 38965, but most my memory of the failure of the plan is verbal from telecons -- perhaps others can retrace the steps.

I have started adding some violin mode bandpass filters for ETMX, but have not had a change to test them out yet.

I added bandpass, +60, -60 degree phase shifts for 508.36 Hz, 508.844 Hz, 510.714 Hz and 516.678 Hz, in the filter banks under Mode2, Mode 11, Mode 12, and Mode 13 respectively.

The 503.085 Hz mode is on ITMY, and has been damped with the MODE1 filter, with -70 gain, no extra phase ( this can definitely be tuned in the future though), and feedback to pitch.

Note: This comment should have appeared after Georgia's comment below. 

The link to the wiki page above isn't working.  Here is a link : Violin mode table v2

504.891 Hz is on ITMX, can be damped with a gain of -3 and a phase of -60 degrees.  (FM1 in ITMX mode 1 is a 50 mHz wide filter centered at 504.891, this is narrower than our other filters because there is a nearby mode at 504.953 Hz.)

Unfortunately it looks like we have two modes on ITMY that are only separated by 4 mHz.  I was able to damp a mode at 501.629 Hz on ITMY with the same MODE4 filter Georgia used to damp the mode at 501.625Hz, but with a negative gain and +60 degrees of phase.  I was suspicious that these could be the same mode, so I compared a spectrum with a resolution of 2 mHz from 2:42 UTC today (before Georgia damped ITMY) with the lock at 5:09 UTC, and it looks like they are actually two modes separated by about 4mHz.  (Screenshot) 

To deal with this, I created a filter called doublet in MODE4 which has 0dB of gain and a phase of -31 degrees at 501.624 and -3dB with -147 degrees at 501.628 Hz.  A brave soul might be able to engage this filter with the bandpass that Georgia made earlier and no additional phase shift to damp both modes at once.  

Edit: I got to try the doublet filter but it isn't very effective at damping the higher frequency mode which is rung up at the moment.  

I also tried to damp 501.755Hz, but I don't think this is on ITMY.  I copied my filter to ETMX MODE3, and tried 2 phases of pitch, but we lost lock before I could try yaw or longitudinal for ETMX. 

I added a few more violin mode damping filters for ITMY. I promise I'll update the wiki table with this information soon.

The 503.198 Hz mode (MODE5) is on ITMY YAW, and was damped easily with -10 gain (haven't optimised any of the phases yet).

I tried to tackle the ITMY forest around 501.6 Hz but had trouble with cross coupling, accidentally ringing up neighbouring modes. I narrowed the filters and had some success damping

 - The 501.625 Hz mode in pitch with a gain of 15 (filter bank MODE4)

- The 501.555 Hz mode in yaw with a gain of 20 (filter bank MODE6)

- The 501.692 Hz mode had some success with a gain of -5, before we dropped lock for other reasons. Will come back to it (filter bank MODE7).

J. Kissel

We don't yet have enough long lock stretches to get more precise than 0.005 mHz resolution, but I attach a few 1 mHz BW ASDs in the recent lock stretches that Sheila mentions (and some that I've found / reported), 
    - 2018-08-21 07:25 - 07:54 UTC,
    - 2018-08-21 02:43 - 03:11 UTC, 
    - 2018-08-20 21:50 - 22:25 UTC,
    - 2018-08-15 19:19 - 20:48 UTC, 
where, because the last from several days ago, was 1.5 hrs, I was able to get a 0.5 mHz BW ASDs.

The message -- I can concur with Sheila's assessment that these two modes at 501.625 and 501.629 are 4 mHz apart -- however, only in 1 of the 4 measurements is the lower, 501.625 mode rung up. Thus, I think (thus far, we're not yet on DC readout) this is a weakly coupled mode that was rung up by our damping exploration attempts, so if we can design a filter that only tackles 501.629, then we might be OK. (It may not even be ITMY, but the 4 mHz mode separation does smell very much like a barely-elliptic mode splitting.)

I think Sheila and Georgia are on the right track of refining the band-pass filter to be that much more narrow. Mode frequencies for these violins are stable to temperature and test mass alignment at the ~10 microHertz level, so I think plant inversion -- or at least a very narrow (0.5 mHz) band-pass -- might be OK (see second attachment; figure 2 from G1601163 and/or G1700038).

Updated the table with some gains and phases for modes 504.606 Hz, 504.719 Hz, 504.85 Hz, all on ITMX and requiring low gains (<~5). And 504.953 Hz on ITMY (high gain, -30).

Posting this screenshot as a message to all the other v-modes out there. Blue reference is before damping, red is after.

In response to Sheila's request to compare the two following locks:

Aug 21 2018 02:42:00 UTC >  Aug 21 2018 03:11:00 UTC [1218854538 > 1218856278]
Aug 21 2018 05:09:00 UTC > Aug 21 2018 05:29:00 UTC [1218863358 > 1218864558]

I ran 1mHz resolution PSDs. Results are below.

LOCK Aug 21 2018 02:42:00 UTC
Freq. [Hz]        SNR
501.3750        77
501.5556        33815
501.6251        10114
501.6913        2815
501.6972        184
501.7553        181884
501.8750        76
502.7849        21376
502.9092        185016
503.0863        111095
503.1984        8470
503.6779        36286
503.7335        82344
504.1442        1609
504.1778        6259
504.6075        19039
504.7205        103589
504.8516        8474
504.8900        26733
504.9529        33514
505.0786        29540
505.1875        230981
507.3609        143884
507.4945        1342
508.8452        783438
508.9403        590
510.7142        915952
510.7255        268
511.1820        147617
513.4059        398372
513.5119        159
516.6806        693925
516.7800        5140

Lock Aug 21 2018 05:09:00 UTC
Freq. [Hz]        SNR
501.5540        147
501.6293        478058
501.6925        2127
501.7553        612669
502.7849        17214
502.9091        183754
503.0860        50717
503.1984        88
503.6779        48336
503.7335        45705
504.1442        5292
504.1778        17130
504.6075        45363
504.7205        529860
504.8516        386988
504.8900        1223
504.9529        534100
505.0786        50532
505.1875        238858
507.3609        269516
507.4945        1032
508.8452        459774
508.9404        271
510.7142        492155
510.7254        108
511.1819        184454
513.4059        193424
513.5119        159
516.6806        423031
516.7799        4596

I am sorry I have just noticed these round of violin modes alog entries.

I will investigate further but as a quick comment to add to the comments above:

Jeff suggested that the frequency separation of 4mHz of the two modes may indicate frequency splitting of modes associated to the same fibre. However based on the data we have from all fibres of both LIGO detectors this is very improvable. Frequency splitting observed so far is on the order of tens to hundreds of mHz, there only one case of about 1mHz separation at LLO ITMX and it is not certain to be associated to the same fibre.