[Matt, Jenne, Evan, Sheila] There is an enormous peak in the DARM spectrum at 4735 Hz. Shown in the DTT printout below is the IOP channel for the OMC DC PD (H1:IOP-LSC0_MADC0_TP_CH12), from 1 kHz to 25 kHz, and this 4.7 kHz peak is dominating by about 2 orders of magnitude. We wonder if this is perhaps an acoustic internal mode of one of the test masses, although we are having trouble finding a listing of such modes. Does anyone know where we can find a listing of test mass acoustic modes? Or, alternatively, does anyone have any thoughts on what this mode might be?
Sort of unsatisfying (because they're not the real deal, or their incomplete) FEA results for the test mass body modes can be found here: http://www.ligo.caltech.edu/~coyne/AL/COC/AL_COC.htm (Only for a right cylinder) and here T1400738 (only shows the modes which are likely to be parametrically unstable). A quick glance through the above doesn't show anything at or near that frequency (including abs(16384 - FEA results)). I've yet to see FEA analysis of non-test-mass optics, but I've been told that Ed Daw and/or Norna/Calum's summer students on working on it. The best I've seen on that is the ancient 2004 document for the Beam Splitter, T040232 which is where we colloquialy get the frequency of the beam splitter's butterfly mode, which was done by eyeballing the current beam splitter's parameter location Figure 2. (But, the modeled dimensions are wrong, and the wording is confusing on whether the listed frequencies are from the model with flats or not.)
It appears to be a 10th order violin mode on EY.
It is damped with a 1 Hz wide butterworth (unity gain in the passband), a +100 dB filter, and a gain of -30. No rotation needed.
Jeff
As you notes there is some data in the links you already included and we have started to fill in the blanks. Refer to https://dcc.ligo.org/T1500376-v1. When we talk I (we) can complete.
Calum
For reference, with a combination of Slawek's (T1400738) and Calum's (T1500376) FEA models, and Calum's video of the test mass internal mode shapes (T1500376), we expect to find the drumhead mode around 8029 Hz, the x-polarized butterfly mode around 5821 Hz, and the +-polarized butterfly mode around 5935 Hz (using Slawek's values for the mode frequencies). The next two modes (at 8102 Hz and 8156 Hz) do not involve distortion of the test mass face in the direction of the beamline.