jeffrey.kissel@LIGO.ORG - posted 09:30, Friday 26 August 2016 - last comment - 12:00, Friday 26 August 2016(29326)
Lines seen at ~20Hz-25Hz in DARM are Intentional Angular Dither Lines
J. Kissel, J. Warner As Jim brought the instrument up this morning, we were alarmed to see a forest of lines around 20 to 25 Hz. As they did not disappear or change height as we went through the lock acquisition sequence, as ASC loops turned on, noticed them in MICH and PRCL, found a whole bunch of ASC-ADS SDF differences, and I heard Shiela suggest that she might try an LLO scheme for dithering the alignment, I began to suspect that these lines were intentional. Indeed, after poking around, I found the dither alignment overview screen and found several oscillators pushing out excitations at 19.1, 19.7. 20.1. and 20.8 Hz in Pitch and 21.3. 21.9, 22.3, and 23.0 Hz in yaw, being sent to PR2, PR3, and the BS. I confirmed that these lines actually make it to the SUS by gathering an ASD of the input control request at the bottom of PR2, PR3 and the BS indeed I see the same features in PR3 and BS (why not PR2?). *phew!* We'll leave these on, in case the plan was to just gather data with these lines present. See attached ASD of input request to the SUS, a screen cap of the Dither Overview screen, and the corresponding DARM ASD.
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For the record and/or future study, this lock stretch -- chilling in DC_READOUT at 2W -- lasted ~1.5 hours, from 2016-08-26 16:10:26 UTC to 2016-08-26 17:47:?? UTC. This would be an excellent lock stretch to, for example, pull out the 3501.3 Hz PCALX to DARM transfer function. From DTT alone, I was able to estimate the TF to be: mag: 2.531e-6 [m/ct] pha: -40.885 [deg] re: 1.852e-6 [m/ct] im: -1.682e-6 [m/ct] coh: 0.607 BW: 0.0025 ENBW: 0.00292 nAvgs: 35 unc: sqrt((1-coh)/(2*nAvgs*coh)) = 0.096 = 9.6%
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