jeffrey.kissel@LIGO.ORG - posted 19:01, Wednesday 19 March 2014 - last comment - 13:48, Tuesday 25 March 2014(10881)
H1 ETMX Optical Lever Confusion
Has the analog whitening settings changed on the ETMX optical lever? I've compared a high-frequency spectrum with a measurement I took in February, and there's reported tons more motion above 2 [Hz], when the ISI motion is roughly equivalent if not better at these frequencies (see attached). Looks there's an extra 1:10 analog filter that's not being compensated or something...
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Looking at a previous OPLEV spectra that was made in February on ETMX ITMX and ITMY (calibrated in urad/sqrtHz), it looks like the signal amplitude @ 10Hz (on the three of them) was similar to what you have on your red curve (~0.1 nrad/sqrtHz). Maybe the green trace (reference) was taken with a digital compensation filter engaged, but no analog whitenning ?
Refer to ALOG-10267 for the changes in whitening settings on Feb 21st 2014. This accounts for the two orders of magnitude difference you see above 10 Hz.