Unusual HF resonances/noise were observed on the transfer functions recently measured on HAM-ISI Unit #4.
Multiple configurations were tested:
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Initial Configuration: Top Mass Sitting on 3x thin Washers, one of them was mistakenly set on an optical table track (we found the washer bent) - Balancing Masses (5kg and 10kg) stacked on thin washers too.
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Top Mass Sitting on 3 Thicker washers instead of the thin ones. Same configuration (thicker washers) for the 5kg and 10kg balancing Masses. -> The unusual 126Hz and 160Hz resonances are pushed towards ~195Hz, where we saw them on the previous Units.
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A bolt, and its washer, that were sitting in a vertical hole of corner #3 GS13's door were removed -> No Noticeable Effect
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A cable, which was possibly rubbing, was set in another position -> No Noticable Effect
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The washers set under the balancing masses were removed -> This considerably reduced the unwanted HF resonances/noise
A presentation for the results of each step is attached.
Transfer function measurements are running overnight on the ISI in its latest configuraton.