Virgo Logbook

Since the beginning of the run, there is an annoying comb of lines at 10.2782Hz visible in the dark-fringe (which is apperently an old problem). Suspecting some electro-magnetic disturbance, I looked in all magnetometers without finding anything. Thanks to the online coherence-monitor, I finally found the comb present in some of the current-monitors of the input mirrors, see fig 1. Lots of harmonics can be seen, see fig 2.

This does not directly give a clue about its origin, but it narrows down the area where to look quite a bit. It might be worth hunting a bit with a portable magnetometer between the DAQ room and the suspension-racks on the platform. Note that the coils of the input-mirrors are not used for the lock, so these channels just serving as fancy antennas to pick up disturbances. We have seen something similar with the 3kHz bump, which was visible in several channels with long cables. It might be interesting to connect some long, unshielded cables to a free ADC channel in all of our labs ...