Yesterday some investigation concerning the low frequency mystery noise has been carried on. Mainly it has been checked the possibility that the noise comes from the electronics of NE and WE mirror actuation.

In order to do that, a configuration using only one mirror to lock DARM has been implemented. After a check that no additional noise is introduced in this configuration, the actuation chain of the unused mirror has been unplugged after the distribution box. No reduction of the mystery noise has been observed in DARM.

Fig 1 - NE unplugged

Fig 2 - Zoom around 357.5 Hz: the NE calibration line is not there, which means that actually the actuators are unplugged

Fig 3 - WE unplugged

Fig 4 - Zoom around 356.5 Hz: the WE calibration line is not there, which means that actually the actuators are unplugged

As an additional test, the actuation has been switched to low noise1 (series resistor reduced by a factor of 20). In the hypothesis that the noise comes from the board, it would have been largely increased in that configuration. In fact there was no evident change of the noise.

Just a remark: when an actuator is unplugged the 50 Hz increases. This fact has been already observed for the input mirrors (especially for NI) when the relay box is put in OPEN configuration. In case of WE, the spectrum around 50 Hz is very bad.