Figure 1. Another channel that sees the noise is SDB1_B5_QD1_H (and all the other channels of the quadrants on SDB1). It sees the noise start when the slow shutter on SDB1 starts opening 15:59:16, then the noise level is constant, and the it stops. On B1 PD3 the noise level stops when it stops on B5 QD1, and it is present only when there is light on B1 PD3, so that is the reason why the level of noise on B1 PD3 varies in time during these 5 minutes.

This is most likely due to the slow shutter of the OMC which is mounted on an Agilis translation stage with electronic end of range stops. The stops are failing and the piezo-motor of the shutter keeps trying to push it further even though the translation stage arrived at the end of its range.

Figure 2. Shows that the OMC thermistor sees a line at ~1721 Hz when the slow shutter is in motion. This is likely the electrical cross-coupling of the voltage sent to the slow shutter showing up on the voltage readout used for the temperature measurement.  In blue is when the shutter moves and in purple when the shutter doesn't move. Unfortunately dataDisplay can't seem to handle the dynamic, and making an FFTtime plot of this channel doesn't seem to work.

FIgure 3. When the shutter is closing after an unlock there is also a 1721Hz line preens in OMC1 T1, but it is 10 times smaller.

Figure 4. OMC T1 also sees the 1721Hz when opening the OMC shutter in single bounce. The line stays on for about 30 minutes, before stopping by itself. The shutter is closed 20 minutes after the line stops by itself according to the shutter log file.

Action items to debug this further and mitigate it:

• To open the OMC slow shutter use a given number of step instead of moving the shutter to the limit. The needed number of steps to open the shutter need to be tested, for example in single bounce after the next unlock.
• Keep using the move to limit command for closing the shutter, so the shutter always goes back to the same position after a round trip.
• Test the operation of the OMC slow shutter in single bounce. The problem can be seen on OMC1 T1 when opening the shuter. Turn on/off the driver of the slow shutter and see if that restores the functioning of the translation stage stops.
• Investigate in past data if the shutter has started to slow down, if it takes more seconds between when the shutter command is sent and when the light appears on B1s.
• Check if the height of 1721Hz line is constant on OMC1 T1 when the shutter is opened, or if it changes in height after a fews tens of seconds when the translation stage arrives at the stop.
• Investigate in past data if the height of the 1721Hz line when the shutter is being opened has increased over the past months.