Gabriele's log entry 71205  pointed out that the OMC is moved a lot (at least in comparison to the LLO OMC) when it is receiving OMC ASC feedback. This observation led to the test of turning of the OMC ASC for some time to look for any effects, see 71422. Although no effect on DARM noise was seen, I was still curious to know: how much is the OMC moving under ASC control?

The attached plots show time series of the 6 BOSEM sensors that sense the upper/top mass of the OMC suspension (relative to the OMC suspension cage). The 5 minutes of data is during the 'OMC pause' test, so in the first half of each plot the OMC is receiving ASC feedback, and in the second half this feedback if off. The plots show:

• T2 & T3: On the top, and at one end, of the (top) mass, so they sense vertical motion. The motion is about 1 micron pk-pk. These 2 sensors are combined to form the Pitch (P) degree of freedom, so the top mass is 'pitching' by about 15 urad pk-pk.
• T1: On the top, at the other end of the top mass. This is moving much less, by about 0.2 micron pk-pk.
• LF & RT: On the long side of the top mass, separated by about 16 cm. The motion is also about 1 micron pk-pk. These sensors form the Yaw (Y) DoF, to the top mass is 'yawing' by about 10 urad pp.
• SD: On the short side of the top mass. This motion is very small, maybe 0.06 um pp, and there's little if any diffference between OMC ASC on or off.

So, even though the OMC is moving a lot compared to that at LLO (which has a different OMC alignment feedbak scheme), the motion is still small compared to the beam, which has: 500 um radius, 680 urad divergence angle.