[Sheila, Evan, Gabriele]

Since yesterday night the IMC has behaved in a strange way. It transitioned from a good state with high and stable IM4_TRANS power, to a bad state with lower and fluctuating power. The IMC remained for all the night in the bad state, and this morning it jumped back to the good one a couple of times. One interesting observation was that IM4_TRANS gets very unstable (30% fluctuations) but the power seen by the ISS second loop PDs is almost unchanged: same mean values and only slightly larger RMS. When transitioning from one state to the other, the MC mirrors are also slightly tilted, but is is cleraly an effect and not the cause of the transition.

We finally discovered that the bad state is triggered when the PRM mirror is aligned (see the plot). Apparently, there is some ghost / stray / scattered beam that reaches IM4 QPD when the PRM is aligned. In addition, this light also affects to some extent the IMC alignment signals, since the MC1, MC2 and MC3 mirrors are clearly moving after the PRM is re-aligned.