Yesterday morning the 4 MHz beat note between LO and CC at the homodyne detector was suddenly lost at 7:38 UTC, see first picture. Since the DC readout of the HD photodiode did not change, this was likely due to a misalignment of the squeezing beam reaching the homodyne detector: at the same time the green beam was sent from EQB1 to SQB1 by flipping a movable retro-reflection mirror quite close to the delay line on the IR optical path. The misalignment is probably due to a mechanical contact between the flip mirror and the translation stage of the delay line.

To recover the alignment we observed the subcarrier beam, which was previously well aligned on the OPA, on the HD cameras. We had to move the DL_M4 mirror horizontally by 24000 picomotor counts to bring the beam within the range of the automatic alignment on the LO beam, see second picture.