Today we fine tuned the rotation of the HWP that rotates the polarization of the beam after the IMC. We followed these steps:

- we used an HWP at the bottom of the PSL periscope to set the polarization approximately "wrong", so to have considerable power transmitted through the unlocked IMC

- we used the HWP after IM1 to make sure that a good amount of power would go in the first forward rejected beam

- we rotated IM2 in yaw so that the first forward rejected beam would clear the FI and be transmitted all the way to (almost) IM4

- we position a power meter there

- we removed the HWP on HAM2, and rotated the HWP in the PSL so to minimize the power in the forward rejected beam. Because of the absence of the HWP in HAM2, this beam contained the "good" polarization: this way we made sure that the polarization off the PSL was as bad as possible.

- we steered IM2 back in its original position, and we placed the power meter in the main beam

- we re-inserted the HWP in HAM2 and optimized it to minimize the power in the main beam, thus rotating the polarization coming out from the IMC by 90 deg, that is what we want.

Unfortunately, during this process we had the opportunity to compare the beam out of the IMC with the re-routed IMC_REFL beam that had been setup couple of weeks ago and used since then as a more powerful reference for alignment. What we discovered is that the beams are not well aligned (more than a beam diameter at the FI). Note that:

• the beam from the PSL, reflecting off of MC2 is still well centered in two irises that we have in HAM2, in front and behind MC3. Thus, is seems that the beam out of the IMC has not drifted.
• To center the beam into the FI, IM2 now needs to be steered in yaw by about 10000 cts, while it's aligned position is supposed to be -520 cts (and still work with the rerouted REFL beam). This suggests that the misalignment between the two has not happened this week, but probably much earlier in the process of IO alignment.

The good news is that the situation can probably be easily recovered, without moving other components, using IM1 and IM2 to make sure that the current IMC beam is aligned, at the FI and further downstream, to the reference we have been using up to now. If the offsets turn out to be big, we may need to relieve them.