Following the installation of the new OMC yesterday ( alog 75529), we started aligning the laser into the OMC.

Preparation:

Vicky enabled the WFS centering to center the SQZ beam (LOCKED_SEED_DITHER state) reflected by the SRM on AS_A and AS_B. At this point almost nothing was visible on the OMC QPDs.

We increased the whitening gain of the OMC QPDs (H1:ASC-OMC_A_WHITEN_GAIN and H1:ASC-OMC_B_WHITEN_GAIN) to 45dB from the original 0dB and reset the dark offset.

Initial beam search. Beam visible on OMC QPDs only when OM3 is totally railed in PIT no matter what:

We started scanning OM3, and saw nice beam at the edge of both of the QPDs but only when OM3 was completely railing in PIT. 

We disabled the WFS centering and started using OM2 and OM1 to make the beam more centered on the OMC QPDs. I was able to make it somewhat better but not really, and more importantly, no matter what I did, I could not releave OM3 PIT offset at all. Whenever I removed a bit of OM3 PIT and tried to use the combination of OM1 and OM2, I was only able to make it worse.

I also used OMCS offsets, but it made no meaningful impact as  the alignment was so bad and the OMCS range so small.

Diagnosis and mitigation part 1, things are making sense:

These all indicated that the OMC has a big PIT tilt relative to the old OMC. This was corroborated by our observation yesterday that one corner of the OMC breadboard was hitting the EQ stop structure and we couldn't release the OMC without moving the EQ stop bracket itself down. According to alog 75529, "We noticed a corner of the OMC on the HAM5 side was bottoming on an EQ stop. We decided to lower the EQ stop by releasing the fixing screws for the EQ stop holder. The holder was tilted, and the OMC became free."

With this knowledge, we went back to HAM6 to rebalance the OMC suspension in PIT.

Rahul and Koji pushed the PITCH Insert mass (part number 14 in https://dcc.ligo.org/D060502) by about 5mm into the direction of HAM7. We immediately lost the beam on the OMC QPDs, but relieving the PIT offset by 1000 counts (out of the total of 3500 counts), we regained the beam, so the adjustment was in the right direction.

The intermediate conclusion is that OMC breadboard is tilted in PIT such that the long edge at -X side was too low. We at least partially relieved it by tilting the intermediate mass to counter that.

I thought that another step or two would be great, but at that point we had to stop our work. We'll continue tomorrow.