[Vaishali, Jenne]
We went again to have a look at the IMC trans path on IOT2L, and we think that the main beam is just not coming out of the vacuum.
Kiwamu pointed us to alog 17310 from March 2015, where he notes that something happened in vacuum and the IMC trans beam went from somewhat clipped but still somewhat normal looking to totally abnormal looking (which we've had consistently for the last 2 years). Vaishali is currently looking into Cheryl's IMC spot measurements to see if the beam spot motion is consistent with the beam moving closer to a position where the trans beam is clipped on the IM1 suspension cage.
We still have the ghost beam on the trans camera, but no beam bright enough (by a factor of ~100, as measured by the PD) to be the main beam.
Drawing attached showing the beam on IM1 that is IMC Trans.
That beam was -6.5mm off center before the vent, and is now -7.0mm off center after the vent, making the clearance through the light pipe even worse.
The -0.5mm change in beam comes from using the MC3 OSEM changes from before to after the vent.
The root cause of this issue with IMC Trans are the positions of the beam spots on the IMC mirrors, and in particular the beam spot on MC3 that I measured recently as -5.7mm from center in yaw along the face of the optic.
The two things that we can control that effect the IMC beam spot positions are the IMC input pointing, and I have an approved ECR to add hardware to the IO beams on the PSL to monitor that beam. That beam can also be corrected from the PSL.
An emerging issue is that the steering mirror to the MC2 Trans QPD may be positioned in such a way that maintaining centering on the QPD with it's servo is not producing centered beams on the IMC mirrors.