Starting where I stopped yesterday, I continued to adjust the mode matching lenses.  Unfortunately, no matter how I adjuted the lens positions I could not improve the power transmitted through the PMC.  Therefore I returned the lens positions to where I started this morning (same as where I ended yesterday).

I believe the degradation in PMC Trans is due to a slight alignment shift through the 70W amplifier.  I base this on the beam profile I took yesterday.  When compared to the one from November 2018 (the last time we tweaked the alignment through the 70W amp) the beam is not as round as it was.  Last November the horizontal beam width was 0.69% larger than the vertical; now the horizontal beam width is ~3% larger than the vertical.  In addition, the halo around the beam is brighter than previous.  To fix this we would need to realign the beam through the amplifier, which would then cause a full realignment of the beam path from the 70W amp to the PMC; this would interrupt all work on the IFO for a few days (PMC and FSS unlocked, ISS AOM removed for alignment).  Before launching on this I spoke with Betsy and Keita.  We decided that there was no rush to launch on this effort for a few reasons:

• No evidence of degradation accelerating.
  • In the ~11.5 months since the last amplifier alignment, PMC trans has only dropped by ~1.5W.  That's pretty good.
  • Regular alignment tweaks have recovered almost all of PMC trans in the past, this 1.5W has been cumulative over the last 11.5 months.
• The H1 IFO currently operates at 37W; we have 53W output from the PMC, so we have plenty of power banked away in case we need it.
  • The PSL/IO rotation stage can be recalibrated to bring power into the IFO back to 37W when necessary.
• Commissioning efforts need to launch on time if we're going to start O3b on 1 November, 2019.
  • There is still other work that needs to take place before full-scale commissioning can begin, and this realignment would delay that work as well.

So, that's where things stand right now.  While we are seeing less light through the PMC, this is not a critical issue worth bringing other necessary work to a stand-still over.  I will continue to monitor the PMC and will continue to take necessary preventive maintenance actions when required.  Thus ends the H1 PSL work for the October 2019 break.