We're seeing another somewhat quick drop in the FSS RefCav TPD.  Back on October 14th I adjusted the beam alignment into the FSS RefCav and got a TPD of 5.4V.  Between the 14th and this last Tuesday, the 29th, the RefCav TPD dropped from that 5.4V to 4.2V.  I attempted to use the picomotor-controlled mounts to return to the 5.4V level, but was only able to get back to 4.8V (subsequently, I think being unable to get back to the previous TPD volatge is an indication that not only are we about to see a quick drop in the RefCav TPD, but that drop is likely to require a PSL enclosure incursion to fix).  In the ~72 hours since I performed that last beam alignment tweak the RefCav TPD has dropped from 4.8V to ~3.2V, a drop of 33% in just 3 days.  The attached picture shows the FSS screen as it is right now; as can be seen, the FSS common gain has been increased by 3dB to account for this loss of optical gain.  At the next available opportunity (earthquake, upcoming Tuesday maintenance window), I will attempt to use the FSS picomotor-controlled mounts to tweak the beam alignment into the RefCav.  If I am unable to get the TPD back to its previous levels, then we will have to plan an enclosure incursion to fix (as was done back in May).

My current best guess for the underlying cause is temperature changes in the PSL causing mounts to move slightly.  We are currently coming out of a mid-autumn cold snap, and temps in the PSL enclosure are slightly lower than they have been the last several months; when we encountered this issue back in May the weather was warming up for the impending summer.  At this point this is a rather serious WAG on my part, so I'll look back through trends to see if there's any actual evidence for this.