I have asked Eddie to have a look at some masking shields for the vacuum tank's 8" viewports. He will look at the drawings and seek out someone to build and polish them. This is to help with reducing the emissivity as viewed from inside the tank looking towards the viewports. Fused silica has a epsilon>0.75, a shiny mask coving all but the spots where light is coupled in/out should improve the radiative thermal isolation from the tank.
I've attached the drawings of my initial sketch. In the process I also did an update of all the Solidworks assemblies that we have in the SVN, these were largly drawn in the student version of Solidworks and were useless for generating drawings and assemblies. I have now updated all the parts and assemblies, there are now no student version files. All of these has been updated in (root)/trunk/CTNLab/current/drawings/dual_refcav/ of the 40m SVN . In an effort to reduce the entropy of this directory, I have broken the reference cavity drawings into subassembly folders so individual sub-systems can be edited separately.
Is 0.5" a big enough hole for our beams to get in/out?
To get the drawings follow instuctions here: https://wiki-40m.ligo.caltech.edu/How_To/Use_the_SVN_archive