Thanks for the further update Gaston, I'm now wondering if the Edmund Specs of 6 lambda is a misprint.
I had a look at Thor Labs Cold Mirror specs and they claim one tenth lambda surface error.
Unfortunately their biggest size is 25mm at this spec in fused silica. Their Soda-Lime mirrors are larger but no mention of surface quality.
Sorry for hijacking your thread Charles but if you intend to still go ahead with building one, have a look at the Thor specs before committing to the Edmund product.
http://www.thorlabs.hk/newgrouppage9...tgroup_id=6108
Regards
Bill