Peter, other makers can do >98% reflectivity coatings as a matter of course and at low cost - it seems reasonable that GSO could do the same. Spectrum use IAD to avoid substrate heating/distortion and don't call the coatings "full dielectric", but that becomes a matter of semantics, since they must have a multi-layer dielectric component overlaid on Al. Spectrum make the point that having an Al undercoat is actually an advantage over a full dielectric, since it allows the optics to be recoated.
With CF trusses, quartz substrate and 99% coatings, the GSO scopes seem to have the basis for quite high end optics.
http://www.spectrum-coatings.com/Tel...ror-Prices.htm