The mirror seat of the diagonal body is machined, to provide a defined position.
We do regard the mirror substrate of our dielectric diagonals a proprietory information. As you may have heard, a dielectric coating with as much as 50 coating layers can extend an enormous amount of stress onto the mirror substrate. Many proudly offered dielectric mirrors suffer under "power" - means the surface bends under the coating stress and over time develops a very weak spherical shape. For this reason we use a hardened variety of borosilicate glass, which can sustain the plane polished surface to remain in perfect order. Admittedly this substrate has somewhat larger index of expansion and does take longer to regain its optical shape when subjected to large temperature differences - but only when compared to our BBHS mirror-diagonals which feature a hard-silver coated glass-ceramics (Astro-Sitall) as substrate.
However - we had extensively tested this combo of coating and substrate to be a perfect match for our dielectric diagonals - especially ensuring a longlived product that does not change in optical quality over time. Please read more about the mechanical and optical properties of our BBHS-diagonals, as well as our marketing philosophy, to not participate in the usual number competition here:
http://www.baader-planetarium.com/en/blog/baader-bbhs-reflective-properties/