Whats better; semi-enhanced or enhanced mirror coatings?
Enhanced coatings offer slightly higher reflectivity over semi-enhanced coatings, but at the expense of a slightly rougher surface profile, and a slight change in the mirror’s figure. This results in some additional light scatter and a lowering of contrast. Semi-enhanced coatings are built up of two layers (aluminum and protective overcoat). Enhanced coatings are made up of many layers of coating. As these coatings tend to build up at the edges and curves, the actual figure of the mirror can be compromised. This is why mirrored flats and diagonals can be coated with enhanced coatings generally without problems. But note, even a $5000 laboratory reference flat with enhanced coatings is only guaranteed to spec in the 90% inner zone, the outer 10% zone is often considered a wash. Most mirror manufacturers advise AGAINST enhanced coatings. Some batches of enhanced coatings are un-strippable, and result in the mirror needing to be completely re-polished. Here is a chart showing the effectiv
