The high optical quality of the Baader narrowband filters guarantees no loss in sharpness or contrast. Each substrate is fine-optically polished plane parallel to deliver a smooth and accurate 1/4 wavefront over the entire surface. Each filter is blocked and polished individually, not cut out of a larger plate of glass. The special custom polishing operation sets Baader filters apart from the lower optical quality provided by standard commercially precision polished filter substrates used by most other filter manufacturers.
Coatings are professionally applied in Germany by the most sophisticated equipment available today, ensuring high consistency for bandpass cutoffs, transmission, and out of band blocking. The anti-reflection coatings are evenly balanced on both sides of the filters to reduce surface stresses and prevent deformation of the substrates. The ultra-hard ionbeam deposition ensures the coatings will never degrade from use or normal cleaning. Unlike many competitive filters, Baader coats each filter individually. Baader filters remain sealed at their coating edges for a long life without degradation (you can see evidence of this process by inspecting for a thin un-coated region at the very edge of the filter). Though more costly, this process avoids the coating failures that can result when filters are simply 'biscuit cut' from bulk coated sheet glass, leaving ragged coating edges for moisture to penetrate and cause coatings to peel away from the glass. Baader filters are intended for a lifetime of real-world use.