Some of the world's most expensive mirrors use Dielectric coating, which can distort the surface and cause aberrations in the reflected light. Though Dielectric coats cost nearly double that of Aluminum-coats, at Olivon Scopes & Optics, we use Aluminum-coats for most of our mirrors. The reasoning behind that is to provide the highest quality materials to produce the mirror, with super precise machines to make it super ultra-flat, and provide the best viewing enjoyment with a coat that doesn't distort your mirror over time.