Dielectric coatings and distortion??

Hi all,
With the recent threads on wavefront errors etc. and being in the market for a new 2"diagonal, I happened across one vendor's website that implies dielectric coatings can distort the mirror surface over time leading to aberrations. The suggestion is that Aluminum coatings are superior as they do not distort the mirror surface over time. I have not heard this before and actually thought one of the main advantages of a dielectric coated diagonal was the durability as well as resistance to scratching and distortion. Can anyone clarify?
Thanks
John

You are falling victim to a marketing ploy. The dielectric star diagonals that we make use a proprietary coating process that does not stress the mirror surface, neither when new, nor years down the road.

Aluminum coatings are fine for general use. For critical planetary viewing they have an advantage because you need a very clean surface on your eyepiece lenses as well as on the diagonal to get the highest possible contrast. Dielectrics have much better longevity and cleanability and lower scatter than any mirror coated with a metallic material.

Diagonals are very prone to accumulation of dirt and dust because one tends to insert eyepiece barrels numerous times during the night. This action drops crud down on top of the diagonal mirror which most often does not come off by simple blowing of air. Shine a flashlight onto your diagonal after only one night's use, and you will see. Dielectric diagonals can be repeatedly cleaned without picking up tiny faint sleeks.

Roland Christen