The main difference is not in the reflectivity. This is not what's important about diagonals, because you will not see the brightness difference visually between a normal aluminum 88% diagonal, and enhanced 92% aluminum diagonal or a 99% dielectric.
There are two reasons for a dielectric 99% diagonal, aside from the high reflectivity:
1) the surface is cleanable without danger of introducing sleeks which can affect planetary contrast. A full dielectric is made from 50+ alternating layers of hard oxide materials. These have very good abrasion resistance because they are similar to thin layers of quartz. A diagonal using aluminum, even when overcoated with a protective dielectric layer, is vulnerable to scratching and sleeking when repeatedly cleaned.
Why does a diagonal need to be cleaned? Dust and dirt will accumulate quickly on a star diagonal as you insert and remove eyepieces, and this is the number one reason for poor planetary contrast. To get the best planetary performance, you need a clean diagonal surface.
2)An all-dielectric diagonal does not have a metal layer, so the overall light scatter is very much lower with the dielectric. Any time you apply a metal layer to a glass surface, you produce a scatter layer. You can see this easily enough if you compare an aluminized surface with a dielectric surface under full sun illumination. If you look through the diagonals, without any telescope, as they are pointed nearer and nearer to the sun, you will see the aluminized diagonal surface grow in brightness very much faster than the dielectric version. The bottom line again is higher planetary contrast. A good clean dielectric is the closest thing to having nothing but air between the eyepiece and the telescope objective.
Is the difference big? It is to dedicated planetary observer, and small or non-existent to the deep sky observer.
Roland Christen
There are two reasons for a dielectric 99% diagonal, aside from the high reflectivity:
1) the surface is cleanable without danger of introducing sleeks which can affect planetary contrast. A full dielectric is made from 50+ alternating layers of hard oxide materials. These have very good abrasion resistance because they are similar to thin layers of quartz. A diagonal using aluminum, even when overcoated with a protective dielectric layer, is vulnerable to scratching and sleeking when repeatedly cleaned.
Why does a diagonal need to be cleaned? Dust and dirt will accumulate quickly on a star diagonal as you insert and remove eyepieces, and this is the number one reason for poor planetary contrast. To get the best planetary performance, you need a clean diagonal surface.
2)An all-dielectric diagonal does not have a metal layer, so the overall light scatter is very much lower with the dielectric. Any time you apply a metal layer to a glass surface, you produce a scatter layer. You can see this easily enough if you compare an aluminized surface with a dielectric surface under full sun illumination. If you look through the diagonals, without any telescope, as they are pointed nearer and nearer to the sun, you will see the aluminized diagonal surface grow in brightness very much faster than the dielectric version. The bottom line again is higher planetary contrast. A good clean dielectric is the closest thing to having nothing but air between the eyepiece and the telescope objective.
Is the difference big? It is to dedicated planetary observer, and small or non-existent to the deep sky observer.
Roland Christen
