Tim LeVane said:
I see that many of the better achromats like Vixen have coatings on the objective lens. How durable are these coatings with normal use? Do they need to be recoated after a few years? How much effect do these coatings have overall on the scopes performance?
Anti-reflection coatings are extremely durable given
some minimal amount of care (avoiding dew, dusting
often with blower bulb, minimal and very careful
"contact" cleaning, etc.). In principle, they should never
need re-coating. Sometimes you may get poorly applied
coatings that quickly wear away, but this is
uncommon, especially for a respected brand like Vixen.
Simple single-layer Magnesium Fluoride coatings will
increase the light transmission by about 12% for
achromat. Fancier multi-layer coatings will add a
further few percent. They also improve contrast by
eliminating stray reflections. For example, one of my
uncoated achromats makes a faint out-of-focus image
of whatever you are looking, right next to the
object (i.e. Jupiter has a faint blurry companion
Jupiter, etc.). Coatings fix this problem.
