Recently received two new prism. Both are copper coated. Vendoir says this is an upgrade over silver coating. Anyone with comments? Thank yhou
What little I know about the situation makes me think it doesn't much matter optically what metal is used, but silver may not stand up as long as copper to moisture that might condense if the housing isn't sealed perfectly.
I tend see the copper ones a lot better in vegetated areas than the silver ones, which tend to "camouflage" themselves by reflecting their surroundings.
Leica's literature states that the copper performs best for the wavelengths that EDMs emit. Celestron's (astronomic telescopes) website talks about the performance of silver for visible wavelengths. Seems reasonable.