Re: [ATM] question about eyepiece coatings

[Dominic-Luc Webb <dlwebb@canit.se>]

I'd like to take a stab, but I am not sure. First, let us
clarify if we are discussing MgF2 (note the 2). Next, note the
reflective properties of MgF2 coatings:

http://www.escoproducts.com/html/single_layer_mgf2_coatings.html

Notice that MgF2 reflects best in the blue. Assuming you are
using a common light, like an incandesent bulb, sunlight, etc, we
assume the other photons (i.e., red) went elsewhere. The answer
probably resides somewhere in this property. Maybe you are getting
one color from the differential reflection (i.e., blue) and the
other from differential refraction and/or absorbance?

That being said, this whole field is very complicated. I looked
for absorbance of MgF2 and found a number of research papers
about spectral properties of things like Ag-MgF, Au-MgF, Cu-MgF, etc.
Pertinent to this discussion, these have distinctive absorbance
and reflective properties that are distinct from pure MgF2 and
I have noticed these are loosely called MgF coatings or films.
You may want to track down what specs you can and consider that
you might have one of these, which in essence could still be a
single coating. Either way, this is a quite interesting subject
and worth a little time reading up on. Some of the research
papers can be downloaded free in PDF format that show reflective
and absorbance properties.

Dominic

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