[ATM] Multicoatings

[Bob May]

Multicoatings are used to "match" the index of refraction of the
glass to that of the air.  Reflective coatingsdo improve the
reflectivity of the glass surface but do tend to be more narrow
band than plain ol' silver or aluminum and thus they need to be
put on for the particular angle that you're going to be using the
mirror for.  The peak reflectivity is put at the center of the
light spectrum so that the last four percent of the light is
highly reflected.  Unless the multicoating is really narrow, you
won't notice the color loss at the edges of the spectrum.
For refractive surfaces, this also ends up being a plus as the
light at the ends of tghe spectrum are less controlled than the
center of the spectrum so not only do they end up letting more of
the light through at the center of the spectrum but also act as a
weak filter at the edges.  Light loss at the edges my be greater
than the four percent noted, depending upon the number of layers
of teh coating.  This actually improves the appearance quality of
the image on a refractor.
For a side note, calculat out the light loss needed to reduece
the light by one magnitude and you'll see how little that a
multicoating will affect the image quality.
Bob May
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