To: atm{at}shore.net
From: tkrajci{at}san.osd.mil
Reply-To: tkrajci{at}san.osd.mil


> From: "John Sherman" 

> >Enhanced coatings do reflect more light...but it's only about 5-10
> >percent more than aluminum coatings.  Are you sure you can see a 5-10%
> >difference?
>
> Is it really as simple as that? What happens to that extra 5-10% if it is
> not reflected? Does some of it contribute to the background glow in my
> eyepiece?

See the links I recently posted - essays by Roland, such as coatings and
eyepieces and scatter.

> >
> >Try this simple test.  Take a piece of clean window glass (no
> >anti-reflection coatings allowed)...look at a typical night sky scene
> >
> >Now look at the scene without the glass.  See how much
> >better it is?
>
> Yup, easily. The sky is darker when looking through the glass, naked eyes
> (1/8" window glass). Stars aren't suffering as much, so it seems there is
> better contrast with the glass.

This raises a good point.  The eye/human/subjective aspect of it all.  If
you are into visual observing, then yes, subjective results can be very
important.  I'm into counting photons with a CCD.  I try as much as
possible to remove the subjective aspect in my work.  If I were to put a
piece of glass in front of my CCD...I could objectively count/measure the
difference.  If I analyzed star images with/without the glass...I could get
an appreciation for the change in scatter (and other abberations) the extra
glass introduces.

> >But what about *scattering* of light by the more complex/multi-layer
> >coatings?  That may be more damaging to contrast...
>
> I think I saw an article in S&T a while back, where he was aiming the
scope
> at a black box in the daytime, and measuring how black the black was in a
> photograph. Is that the one?

It could be...it sounds right.  Unfortunately I don't have easy access to a
decent library here in Tashkent.  (One of these days I hope to visit
some...just to see how libraries work in a former-Communist-infant-nation.)

Tom Krajci
Tashkent, Uzbekistan

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