Re: [ATM] The standard aluminium coating: How much scatter?

["tom bouthillier" <athenabout@hotmail.com>]

I've always thought that the scattering is due to the mirror or cleaning etc 
after the fact of aluminizing. When you consider that the aluminum and 
coating is layed down by evaporation - on an atomic scale, it's had to 
picture that the coating could induce scatter unless the underlying surface 
has scratches.


>From: "John Sherman" <atm@johnspics.com>
>To: <atm@atmlist.net>
>Subject: Re: [ATM] The standard aluminium coating: How much scatter?
>Date: Tue, 15 Aug 2006 13:21:23 -0700
>
>
>
>
>Hi Guntram,
>
>Welcome to the list! I haven't seen an answer to your question, perhaps
>because no one has facts. I don't have the facts you want either, but you
>said:
>
> >I would be happy even for a good "order of magnitude" estimate
>
>Consider that the standard aluminum coating reflects about 86%. The rest is
>absorbed or scattered. I believe much more is scattered than absorbed. You
>can take a flashlite and shine it on a mirror from the ROC, and put your 
>eye
>next to the returned image. You will see plenty of scattered light, the
>closer you get to the image the brighter the scatter. Having done this, I
>estimate that more than 10% is scattered from a standard coating. Perhaps
>you could shine a light on a mirror, in a very dark room, and record the
>scatter with a camera or photometer. If the image also falls on the
>detector, you can probably get a good idea of the amount of scatter, and 
>how
>it varies with distance. You could also use a photographer's light meter to
>gather data.
>
>I can easily picture how light scattered off of the primary can also 
>reflect
>or scatter off of the secondary and then enter the eyepiece as veiling 
>glare
>(background light).
>
>I have a 22" scope with a standard coating on the primary. The mirror which
>was previously in the scope had enhanced coating. Unfortunately I never had
>the two mirrors at the same time to swap in and out, and a further problem
>is that the previous mirror did not focus light very well. But after years
>of observing, it seems to me that the enhanced coating provided much better
>contrast, because the background sky was darker, and the image was a tad
>brighter. If ever I have a choice, I will always choose enhanced. Consider
>that enhanced might reflect 96%. That would mean there is less light to
>scatter back into the eyepiece.
>
>It is an interesting question, please let us know if you learn any facts.
>
>Thanks,
>
>John
>
>
>
>
>
>
>
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