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

["John Sherman" <atm@johnspics.com>]

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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