From: "Tom Krajci" 
To: 
Reply-To: "Tom Krajci" 


From: "John Sherman" 

>...And if this is true, why do R&vV, in Table
>19.3 of Telescope Optics, claim that 2.8% to 9.6% of the incident light
is
>reflected (much more at higher incidence) before entering the lens? I
know
>I've read in other sources that 4% reflectance is considered normal.

The amount of reflection at a bare glass surface depends on the index of
refraction of both mediums (in this case air and glass).  Higher index
glasses reflect more light.

>There
>is also a reflectance/scatter at the exit surface of the lens, and a
>cemented achromat has three surfaces, etc.

Yes, with more surfaces you get more loss from reflection and scatter.
That's why I prefer simpler designs that uses fewer mirrors, and fewer
eyepiece elements.  If you need more surfaces, then you need to probably
spend more effort or more $$ to reduce scatter/reflection on each surface.

>So maybe enhanced coatings, if done properly, do indeed improve the
view in
>a reflector telescope, by providing less scatter.

Or techniques to lay down a single aluminum coating that is very smooth at
the microscopic level...such as ion assisted deposition.  Depending on how
much this process is used, it can get expensive.

>>The amount of scattered light can be  seen if you illuminate a
>>mirror  with a bright light, and note the amount of back reflection
>>well away from the axis of the mirror. As you approach the  axis,
>>the amount of light scattered to your eye gets very large.

>I tried this. It just happens that I have a 7" spheroid laying around
with >a brand new Beral coating on it. So I put new batteries in my
Maglite and >gave it a try in a dark room. And yes, there is a lot of
scatter. Perhaps >there are some folks out there who have mirrors with a
variety of coatings >who can do comparisons for us?

We need to standardize and make objective this comparison.  If not, we
won't prove much.

There are many variables to control here:  quality of surface polish of the
glass, age and cleanliness of the coating, etc., etc.....

Tom Krajci
Tashkent, Uzbekistan


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