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

["Guntram Lampert" <glampert@aon.at>]

John, Tom,

thank you for your input!

John: Yes, I have heard of this "test" too, and if I have time, I will try it out this weekend. Unfortunately, I am not equipped for quanitative measurements.
While thinking about your answer, it occured to me that the scattered light the test shows might simply be due to the residual roughness and pits of the polished glass surface, made much more easily visible because of the reflective coating.
I have a polished, but uncoated mirror lying around. It will be interesting to see if it produces the same phenomenon when intensely illuminated compared to a similar, but aluminized mirror illuminated much more dimly to account for the big difference in reflectivity. Stay tuned!

Tom: I have a visceral feeling that you are on the right track. As far as I know, it is necessary to coat specimen for scanning electron microscopy wit metal films by evaporation, and no "roughness" whatsoever is revealed in magnifications of many thousand times.
Dusty and dirty mirror surfaces almost certainly scatter more light than a well applied coating.

However, I would be most interested in quantitative data. How much light does a surface scatter before it is put into the coating chamber, and how much does it scatter when it comes out?

Personally, I tend towards Texereau´s point of view. He did such thorough and comprehensive work on the topic, and never blamed the coating itself for stray light, to my knowledge.
This whole scatter topic emerged when I read older german literature (~ seventies and eighties) concerning prominence observing. The literature flatly dismissed reflectors as being incapable of showing them because of scatter due to the reflective coatings. Now, some years ago, a Newtonian reflector was adapted very successfully for prominence viewing, and I intend to try the same with my 110mm unobstructed Kutter reflector.

Guntram


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