Re: [ATM] How dark is dark?

[GF <gary@rcn.com>]

 >> Flare is largely a neglected issue with camera lenses,

It is?

 >>Well, we mainly build Newtonians so that is what much of the 
discussion should turn on.

Agreed. In, so far, over two dozen posts, cameras were mentioned twice. 
Three times or more if you count mentions of densitometers and emulsions...

I have found the discussion interesting. It does seem, intuitively, that 
the idea of reflecting stray light to places where it won't reach the 
primary or secondary or focuser tube is a good one. Though carried to 
extremes it might mean a polished aluminum tube would be even better 
than any black.

I didn't ask or notice at the time (or did but don't remember) but I 
wonder if the scope maker I mentioned also used gloss on his secondary 
support and inside the focuser tube and eyepiece barrel and exposed 
parts of the mirror support? I'll bet that for the gloss method to work 
it woud have to be used only on the tube and maybe on the baffles also. 
I seem to recall that the baffles on that scope were slanted rather than 
perpendicular to the tube so that light striking them wouldn't be 
bounced back up.

Cheers,

Gary Fuchs



hermit wrote:

>GF wrote:
>  
>
>>I think three years ago at Stellafane a fellow showed a 6" or so 
>>Newtonian that was baffled and gloss black inside. He claimed that with 
>>the right baffle design less light reached the mirrors or eyepiece than 
>>with flat because the reflections were directed away precisely and more 
>>fully.
>>
>>It was a very bright day and compared to other scopes, his did indeed 
>>look darker. With other scopes I could see down to the mirror support, 
>>with his I could just barely make out the first baffle.
>>  
>>    
>>
>
>Well, that is what I was 'surmising' in an earlier post.  Glossy black
>always looks darker than flat.  Depending on the design of the scope it
>should/could work better as in this example.
>
>  
>
>> I do also think though that Leitz, Zeiss, Haselblad, Nikon, Canon, and 
>>so on would have used this in their cameras and lenses years ago if it 
>>was truly better.
>>
>>  
>>    
>>
>Well, we mainly build Newtonians so that is what much of the discussion
>should turn on.  Flare is largely a neglected issue with camera lenses,
>but Phil Davis in his "Beyond the Zone System" book suggest 10% is not
>an unreasonable amount to plan on in a large format camera.  Remember,
>these are more refractors than Newts.
>
>But you also illustrate that some one has done the 'test' and indeed,
>you noted a remarkable difference when using gloss paint.  My scope is
>about due for an overhaul, and I will use it myself.  I have never
>properly baffled it, but that will also happen.
>
>I can paint some thin surfaces and pop them into the densitometer when I
>do the rebuild. 
>
>Ken Lowther
>
>  
>
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