Re: [ATM] How dark is dark?

[hermit <hermit@outofoptions.org>]

GF wrote:
>  >> Flare is largely a neglected issue with camera lenses,
>
> It is?
>
>   
Well, it is by folks using them.  Only one author that I have read even
mentions its effects on the final print.  Again, Phil Davis in his book
"Beyond the Zone System".  He gets lots of criticism for being too
technical.  All of those 'artist' photographers think  knowing the
technical end ruins you as an 'artist'. ;)


>  >>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...
>   
With emulsions you have something that is pretty much identical EXCEPT
the surface of the paper.  So you can compare apples to apples.  If you
go out and buy flat black and glossy black, you probably won't get the
other parts held as constants.  If you have a better way of measuring
reflection than a densitometer, let me know.  But remember, this is what
got me thinking about it again.

> 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
>
>   
Again, this is the point.  I'm not saying use it on everything,
everywhere in the scope.  Without baffles, I don't think it would be
good in a tube unless you had a very oversized tube with plenty of
length ahead of the secondary to limit the angle of light.  I think you
said you looked down the front of the scopes.  Looking into the empty
draw tube would have been more telling perhaps.

Ken Lowther
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