Re: [ATM] How dark is dark?

[hermit <hermit@outofoptions.org>]

Michael Wheatley wrote:
>
> The densitometry from gloss and matte photographic may be 
> missleading.  The readings would depend on the angle of incident 
> light and the optics of the densitometer.  It was said that the gloss 
> black had a lower densitometer reading but I would expect gloss black 
> to have much higher reflectance, especially at a low angle.
>
> If the above is valid then whether you want gloss black, flat black 
> or a mirror surface in a telescope tube would depend on the latout of 
> the surfaces.  You would probably need a graphics ray tracing program 
> like Povray to sort it all out.
>
>   
Well, I believe it does all come down to surface.  Overall, I think the
mat reflects more because it scatters BEFORE the black can absorb it. 
The point is, it does scatter about double what the glossy does.  Each
fstop is halving the light if  you are stopping 'down' or doubling if
you are stopping 'up'.  I think I was on the opposite side of this
debate first time around, but I can't sit here with an instrument to
test reflectance and ignore what it tells me. 

I think your observing conditions play into this too.  Not many of us
have to worry about getting visible reflected light opposite the
focuser.  If we do, then there is a problem with the scope design to
start with.  Now the question becomes, "Do any of us have good enough
eyes that we will be able to see the difference?". ;)

Ken Lowther
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