Re: ATM Internal reflections in Obsession Truss type designs???

["Bill T." <twentiethwave@hotmail.com>]

Fred Hermann wrote:
>
> I'm looking at constructing a large aperature truss scope and noticed that
> Obsession type Dobs don't bother to paint their truss tubes a flat black
> color, yet provide optional light shrouds.   Also programs like Newt
provide
> information for placement of a large number of light stops.  I've always
> been under the impression that internal reflection sources and decreased
> contrast were a big concern, yet when I look at these open truss designs
> these reflection sources are completely ignored.   Am I missing something
> here???

I and others I know have experimented with baffling the tubes of Newtonian
reflectors.  Although baffles are highly effective in refractors, the
results I have observed have not supported the notion that tube baffling
gains anything contrast-wise for Newtonians.

However, simple baffles located in the focuser tube to prevent non
optical-path light from reaching the end of the eyepiece are very effective.
If you feel like cutting some baffles, that's were to conentrate your
efforts.  There's discussion about sizing and placing of such baffles in the
ATM archives.

I have no data to judge whether or not truss tube painting is useful, except
that I have never seen any in-the-eyepiece effect which I thought was caused
by bare aluminum tubes.  At the high grazing angles light hits the tubes,
almost any surface will be highly reflective, even black matte paint.  You
could add granular material to the truss tube paint to handle the grazing
problem but this would make the tubes very difficult to manage.  Some people
put black pipe insulation foam over the tubes.

Bill T.