Re: [ATM] Baffles

[Andy Saulietis <iss@pvtnetworks.net>]

Re baffles


>How is it that making the mirror end of the tube
>larger in diameter will reduce the number of baffles?
>I don't get it, or Astronomy Magazine either.  It
>seems to me all this does is replace a light weight
>baffle with more heavy tubing.


I'm the guy who designed optical baffling for Apollo, Skylab, and the 
Shuttle star
tracker programs.

The design goal for a simple telescope baffle is to have the stray 
light take 2 bounces before
reaching the focal plane, and also minimize the number of baffles to 
reduce the amount of
light that bounces directly into the focal plane from the interior 
edges of the baffle. A larger
tube diameter has fewer baffles because they can be farther apart and 
still make the tube
interior not visible as seen from any point in the focal plane. 
Baffle layout is done
starting at the focal plane, spacing them to meet the tube ID not 
visible criterion.
Baffle edge scattered light can be minimized by masking the baffle 
edges sharper. This
will result in a 2-bounce baffle, assuming 2% reflective flat black 
on all interior surfaces
the baffle would scatter about .04% of the extraneous stray light 
into the focal plane.
For scopes with central obscurations and spiders it may not be 
possible to have total 2 bounce
performance, but using good flat black surfaces will help.

For my own scopes, I use Krylon 1602 Ultra Flat Black spray paint. 
Works great if
applied just thick enough to cover the surface. Additional coats 
actually increase the
reflectivity.

If better baffle performance is required, one can a) use blacker 
surfaces and/or add an
external baffle that shades the main baffle..we made some that had 3 
to 6 bounce
geometries. Wide fields of view can make the baffle size increase to unworkable
lengths/diameters.

The blackest surfaces I've seen were acetylene soot (fragile & 
messy), and a proprietary
anodizing process called Martin Black (horrendously expensive and 
only works on some
aluminum alloys). Both of these reflect <0.5%. Black velvet or black 
flocking is in the 1%
range but can be difficult to apply.

Clean optical surfaces also help a lot.

If you have a lot of light pollution, baffling will help but only to 
the degree it doesn't contribute
significantly to the amount of sky brightness. One of the reasons I 
now live in the NM mountains..:)



_______________________________________________
ATM mailing list http://www.atmlist.net/