Re: [ATM] More on my Mak-Cass project

[Michael Peck <mpeck1@ix.netcom.com>]

At 20:55 25-06-04, David Whysong wrote:

>Putting knife-edge baffles outside a cylindrical primary baffle would
>stop light from reflecting off the outside surface of the baffle tube
>at grazing angle.
>

David:

The primary baffle should, by design, lie entirely within the shadow of the 
secondary obstruction even for off-axis light. That's a basic constraint 
that needs to be satisfied if you want no differential vignetting. So you 
shouldn't have to worry (much) about light traveling from the primary to 
the secondary scattering off the outside of the primary baffle. On the 
other hand some portion of the primary baffle could be illuminated by 
extreme off-axis light, and that might be a consideration. My guess though 
(and it's only that) is that a simple flocked or ribbed tube should be good 
enough.

If you want to see a really thorough analysis of a baffle design problem 
where all sources of direct and scattered light were considered here's one 
among the SDSS survey telescope's engineering documents: 
<http://www.apo.nmsu.edu/Telescopes/SDSS/eng.papers/19960616_Baffles/19960616.html>.

There is also a gallery of photos of the telescope at 
<http://www.sdss.org/gallery/gal_photos.html>. In particular this photo 
<http://www.sdss.org/photos/00_514.72dpi.jpg> shows the top of the primary 
baffle. Notice they did use a stack of annular baffles with the outer edge 
having a conical profile.

Mike Peck


_________________

Michael Peck
email mpeck1@ix.netcom.com
Wildlife photography page http://home.netcom.com/~mpeck1/index.html
Amateur telescope making http://home.netcom.com/~mpeck1/astro/astro.html 

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