Re: [ATM] Baffles

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Anthony Stillman wrote:

> It seems the problem has been redefined to include
> multiple scattering reflections.  OK, I'll bite.  Just
> don't hold your breath, I chew slowly.

As I recall, the start of this thread was the question "How is it that 
making the mirror end of the tube
larger in diameter will reduce the number of baffles?". By now, I think we 
have clarified the geometry enough to answer this, even if we have had a few 
slightly different approaches.

But the broader question is what problem(s) do we want to solve by baffling? 
Some aspects of secondary obstruction or of collimation *can* be solved, and 
it is it is all too easy to jump to the conclusion that they also are 
serious enough that one *should* solve them. Maybe, maybe not.

In the case of baffling, we have discussed how to baffle for single grazing 
reflections in the lower end of the tube, but we haven't discussed under 
what circumstances they are serious enough to matter.
When observing Venus or Mercury before sundown? Likely, but for safety 
reasons, an external baffling such as a house to the SW might be an 
important adjunct. When observing faint galaxies five or ten degrees from 
the full moon? Doubtful. Reducing the skyglow coming as a veiling glare over 
the sky background itself? Possibly, but how much might a few percent 
reflection matter? That lower part of the tube is only diffusely illuminated 
through a small hole - most of the sky does not contribute.

Also, how much of the problem stray light comes via the secondary, and how 
much outside it directly to the EP (or camera of course)? I find it likely 
that the baffling of the focuser is much more important(including a fixed 
baffle in the tube inside it - and this, opposed to the tube botttom, is a 
refractor type baffling where you see the unlit side of the baffles). Do you 
observe in the vicinity of streetlights that can directly illuminate the 
parts of the upper tube directly visible from the EP? If so, would it be 
better to use an external baffle such as a garden parasol?

"Baffling" the lower end of the mirror box, if open, can be important too - 
some light may sneak by from the ground behind the primary - would a bent 
baffle attached to the rocker box be an answer?

What about the minimalist telescopes with a pole carrying the focuser and 
secondary, but no baffling "behind" them at all? would they benefit from 
having a "flag" of black velvet suspended by other means? Separately 
suspended baffles might make the telescope much less sensitive to wind.

Lots to think over....

Nils Olof


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