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Re: [ATM] Baffles
Anthony Stillman wrote:
>>>>>>>>>
<http://www.atmlist.net/contrib/atmer-at-flash-dot-net/6f10m.jpg>
<http://www.atmlist.net/contrib/atmer-at-flash-dot-net/6f5m.jpg>
Two more baffling examples, limited as Nils Olof has
done, to only what can happen. In both cases, in
exchange for inflating the heavy tube from 8 to 10
inches, two thirds down it's length, the baffle count
drops by one.
>>>>>>>>>>>
I can't see the baffles at all! But I'll calculate for the 6 in. f/5,
assuming a 2 in. secondary 6 in. from the focal plane.
The upper limit (measured from the primary) for visibility of the tube wall
via the secondary is 12 in. for a 8 in. tube, and 8 in. for a 10 in.tube.
With the approximation x=tan x for small angles, the lowest reflection
strikes the wall:
for 8 in tube, 7.5 in. up, this is in the visible area and you could use
baffling.
(In yesterday's example, I falsely assumed that the "worst" ray to baffle
comes from the edge of the primary - it doesn't but it arives at the upper
end of the visible part of the tube by the same angle.
if hopefully correctly calculated, a single baffle here would be placed 12.5
in. up in the tube and have a 6 in. inner dia - a bit too narrow for a
single baffle. In yesterday's example 6 in. f/12 with 8 in. tube, the baffle
would be 6.25 in. inner dia and placed 29 in. up in the tube, also a little
too narrow for one baffle. A 7 in. baffle at the top end of the tube would
serve for the one.)
for 10 in. all the way 12 in. up = not in the visible area and indeed the
baffle count drops by two, from 2 to 0!
for 8 in. at the upper end but 10 in. at the lower end it is 15 in. up, well
above the visible area, thus baffle count=0.
For a 9 in. tube all the way, both measures are 10 in., and the baffle count
=0.
this assumes the tube wall is coated to suppress double grazing reflections
sufficiently not to matter -even if a streetlight is shining directly into
the tube! And anyway, baffling of the upper part of the tube against stray
light not reflected in the secondary is another thing.
The analogy with a refractor is false. Hope this example shows that a bit of
calculation can be worth the effort, and that the requirements for baffling
a Newtonian is often, by this false analogy, exaggerated.
Nils Olof
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