Re: [ATM] Baffles

[Anthony Stillman <atmer@flash.net>]

Last night I took a close look at baffling a newtonian
by making some accurate drawings.  I no longer
question whether its a bad idea to flare the mirror
end of the tube, I know it is.

I?m not contradicting Andy.  If reducing the baffle
count is paramount then a larger tube will do that. 
It will also increase the weight, complexity, and
awkwardness of the OTA (outer tube assembly).

I uploaded a optically simplified drawing of a 6 inch
f/12 system at
<http://www.atmlist.net/contrib/atmer-at-flash-dot-net/6f12.jpg>.
 The fully illuminated field is 0.7 inches and the
secondary (identified by the arrowed lines), which
functions as a baffle, is six inches from the focal
plane.  The top of the drawing shows baffling for a 10
inch tube, the bottom for an 8 inch tube.

The first obvious observation is that changing the
tube size will at best reduce the baffle count by one.
 A slightly less obvious observation is that the large
baffles are two and a quarter times heavier than the
small ones.  By going to a larger tube, and fewer
baffles, the baffle weight goes from four to five and
a half.

Of course baffles don?t weigh much, but tubes do.  An
identical but ten inch diameter tube weighs 25 percent
more than an eight inch diameter tube.  Truncating the
10 inch tube at the first large baffle and using an 8
inch tube from there on will reduce this a bit, but
the coupling between the two tubes more than makes up
for any savings.

There are good reasons to oversize a tube, tube
currents, boundary layer interference with the light
cone, it's all the hardware store had, even reducing
the baffle count, but it's at the cost of increased
weight, complexity, and awkwardness.

If anyone would like to draw or describe a counter
example, I?m open to contradiction.

Anthony


fallaci nimium ne crede lucernae    - Ovid 
Literally: Trust not too much to deceitful lamplight.
What it means: Don?t commit till you?ve seen her
during the day.

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