Re: [ATM] Baffles

[johncdeitz@comcast.net]

Somehow reducing the number of baffles by one seems incorrect. Of course, when working at f/8 there will be a greater reduction seen in the baffles replaced by a wider tube. during the winter time I found I needed an additional fan to eliminate air trapped by the baffles in conventional tubes . As a minor added advantage I installed eyepiece holder in the space left by the difference in diameters and so had ready access to eyepieces at the front of the tube.
John
 -------------- Original message ----------------------
From: 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.
> 
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