Re: [ATM] Dob Design Questions

[Dale Eason <atmpob@yahoo.com>]

You can't really know the balance point unless you
know the weight of all your components and their
placement on the optical tube assembly. What I did was
to built it without the altitude bearings and then
find the balance point.  This then I made the altitude
bearings big enough so that thier centers where close
to the balance point.

Don't get too hung up on having this perfect because
the balance point will change as you add barlows and
different weight eyepieces during use.  I added velcro
strips on the mirror box that let my rebalance by
attaching 1/2 lbs weights on the mirror box of my 16"
F5 scope.

Dale Eason

--- Jan and Rod Shea <jnrshea@comcast.net> wrote:

> I am getting farther into designing a 12.5"
> lightweight dob, something like Dan Gray's 28"er's
> baby brother.  I am getting more and more confused
> about how to design it.  If you get a chance, could
> you let me know what you think of this?  These are
> the things I'm kinda stuck on right now.  I get the
> feeling there is/are some basic things I am missing,
> even after reading Dave Kriege's book, and
> everything on the web I can find. 
> 
> How should I decide what radius rocker bearings I
> should make, and where should I put the CofG? 
> Clearly, the larger the bearing radius, the farther
> from the CofG it will be, assuming a balanced scope.
>  To know the CofG, it would seem I need to know the
> weight of the secondary assembly and, in fact, every
> part of the scope, before nailing down the rocker
> dimensions.  Should I be making the secondary
> assembly first, if only so I can weigh it? 
> Following this line of thought, Do I need to build
> most of the scope before designing the rockers and
> the rocker box?  Worse, as the rocker's radius
> increases, it gets heavier, and farther back from
> the CofG.  How do I deal with this?
> 
> I have looked at Hughes Larouche's and Ken
> Bertapelle's spreadsheets, and they seem to define
> the CofG at the top of a traditional mirror box,
> which this scope will not have, and they just need
> to know the weight and position of the rockers.  Has
> some clever person done a spreadsheet which
> calculates an optimal rocker radius taking into
> account the weight changing with the radius?
> 
> I get the feeling some designers choose a radius
> based on a rule of thumb, and then adjust the mirror
> up or down to get the CofG and radius to line up. 
> What rules of thumb are out there?  
> 
> I think I want the rocker radius such that the
> rocker is as close to the plane of the mirror as
> possible, to minimize the material connecting them. 
> It would be easy enough to simply build it this way,
> and then counterweight (real or springs) it until it
> balanced, but there must be a better way!
> 
> Thanks,
> 
> Rod
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> ATM mailing list http://www.atmlist.net/
> 



		
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