[ATM] Dob Design Questions

["Jan and Rod Shea" <jnrshea@comcast.net>]

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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