Re: [ATM] Dob Design Questions

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

Dale,

Thanks for the reply.  I am getting the strong feeling I will be doing 
something similar.

Rod


----- Original Message ----- 
From: "Dale Eason" <atmpob@yahoo.com>
To: <atm@atmlist.net>
Sent: Thursday, September 16, 2004 10:22 PM
Subject: Re: [ATM] Dob Design Questions


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