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Re: [ATM] Dob Design Questions
Hi Rod.
What I did on my 24" is :
1) I made an excel sheet with the theoritical weights
2) made a scale drawing of the entire scope (a first version)
3) adjust the excel sheet with the information you get from the drawing :
for example, when you do the scaled drawing you can then adjust many of the
dimensions of the scope that you just had assume at first (height of
secondary, height of the mirror box, size of the radius rocker bearing). For
the size of the radius rocker bearing, you know from Kriege book that the
bigger will give the best buttery feel with FRP or ebony star. But when you
draw the hole scope, you then see that a huge radius rocker bearing leads to
bearings that will be much larger than your mirror box => the global
assembly will then be less strong. So just using a drawing you find an
optimum between the larger rocker bearing and the problems a very large
rocker bearing means (assembly less strong, global telescope size when
stored, visual aspect of the scope)
4) Then you're reading to start building the upper cage. When it's finished,
you know have it's real weigh. I then made a new excel sheet with the real
weight. Then build the mirror triangle assembly, wheight it, put it's real
weight in the excel sheet, then build the mirror box wheight it, put it's
real weight in the excel sheet.
You see that each time you build one part, your excel sheet gives you a
better and better estimation of the final COG. The last parts you do are the
radius rocker bearing. You finally put their weight in the excel sheet and
the final COG is there.
I send you my excel sheet privately. The first sheet (with only theoritical
weights) is named "600 mm f 3.3 (PTFE)". The second (with some real weight)
is named "600 mm f 3.3 (PTFE),poids réels".
Last thing : don't forget to put the AVERAGE weight of your eyepieces (in
fact average = (your heaviest ep+ "no" ep (when you change ep!) ) /2 => your
heaviest ep /2
Raphaël Guinamard
----- Original Message -----
From: "Jan and Rod Shea" <jnrshea@comcast.net>
To: <atm@atmlist.net>
Sent: Friday, September 17, 2004 6:54 AM
Subject: [ATM] Dob Design Questions
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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