Re: [ATM] New 16" telescope design

["Alan Scott" <alanne1956@hotmail.com>]

Mark,
  Thanks for the advice on the cell.  I am basically trying to scale up the 
12", and may have gotten too simple.  I will take a much harder look.

  Also, thanks on the advice on Albert's cell.  He does wonderful work, and 
I have to admit he has been an insporation for many of these ideas.

  I love many of the ideas on dobs from Kriege & Berry, but find much of it 
over build and/or heavy.  I am trying to make a scope that is lightweight 
enogh that I can pick it up out of a hatchback.  Thus, many of the 
lightweight ideas.  I will take a look at their work (it is on my shelf 
behind me) however.

  Thanks again,

  alan

p.s. - I corrected the web page to include the type and source for the 
composites.  They are going to be thin skins of plywood with a lightweight 
core.



>From: Mark Holm <mdholm@telerama.com>
>To: atm@atmlist.net
>Subject: Re: [ATM] New 16" telescope design
>Date: Sun, 20 Nov 2005 17:00:27 -0500
>
>I am a bit concerned about the cell.  I'm worried you will run into
>alignment problems with this large a mirror on a screw and spring setup.
>  Also, I worry about the tilting arms being mounted on the collimation
>screws.  It sounds like it will be easy to get undesired forces on the
>arms, and thus to the mirror.  I do not understand your description of
>the arm to screw junction.  You need the arms to be free to tilt (a
>little).  Can't tolerate much torque on the arms at all, best if zero.
>
>I would be happier if you copied Albert Highe once more.  See the way
>his cell has the arm pivots entirely separated from the cell
>mount/collimation screws?  I think that is a wise design.
>
>One other potential problem, though this may be over-caution speaking:
>the thermal expansion, not to mention humidity expansion of wood is a
>lot higher than that of glass.  As the wooden arms expand and contract,
>are they going to put loads on the mirror?
>
>One other thing, if you are going to do a cell with arms (bars) such as
>a six point design, it might be better to use good hardwood, such as
>maple, for the arms rather than plywood.  You don't really need the 2-d
>strength and stiffness of plywood, since you will be loading only in one
>dimension.  This is also an easy place to use simple aluminum extrusions
>from the hardware store.  Albert Highe's cells are nicely cast, probably
>in aluminum.  You can probably make up something similar in square
>aluminum tube and have it TIG welded at a local shop.  Use the ideas
>that Kriege & Berry advocate for tailgates, but translate into
>triangular instead of square, and aluminum instead of steel.  Shoulder
>bolts through the square tubing would make nice pivots for support bars
>made of L-channel.  (A hole in the middle of the L rides on the shoulder
>of the bolt.)
>
>Mark Holm
>mdholm@telerama.com
>
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