Re: [ATM] New 16" telescope design

[Mark Holm <mdholm@telerama.com>]

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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