RE: [ATM] 24" f/4 lightweight Dob

["Cord Scholz" <cord@astro-image.com>]

Mark, I'm on the list, though mostly lurking. 

For the cage rings I used a core of 20mm styrofoam between layers of 3mm
plywood, for the focuser board cross linked expanded PVC foam (Airex C70)
and two component expanding polyurethane foam for the altitude bearings.  
The choice of the core material is a tradeoff between cost, density, shear
and compressive strength, bonding properties and ease of use. One of the
best core materials is aramid honeycomb core, however it's expensive and
more diffcult to use than foam cores - vacuum bagging is mandatory. That
said I recently made a few parts with honeycomb core and it's amazing how
stiff and lightweight the parts are, e.g. the triangles of a 27 point cell
for the 24" weigh only a few ounces.
PVC foam or Styrodur (extruded polystyrene) on the other hand are cheap,
very easy to use and unless you really want to push the envelope more than
adeqaute for typical ATM applications.
Expanding PU foam is excellent for irregular shapes, e.g. the hollowed out
wood parts of my altitude bearings.

I used exclusively high quality epoxy resin and 245g/m^2 twill weave carbon
fabric. Glass fabrics are a good alternative for bigger parts if cost is an
issue and they're somewaht easier to work with, e.g. glass is isotropic and
it's easier to see whether the fabric is properly wetted.

Cord Scholz
 

   

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Mark Holm
Sent: Saturday, December 25, 2004 3:16 AM
To: atm@atmlist.net
Subject: RE: [ATM] 24" f/4 lightweight Dob

Is Cord on the list?  That is a great looking scope.

I would like to know what the core materials for the sandwiches are and as
well the types of fiber materials and resin.

It looks like there are two core materials in the alt bearings, or is that
core with holes?

--
Mark Holm
mdholm@telerama.com


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