Re: ATM LARGE ALUMINUM TUBES

[Aplanatic@aol.com]

Curtis MacDonald wrote:

> Has anyone had any experience with large aluminum tubes? I'm building a 20"
>  cassegrain and needing a tube 24 inches in diameter and 80 inches long.
>  What I need is information on what wall thickness to use.   Also if you
>  have any other ideas on what tube materials to use in this size let's hear
>  your ideas.

If you are planning on using the large Cass for photography, you might 
consider a carbon-fiber (graphite epoxy) tube or, better yet, a carbon-fiber 
truss structure to hold the secondary spider.  The reason for using a low 
thermal expansion material to support the secondary in an astrograph is to 
reduce the change in focal plane location with temperature.  As you know, the 
focal plane moves a factor of (M^2+1) times the change in distance between 
the primary and secondary, where M is the secondary magnification.  Since 
Aluminum has a TCE of 24 ppm/C, a long Aluminum tube in conjunction with a 
high secondary magnification can cause big problems during long exposure 
photography.

Even if the Cass is not intended for photography, you can greatly reduce the 
mass, moment of inertia, and lower the center of gravity by using a truss 
structure to support the secondary spider.

--Dave Rowe--