Re: ATM long truss tubes.

[Bruce Swayze <swayze@europa.com>]

At 09:58 PM 7/16/97 -0600, you wrote:
>Specifically, how long is long when talking the usual uncrossed truss 
>tube design(Partial Serrurier?)? My dilemma arises from the mirror I 
>acquired as surplus. It 
>was used in a missile tracking system for video recording of the flight. 
>It's a 16.5" f5.6 full thickness pyrex. Near as I can tell the truss 
>tubes will be quite long. Maybe I'm being overly critical, but most large 
>designs I see are in the f4ish range.

Actually, most truss-tube systems are not this short.  We have been making
them for a long time in the f/5 to f/6 range.  My current 15" scope is an
f/5.5 and it works beautifully.  I don't have any photos of it scanned yet
since I don't have a scanner, but hope to have some on the web page soon.
The huge 40-inch scope is an f/5.  And there is also a photo of the 12.5"
f/6.6 truss-tube on our web page.  Your scope isn't too long at all for this
wonderful design.

>Short of smashing the Al tube ends and bolting, is it the practice of the 
>commercial makers such as Obsession to make an insert for the tube ends?

I don't know what Obsession does, but you can heat the ends of the tubes and
carefully flatten the ends.  We have it commercially done locally at a metal
shop, and they make it look very nice.  As an alternative, however, you can
make your own inserts out of tabs of metal.  We used some West Systems epoxy
to fill in the end of the tube around the metal tab, and it works great.

>Also, on an open tube Newt design such as I'll build are there any 
>considerations as to the diameter of the tube at the primary mirror cell?
>I Imagine ventilation wont be a prob, neither will tube currents,so what 
>else? 

You don't want to make it too small, of course, but you can make it as large
as is practical.  I don't know of any upper limit other than what's
practical and reasonable.  It will be a bit larger than the minimum
clearances needed if you make it to nest the upper cage inside when packed away.

You can check out our photo page for a few ideas.  It's at:
http://www.europa.com/~swayze/photos.html

Good luck with your project,

Bruce Swayze