RE: [ATM] Celestron 20" R-C telescope

["Richard Brodie" <richardbrodie@optusnet.com.au>]

This is very interesting because I am just starting to build my 8" scope
using some of these same principals, though I hadn't noticed them used
by others before ( I am still new to this).

I can't comment on Celestron's reasoning for the double truss but mine
are:

1. More smaller triangles are stronger than fewer larger ones, so less
flex in the overall assembled truss, by doubling the truss.

2. For transport (in my design), the cell/lower truss stay as a unit and
the top ring will be placed on the lower truss which is designed so that
the secondary mirror clears the primary with a bit of room to spare for
the primary's cover.  The larger truss collapses as one assembly (as per
Chuck's) and fits into a pvc tube for protection.


Richard


-----Original Message-----
From: Chuck Shaw [mailto:cshaw77072@ghg.net] 
Sent: Wednesday, 5 May 2004 1:57 PM
To: atm@atmlist.net
Subject: Re: [ATM] Celestron 20" R-C telescope

Art,

I built my 14.5"f/5 newtonian years ago using two truss sections.  See a
picure at:  http://www.ghg.net/cshaw/truss.htm

The rationale was that because I was planning on carrying a lot of
weight on
the sec cage (image plane de-rotator, finder, and CCD camera, etc. the
balance point was well up the OTA.  That would have resulted in a large
mirror box.  I tried "struts" to attach the alt sectors up higher with a
conventional box, but there was far too much viabration.

Because the balance point is not in the center of the OTA's length, the
two
sets of trusses are not of equal length.

The two sets of trusses release with thumbscrews (no loose parts) and
then
collapse like those folding chairs that slip into a thin sock like bag.

It is a design that has worked really well......  A side benefit is the
wind
does not affect the scope as much as the long dobs that act like a
weathervane.

Chuck


----- Original Message -----
From: <artbianconi@blast.net>

> Why Two Truss Sections?
>
> Most every open truss telescope I've seen utilizes one set of tubes
> to secure the two mirror assemblies. For some reason, Celestron has a
> seperate, short lower cage for the primary and a seperate upper cage
> that is about 2-1/2 times longer than the primary one. Then joined
> them together.
>
> While two cages break down the truss tubes into something more
> compact and thus more transportable, that does not explain it for me.
> Two sets of truss tubes of the same length would be shorter and
> easier to move about than the tubes resulting from the cage ratio
> chosen.
>
> I can see no manufacturing advantage either. In fact the design
> requires two additional machined rings of considerable size and twice
> as many tube attach points. It's also heavier because of the added
> components and fasteners.
>
> With so many additional fasteners and components, ease of assembly
> seems hardly likely although I suspect that machines with this degree
> of sophistication do not get packed up and moved very often.
>
> Superior mount rigidity could explain it but there are simpler ways
> of accomplishing that.
>
> Could they be trying to place a mounting point closer to the CG?
>
> Can any one tell me what might have been the reasoning?
>
> Thanks
>
> Art Bianconi
>
>
>



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