Re: ATM Tube material - definitive advice please

["B. Pepin" <atmjedit@conceptsnet.com>]

Not to drop another rock in the pond at this point, but please allow me to
point out that there is a very long UK tradition of using WOOD for Newtonian
tubes. This goes back to Hadley, the first maker of a parabolic Newt.

Pros: Cheap, lighter than PVC, relatively easy to get hold of. Grows on
trees, in fact.
Available in a great variety of thicknesses, from 1/8-inch aircraft or
doorskin plywood, on up
Tightly screwed and glued it does not sag (unless you use such thick
material that weight exceeds stiffness),
Works with common hand tools, mistakes can be filled and sanded,
Easy-to-obtain finishing products for a neat appearance,
Thermal qualities are good in thin-to-medium plywood or pine board
(and not that critical in any case with a ventilated 6-inch).

Cons: Heavier than aluminum.
Splinter in finger?
Nesting birds mistake it for a chimney?

If you must go with plastic: I would never use PVC for anything (finder
tubes, focuser parts, etc.) now that ABS is available in nearly all fitting
types.

Black ABS Pipe
Pros: A lot less dense and stiffer than PVC,
Saws and drills more cleanly without dangerous binding and wandering,
Doesn't require finishing since it is a nice semi-flat black,
Takes latex or enamel paint beautifully if you do.

Cons: Slightly more costly (pennies per length unit).

For good measure: PVC over 3 inches really does sag, horribly in my
experience with four-inch. I had to reinforce the tube with aluminum struts
which cost more than the rest of the mounting.

Barlow Pepin
Managing Editor, ATMJ


----- Original Message -----
From: Clive Beale <happysceptic@freeuk.com>
To: <atm@shore.net>
Sent: Saturday, October 21, 2000 5:14 AM
Subject: ATM Tube material - definitive advice please


>
> I'm a newbie who has spent the last week trawling the ATM mailing list
> archive (and Web) for advice on the best OTA material for my planned 6"
f/8
> Dobs.  I've now got to make a choice, but the subjectivity/personal
opinion
> (all construcive!) re this subject makes it difficult!  I've tried to sum
up
> the pros and cons - am I on the right track? Any contributions and
> recommendations are welcome...
>
> Aluminium
> ^^^^^^^^^
> Pros: light; tracks ambient temp well; easy to work with.
> Cons: rel. expensive?
> Questions: What would be the min. recommended wall thickness for a  8" x
> c.4ft tube?
>
> PVC
> ^^^
> Pros: rel. easy to get hold of; easy to work with;
> Cons: heavy; rel. hard to paint (though I am told by a professional TM
that
> Hammerite paint chemically cures/bonds to PVC after a few weeks); may
flex?;
> bad thermal qulities?
> Questions: 1) Would a 8"/4' tube flex enough (laterally - it would of
course
> need strengthening rings at each end) to cause problems? The same
> (reputable) TM uses 3mm-walled PVC to build 'economy' Dobsonians.
> 2)How do 'schedule' gauges (US) convert to mm?  We don't use schedule in
the
> UK...
>
> Sonotube/forming tube
> ^^^^^^^^^^^^^^^^^^^^^
> Pros: cheap & cheerful; rel. light
> Cons: need a lot of work to make waterproof/durable; harder to work with
> (need to strengthen holes, ends, generally messy etc); hard to obtain (in
> the UK).
>
> So  - what do I buy?!!
> Cheers,
> Clive Beale, London, UK.
>
>
>
>
>