Newtonian tube drama

Well, I'm getting conflicting advice on what
kind and what size tube I should get for
my 12.5 inch f/5 newt so I think what I'm going
to do is buy a raw 14 inch I.D. 0.4 inch thick
Sonotube and make a test version of the scope
so I can find out first hand what the problems
will be for my particular situation. If the tube
does happen to work well, then I'll go ahead
and seal it and paint it. Otherwise when I order
my second tube, I'll have more confidence about
exactly what I should get.

Thanks to everyone that responded. Now I at
least have a good idea about what problems to
look out for?

take care,

Rockett Crawford
http://web2.airmail.net/capella

"Rockett Crawford" wrote


Well, I'm getting conflicting advice on what
kind and what size tube I should get for
my 12.5 inch f/5 newt so I think what I'm going
to do is buy a raw 14 inch I.D. 0.4 inch thick
Sonotube and make a test version of the scope
so I can find out first hand what the problems
will be for my particular situation. If the tube
does happen to work well, then I'll go ahead
and seal it and paint it. Otherwise when I order
my second tube, I'll have more confidence about
exactly what I should get.

Thanks to everyone that responded. Now I at
least have a good idea about what problems to
look out for?

Inside tube hugging air currents. I don't know how you will know
whether or not it is a problem, but... if you're looking to get a 14" I.D.
tube as an experiment, fine. I don't know what spider and primary cell
you're planning to use, but if they are commercially made and specified to
your tube, those parts that fit one tube won't fit any other size I.D. and
will be practically worthless. Again, if you want to experiment, fine. After
all, we ARE amateurs and "Experiments R Us." But if I were you I'd just do
it right the first time and avoid the variability that tube I.D. may throw
at you. Get one that is at least 14.5" I.D. and be done with it. Just
because others, including commercial makers, are doing it wrong, doesn't
mean you have to, too.

Howard

Howard

You'll do quite well with your current design.

"Howard Lester" wrote in message
...

Inside tube hugging air currents. I don't know how you will know
whether or not it is a problem, but...

I can tell by star test whats going on in the tube, if the mirror is
thermally stable and if there's tube currents.

if you're looking to get a 14" I.D.
tube as an experiment, fine. I don't know what spider and primary cell
you're planning to use, but if they are commercially made and specified to
your tube, those parts that fit one tube won't fit any other size I.D. and
will be practically worthless.

Thanks, but I've got a spider already that will work.

Again, if you want to experiment, fine. After
all, we ARE amateurs and "Experiments R Us." But if I were you I'd just do
it right the first time and avoid the variability that tube I.D. may throw
at you. Get one that is at least 14.5" I.D. and be done with it. Just
because others, including commercial makers, are doing it wrong, doesn't
mean you have to, too.

I appreciate it Howard. If weight weren't a problem in my situation,
yes, I would play it safe, but just going from 14 to 15 inch Sonotube
adds 4 lbs. If I have to, I have to, but Sonotube is cheap so I think
it's worth a shot? If I can find 14 1/2 inch I.D. Sonotube, I'll give that
a shot instead. Should only be a couple of pounds?

take care,

Rockett Crawford
http://web2.airmail.net/capella

Rocket:

The standard tube size for a 12.5 is 14", however, 12.5 is just below
the size
where one abandons tubes and builds a turss scope instead. You will
find
that building a truss is not so hard, and the whole arrangement is
considerably
in both your car and in storage.

The size of the tube also interacts with the type of mount used for the
mirror,
so switching tube sizes pretty much requires rebuilding parts of the
mount.
The same argument goes for the rocker box.

I have seen scopes with small tubes and a few air holes that delivered
excellent
images, and some 12.5"s with 14" tubes that did not (bottom sealed
and/or
insufficient air flow). Attention to detail is the dividing line
between a working
scope and a scope that is a joy to use.

A final consideration is how far are you going to push the scope in the
baffling
department. A fully baffled 12.5"er might want a larger tube just to
simplify
the baffleing! And baffeling does improve contrast.

Berry and Kriege wrote "The Dobsonian Telescope" you might want to pick
up
a copy if you head down the DOB route. They discuss the advantages and
disadvantages of tubes and trusses. and the engineering decisions
needed to
build a good stiff telescope that does not weigh a ton.

Mitch

Thanks for the advice Mitch.

Actually I'm building a newt for imaging instead of making
a Dob. That's why weight is a big factor. I do need
the scope to hold collimation almost perfectly, allow
the mirror to stabilize thermally, and to be free of tube
currents. A truss tube has been suggested to me and
I might look into that possibility after the test version?

thanks,

Rockett
http://web2.airmail.net/capella



wrote in message
ups.com...
Rocket:

The standard tube size for a 12.5 is 14", however, 12.5 is just below
the size
where one abandons tubes and builds a turss scope instead. You will
find
that building a truss is not so hard, and the whole arrangement is
considerably
in both your car and in storage.

The size of the tube also interacts with the type of mount used for the
mirror,
so switching tube sizes pretty much requires rebuilding parts of the
mount.
The same argument goes for the rocker box.

I have seen scopes with small tubes and a few air holes that delivered
excellent
images, and some 12.5"s with 14" tubes that did not (bottom sealed
and/or
insufficient air flow). Attention to detail is the dividing line
between a working
scope and a scope that is a joy to use.

A final consideration is how far are you going to push the scope in the
baffling
department. A fully baffled 12.5"er might want a larger tube just to
simplify
the baffleing! And baffeling does improve contrast.

Berry and Kriege wrote "The Dobsonian Telescope" you might want to pick
up
a copy if you head down the DOB route. They discuss the advantages and
disadvantages of tubes and trusses. and the engineering decisions
needed to
build a good stiff telescope that does not weigh a ton.

Mitch

wrote:
Rocket:

The standard tube size for a 12.5 is 14", however, 12.5 is just below
the size
where one abandons tubes and builds a turss scope instead. You will
find
that building a truss is not so hard, and the whole arrangement is
considerably
in both your car and in storage.


Hi,

I agree with Mitch. A truss is the way to go with 12.5" or larger.
Weight on a tube scope is heavy, but even more than the weight is the
bulk. Very unweildy to move around. A truss scope isn't much harder to
build than a tube, nor does it take much longer to set up and take
down.
Kreige and Berry book is an excellent reference. I built mine(12.5"
f4.8) from this book. It's a joy to use, only weighs about 50 lbs. or
so, and will fit behind the seat in my extended cab Nissan truck.

Regards.
Bobby

Rockett, the experimental approach, Sonotube first, see how you like it,
makes good sense.

Now, others know better, but I had always heard that one wanted a good
1" clearance around the mirror for good ventilation and to dramatically
reduce tube currents. Hence, 14.5" would be much preferred to 14".

But others with real experience, not hearsay, can, should, and, I'm sure
will, chime in.

OTOH, if 14" works with the cells you have or are getting, and the
Sonotube is relatively cheap and readily available in that size, then
what the heck, try it and see how it works.

Larry Stedman
Vestal

Rockett, the experimental approach, Sonotube first, see how you like it,
makes good sense.

Now, others know better, but I had always heard that one wanted a good
1" clearance around the mirror for good ventilation and to dramatically
reduce tube currents. Hence, 14.5" would be much preferred to 14".

But others with real experience, not hearsay, can, should, and, I'm sure
will, chime in.

OTOH, if 14" works with the cells you have or are getting, and the
Sonotube is relatively cheap and readily available in that size, then
what the heck, try it and see how it works.

Larry Stedman
Vestal

Well, I got the sonotube at lunch so I am in business. I agree with
you that I'll learn a lot from making this prototype and if I make a
mistake I'm not out hundreds of dollars?

take care,

Rockett
http://web2.airmail.net/capella

Actually, you have to filter out some of the "advice" that you get when you
get people like Rocket Crawford that doesn't understand that a Newtonian
design (optical design) is the design of the Dobsonian mount (mechanical
design).

Basically, build your telescope so that you can easily move it about as a
heavy telescope won't be used as much as a lighter one would. Take care to
allow the tube to breathe and get rid of the excess heat from the day and
you will be happy. All else is nuances in the construction of the scope.

--
Why isn't there an Ozone Hole at the NORTH Pole?