Tube suggestions for 8" Dob/ Newt

[Swoop1]

At my last society meeting, a member returned an 8" mirror that belongs to the group that had been in a society made Dob before it got dismantled due to the unwieldiness of the tube itself. Along with the primary, which is apparently in a mirror box equipped with repurposed Triumph Vitesse valve springs, is a secondary on its holder and a focuser.

I am thinking of having a go at making a scope from these components but will obviously need input from The Throng.

What would be acceptable tube material to use? Would 8" plastic pipe be sturdy enough?

How do I work out what is the correct length of tube for the primary/ secondary combination?

Thanks.

Matt.

[Carbon Brush]

Do you know the mirror FL? That determines the tube length.
I think that is the first thing that is going to determine construction.
It sounds like it may be long FL, given you describing the original tube as 'unweildy'.

If you are in luck and it is F5/F6, then you can probably upcycle an old (Skywatcher etc) tube.

I would really like to see photos of valve springs holding a mirror.

Thanks, David.

[johninderby]

Have a read of this. Useful info and some good links. .

https://stellafane.org/tm/dob/index.html

Edited December 7, 2021 by johninderby

[markse68]

You can measure the focal length of the primary ala John Dobson in one of his mirror grinding vids on youtube- just need a sunny day... Then you cna use Newt to figure out how long to make the tube allowing for your focuser etc

The tube on my 8.75" Fullerscope is a 10" od grey PVC drain pipe and it works well- I don't think it changes dimension much with temperature which is nice. I think you need to leave a bit of room around the mirror diameter for a few reasons- thermal currents and vignetting

Mark

[Peter Drew]

A plastic tube thick enough to be suitable would "weigh a ton"!  Better to consider a truss tube?.  OOUK rolled me a couple of aluminium tubes, bare with no drillings some years ago, might be worth a call.     🙂 

[markse68]

9 minutes ago, Peter Drew said:

A plastic tube thick enough to be suitable would "weigh a ton"!  Better to consider a truss tube?.  OOUK rolled me a couple of aluminium tubes, bare with no drillings some years ago, might be worth a call.     🙂 

My Fullerscope tube weighs about 6.5kg I think- quite heavy but not too heavy. It's F8 so quite a long one

Mark

[johninderby]

There is this maker of tubes to any size.

https://www.gerdneumann.net/english/instrument-building-parts-teile-fuer-den-fernrohrbau/fernrohrtuben-aus-hartpapier-phenolic-telescope-tubes-hard-paper.html

[Captain Scarlet]

If it’s not a sunny day, I’ve used this method to roughly measure focal length.

Carefully set the mirror up on on its side on, say, a kitchen surface. Obviously in such a way that it can’t fall flat or roll off 😱

Get a piece of board about the same height as the mirror, and drill a 2-3 mm hole approximately in its centre.

Arrange the board vertically with a bright LED torch behind the hole. Adjust the distance of the board from the mirror until the reflection of the lit-up hole back onto the board is in best focus.

Measure that distance, which is known as the centre of curvature, and halve it to get the approximate focal length. The beauty of it is too that any error you make in the measurement is halved as well.

M

Edited December 7, 2021 by Captain Magenta
Typo

[Merlin]

Years ago, I used a length of aluminium factory ducting, procured from a scrap metal yard, for a  8-inch reflector.