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DIY CARDBOARD sight tube idea?

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#1 CosmoJoe

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Posted 30 January 2013 - 02:31 AM

I know it sounds silly at first but hear me out.
I want to use a cardboard paper towel tube to make a sight tube. I found out that a m42 t adapter fits perfectly in the end of the tube, making it 100% perfectly round. I also found out that the retaining ring from a m48 filter fits perfetly around it, allowing it to be screwed to a 2 - 1.25 adapter and making the other end perfectly round. Stick a collimation cap in the adapter, verify everything is level, and you have yourself a pretty much free sight tube. I have an idea to use string for the crosshairs as well if the sight tube part works.

Is that a silly idea or do you guys think it could work? Advice would be very appreciated as I don't really want to spend 100 dollars on a sight tube, thanks. :bow:

#2 furrysocks

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Posted 30 January 2013 - 03:57 AM

Consider the use of fibreglass resin (like polyester resin) - mix it up, brush it on. Your cardboard tube will no longer be cardboard, it'll be a composite of cardboard and polyester and so will be waterproof, rigid, etc... It cures in 30-90 minutes, so let it set up until it's no longer tacky and then gently tease the rings into place to ensure the round shape is there, but not mounting the rings in permanently.

You may be able to buy aluminium tubing or even PVC plumbing pipe with an appropriate internal/external diameter, but if you've found something that fits, go with it. Call it a prototype and no-one can say nothing! ;)

Not speaking from experience - I've not made a sight tube like this, but just some ideas for you. Good luck.

#3 CosmoJoe

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Posted 30 January 2013 - 04:00 AM

Consider the use of fibreglass resin (like polyester resin) - mix it up, brush it on. Your cardboard tube will no longer be cardboard, it'll be a composite of cardboard and polyester and so will be waterproof, rigid, etc... It cures in 30-90 minutes, so let it set up until it's no longer tacky and then gently tease the rings into place to ensure the round shape is there, but not mounting the rings in permanently.

You may be able to buy aluminium tubing of even PVC plumbing pipe with an appropriate internal/external diameter, but if you've found something that fits, go with it. Call it a prototype and no-one can say nothing! ;)

Not speaking from experience - I've not made a sight tube like this, but just some ideas for you. Good luck.


thank you :jump:

#4 careysub

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Posted 30 January 2013 - 05:54 AM

I was going to suggest the same thing, but you beat me to it.

In the 19th century the best racing shells were made out of a material that was less tough and durable - paper boiled in linseed oil.

#5 CosmoJoe

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Posted 30 January 2013 - 09:32 AM

so it should be sufficient to squarte the secondary under the focuser? another quick question; i have a laser coll and coll cap, so do i need a cheshire?

#6 furrysocks

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Posted 30 January 2013 - 09:36 AM

so it should be sufficient to squarte the secondary under the focuser? another quick question; i have a laser coll and coll cap, so do i need a cheshire?

I'm afraid my knowledge only goes as far as how to make cardboard waterproof. :p

#7 StarStuff1

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Posted 30 January 2013 - 10:56 AM

If your laser collimator is sufficiently accurate you shouldn't really need a cheshire. I don't have a laser collimator but do have a cheshire. But then I always do final tweaking using an out of focus star.

#8 Crayfordjon

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Posted 30 January 2013 - 10:59 AM

It is a brilliant idea, an example of a low tech solution to a high tech problem, full marks. :bow: :bow: :cool:

#9 rboe

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Posted 30 January 2013 - 11:19 AM

I know it sounds silly at first but hear me out.
I want to use a cardboard paper towel tube to make a sight tube. I found out that a m42 t adapter fits perfectly in the end of the tube, making it 100% perfectly round. I also found out that the retaining ring from a m48 filter fits perfetly around it, allowing it to be screwed to a 2 - 1.25 adapter and making the other end perfectly round. Stick a collimation cap in the adapter, verify everything is level, and you have yourself a pretty much free sight tube. I have an idea to use string for the crosshairs as well if the sight tube part works.

Is that a silly idea or do you guys think it could work? Advice would be very appreciated as I don't really want to spend 100 dollars on a sight tube, thanks. :bow:


First book on Messier Marathons I bought mentions a gal doing that with her C8. Worked like a champ. No comments on whether Charmin tubes were better than Northern so you're on your own there.

I've been collecting the very heavy tubes that plotter/larger format rolled paper comes on. One of those, cut into shorter lengths would last a life time I would suspect; with no varnish or epoxy or sealing of any kind.

#10 Mirzam

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Posted 30 January 2013 - 04:42 PM

Maybe there is another way to do it, but I always use a sight tube to make the up/down (axial) and rotational adjustments of a newtonian secondary. I use lasers and autocollimators for everything else.

In many cases it is even possible to simply use the focuser tube as a sight tube (with a pinhole device at the top). Rack the focuser in/out until there is a narrow annulus between the secondary mirror and the bottom of the focuser tube. Then adjust things so that the annulus is the same width all the way around the secondary. The secondary will then appear perfectly round and be centered and squared to the focuser.

Note that it is important for your sight tube to slide into the focuser, or be cut to proper length, so that you can produce the annulus mentioned above.

JimC

#11 CosmoJoe

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Posted 30 January 2013 - 09:41 PM

it would of course be cut to size, but using the focuser tube sounds like an easy solution if it works with your scope.






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