What Size Sonotube?

[michael8554]

Please advise what thickness and inside diameter of cardboard "sonotube" I should use for a 215mm f8 newtonian.

Also will the tube need reinforcing at the focuser, mirror, and spider mounting points?

And finally, is there available some black plastic or rubber channel to protect the top and bottom edges of the tube?

I'll be asking for quotes from Essex Tubes and Siteright Construction, any other sources you can suggest please?

Michael in Pewsey Wilts

[Darkmatter]

I found this link in the States and the supplier is recommending for the 10" tube a wall thickness of 0.19" which is just under 5mm

The site also carries some good information.

How did you get on with your quote?

Telescope Tubes Hubs and Bearings

[michael8554]

Thanks for the reply Darkmatter.

Useful info on the link too.

Haven't contacted anyone yet, I was waiting (in vain it seemed) for some figures to put in the quote.

I'll add some prices when I get them.

Ta

Michael

[Trull]

Personally... I'd build a frame using four lengths of aluminium

This helps with weight, wind performance and you can pack it really small.

My 222mm 1625mm fl dob uses just two truss arms onto the mirror box, secured using bicycle quick release clamps. Works a treat!

Just a thought!

[Peter Drew]

I wouldn't be too confident about the long term stability of a cardboard based tube in the UK's climate. You would need a minimum of 10" diameter for your mirror size.

[swamp thing]

Hi Michael

I used Sonotuube for my 10" Dob and its still as good as the day that I made it (three and a half years ago). As long as its sealed properly there's no reason it won't last as long as any wood.

I'm not at home at the moment. But when I get there I'll send you the details on the thickness of tube I used. I kept it well oversized (to help with tube currents) and plenty thick to keep the strength up.

Regards Steve

[michael8554]

Thanks for the replies guys

Yes Trull a strut system is another option to consider.

For me the advantages would be:

Not having to source a cardboard tube

Not having to paint said cardboard tube

Speaking of which, Peter, what paint would you recommend?

Did you need to add reinforcement at focuser and spider mounting points?

Michael

[Peter Drew]

Better ask Steve about the weatherproofing as his has lasted well.

[Trull]

I've dug out an old photo of TorcDobIII so you can see the simplicity of my telescope. Its more wind resistant and packs down smaller than a solid tube design.

Whatever design you go for - it will give you kudos over any other shop-bought scope, have fun and relish it!

[swamp thing]

Hi Michael

I've sent you a PM

Regards Steve

[michael8554]

Hi guys

Thanks for the replies

Ooops, meant Steve for the paint etc questions.

Trull, your design presumably works, so you have to ask, is the average OTA over-engineered?

[Trull]

I think some others are far bulkier than they need be... incidentally in TorcDobIV I added a second truss arm for stability to take some of the spring out of the cantilever. You have to consider what you are pushing against, I use a virtual counterweight to keep the bottom trunnion as low as possible - which has increased the force needed which in turn means the cantilever needs to be stiffer.

The next version builds on this to work with no secondary mirror and convert it to a Chiefspiegler (lenses bought, optical flat bought...just need a full night's sleep and I'll get it sorted).

[callump]

Generally for a solid tube, its recommended to have about an inch of radius over the mirror size. So if you've got about an 8" mirror a 10" tube should be fine.

I guess it depends on the thickness of the tube, but you should not really need to reinforce the points where the focuser and spider attach.

Richard Berry in his "Build your own telescope" book recommends using a reinforcing ring of ply at top and bottom of the tube.

The american Sonotube is finished with wax or other coatings so that concrete does not stick to it - removing that coating is a big pain, so if you can source an uncoated tube that would make the job a lot easier.

Provided the tube is properly sealed and painted inside and out (and especially at the ends) it should last for years - even in the UK. I have an old Meade Starfinder 8" dob which is just a cardboard tube, and there are no issues with the tube.

Happy atm'ing

Callum

[Gina]

You can get "mailing tubes" which are plain cardboard. I used one for the mould for my concrete pier. It didn't fall apart when filled with wet concrete mix.

[callump]

You can get "mailing tubes" which are plain cardboard. I used one for the mould for my concrete pier. It didn't fall apart when filled with wet concrete mix.

sounds ideal!

Callum

[michael8554]

Well the quotes are in:

1800mm long, 250mm ID, 5mm thick, unlined.

Essex Tubes £54 delivered

Siteright Construction £72 delivered

On advice from Steve (swamp thing) might get a requote for 7mm thick