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Newtonian Tube ID

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14 replies to this topic

#1 CygnusBob

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Posted 21 May 2019 - 11:23 AM

Any opinions about how close the tube ID can be to the mirror diameter?  For example, for an 11 inch primary in a tube with an ~ 11.8 inch tube ID.  Is this too close?  I realize having space for ventilation is a good idea, but having a large and heavy tube is not so great.



#2 Ian Robinson

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Posted 21 May 2019 - 11:31 AM

Too close .

You need to allow room for side clamps that hold the mirror in the mirror cell.

For my 10" Newtonians with Orion Optical mirror cells I have a tube 299mm ID.
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#3 ShaulaB

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Posted 21 May 2019 - 12:06 PM

Maybe consider a different material for the tube. Does your hardware store have Sonotube? Would you consider making this a truss tube scope, if you are concerned about the size of the OTA?


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#4 CygnusBob

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Posted 21 May 2019 - 12:33 PM

I am trying to reuse a 0.09" wall aluminum tube 50 inches long.  My last telescope was an 8 inch Newtonian.  I used a 0.06" wall carbon fiber tube.  The carbon tube was great from a thermal expansion perspective, but the stiffness in the radial direction was very poor.  I had to build an aluminum cage around the spider/ focuser area to obtain an adequate stiffness.

 

The 0.09" aluminum tube seems to be plenty stiff as it is.



#5 Ian Robinson

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Posted 21 May 2019 - 12:36 PM

My first 10" was based around a f4.66 mirror and I was given a length of PVC storm water pipe (the stuff that goes underground and is white). Very heavy but stiff tube material.

My second 10" was based around a f4 mirror and I had a custom made foam cored carbon fibre tube made for it - much lighter and much much stronger and stiffer.

I don't think a truss is worth the bother for a 10" Newtonian.

When I eventually upgrade to a 14" Newtonian likely a f2.8 system I'll likely have a truss tube.

#6 Richard O'Neill

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Posted 21 May 2019 - 12:39 PM

 If mirror cell and secondary holder fit I'd use it. Tube ventilation is easily accomplished with a rear mounted exhaust fan.



#7 macdonjh

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Posted 21 May 2019 - 12:56 PM

The simple mechanical reality is your tube has to be big enough to mount your primary mirror cell.  If you're going to custom make your mirror cell, you can likely use whatever diameter tube you want.

 

Old-school wisdom said you want your Newtonian OTA to be at least 2" larger in diameter than your mirror, preferably 4", to allow tube currents room to exist without infringing on the light cone.  However, as Richard O'Neill says, fans are more common these days, so this may not be a big deal anymore.

 

Some other considerations at the other end of the scope:You need enough room for whatever hardware your secondary mirror spider has to prevent it from invading the light cone and causing a diffraction mess.  Same goes for your focuser.  It is preferable that it not extend into the light cone when it's racked all the way in.



#8 Garyth64

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Posted 21 May 2019 - 01:13 PM

On the 10" f/7 homemade newt that I had, (just sold recently after 40+ years),  I used a 12" x 72" aluminum tube from Hastings.  I lined the tube with thin cork, and used a Novak mirror cell.  The tube was not painted.

 

I never had any problems with thermal currents, and I never used a fan.  My "cool down" time was negligible as it was kept mostly in an unheated garage.  Even after two hours, I may get dew forming on everything, but everything was fine inside the tube.



#9 CygnusBob

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Posted 21 May 2019 - 02:02 PM

Both my primary mirror cell and spider fit the tube.  I bought them to use Dall-Kirkham optics from Robert Royce.  However, the Royce primary mirror had a badly turned down edge.  So I am thinking about doing a Newtonian with either a 10 inch or 11 inch primary mirror.  I think I could go either way however the 11 inch would be a tight fit.


Edited by CygnusBob, 21 May 2019 - 02:03 PM.


#10 TOMDEY

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Posted 21 May 2019 - 02:37 PM

So the mirror, cell, spider and tube all fit, without mechanical interference...

 

Well, on the  + side, this may be a great opportunity to write a paper on the behavior of Tube Currents!

 

Back when I was a ~poor kid~ trying to make stuff out of whatever I could scrounge up, that led to some pretty perplexing concoctions, which generally worked OK. The one question to ask oneself is when the apropos metaphors fit too well --- "Tail wagging the dog" "Beating a dead horse" "Taking up Astronomy for relaxation"    Tom

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#11 CygnusBob

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Posted 21 May 2019 - 02:53 PM

Tom

 

It just a hobby.  I like tinkering around.  I don't know how many amateur astronomy products and components I have played around with that did not have some problems and issues.  Many of those components came from well known vendors.  I probably should not name names.

 

It might be interesting to study tube currents.   Maybe, I will have to drill some holes for ventilation later.



#12 dan_h

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Posted 21 May 2019 - 03:00 PM

Any opinions about how close the tube ID can be to the mirror diameter?  For example, for an 11 inch primary in a tube with an ~ 11.8 inch tube ID.  Is this too close?  I realize having space for ventilation is a good idea, but having a large and heavy tube is not so great.

You may also want to consider off axis performance. If the tube is too small, then any target off axis will be vignetted. The loss of light isn't as big a deal as the unwanted diffraction resulting from the tube walls. 

 

You didn't say what the f ratio of your scope was. Assuming your scope is f5, then the tube would be about 48" long. Your field would be less than 1 degree before the tube walls interfered with the light.  That's a reasonable field as not many targets are that big.

 

dan



#13 CygnusBob

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Posted 21 May 2019 - 03:31 PM

dan h

 

I am looking at F 4 to F 4.5 primary mirrors.  A little vignetting is not a big problem.  I only need about 1/2 degree FOV of "good " imaging coverage.  I will be using a TeleVue Parcorr Type 2 comma corrector.


Edited by CygnusBob, 21 May 2019 - 03:36 PM.


#14 tommm

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Posted 22 May 2019 - 10:50 AM

This (long) post gives a good discussion on tube diameter effects:

 

https://www.cloudyni...or/#entry305201

 

Sounds like you've made up your mind to use what you have, but it has some interesting observations in it which are good to know.



#15 CygnusBob

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Posted 22 May 2019 - 01:46 PM

tommm

 

I decided to go with a Zambuto 10 inch F4 primary mirror and use the tube, primary cell and Van Slyke Spider I have.  I will also add a Moonlite focuser.  You guys helped my avoid making a rash decision.  No sense trying to cram an 11 inch mirror in the tight space and wind up with a heavier package that has air turbulence problems.  Thanks!


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