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Risks of 6-pole truss. Not doing Stewart Platform

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#1 ppg677

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Posted 29 November 2020 - 02:45 PM

Hi,  I'm building a 13" Dob of the style of Kriege&Berry.  

 

I fat-fingered  my order for aluminum poles and stupidly ordered 6 instead of 8.  Now I had resigned myself to paying shipping again on two more poles, but then the thought occurred to me that I had read about all the new-fangled designs like  Merope or other Stewart Platforms that only use 6 poles.  I've already made my mirror cell that assumes collimation bolts so I don't plan on taking advantage of a Stewart Platform.  

 

I will probably attempt block-style wood clamps on the outside of the mirror box as pictured below.   This means I can do any angle I want.  

 

I've read a couple threads here indicating a 6-pole truss is just as strong.   I've made my secondary  rings with an inner diameter of 13.75" for the 13.1" mirror.  The aluminum I bought is 1" OD, 0.058 thickness.  

 

If there were truly no downsides I might  just attempt the 6-pole truss rather than buying 2 more poles.  But if the 8-pole truss is really stronger, then I'll just order 2 more poles.

 

obsession12.jpg

 



#2 Sean Cunneen

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Posted 29 November 2020 - 03:34 PM

There are two downsides to 6 pole trusses. The first is that the mirror box and UTA need to be larger diameter because the poles can vignette the light path. The second is that from certain angles, the whole OTA has a strange look as the pole orientations have an offset that is assymetrical and can look awkward.

#3 Oberon

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Posted 29 November 2020 - 04:07 PM

If you are doing a square wooden mirror box K&B style with wooden blocks, and aren’t doing a Stewart Platform, then don’t stuff around, just buy another 2 poles and keep going.


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#4 MikiSJ

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Posted 29 November 2020 - 04:28 PM

If you are doing a square wooden mirror box K&B style with wooden blocks, and aren’t doing a Stewart Platform, then don’t stuff around, just buy another 2 poles and keep going.

This!

 

It seems to me you are trying to stuff a round something into a square something. Go for what everyone else does.



#5 ppg677

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Posted 29 November 2020 - 04:46 PM

Sounds good. Will order 2 more poles!

#6 Don H

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Posted 29 November 2020 - 07:06 PM

You might consider inside corner blocks as an option. It requires only 4 clamps and is a little bit more streamlined. This was my 13.1 done that way.

 

 

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  • 13blocks.jpg
  • 13.1new.JPG


#7 ppg677

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Posted 29 November 2020 - 07:42 PM

Looks like I may have found a local source for 2 more tubes... assuming that 0.065" wall thickness for 2 tubes won't matter versus the 0.058" wall thickness for the other 6.

7 thousandths! Seems fine to me? :-)

#8 ppg677

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Posted 29 November 2020 - 08:07 PM

You might consider inside corner blocks as an option. It requires only 4 clamps and is a little bit more streamlined. This was my 13.1 done that way.


That's a great looking scope! 13.1" f4.5 by chance? What size secondary did you use and where did you get a spider?

#9 MitchAlsup

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Posted 29 November 2020 - 09:05 PM

Design for 6 poles or design for 8 poles. Don't design for 8 and use 6.

 

There are differences in the design--and while 6 poles prpoerly done is just as strong as 8 poles properly done--designing for one and using the other is a recipie for "bad things happening"/


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#10 Don H

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Posted 29 November 2020 - 10:02 PM

That's a great looking scope! 13.1" f4.5 by chance? What size secondary did you use and where did you get a spider?

Thanks for the compliment. I actually made the mirror f/5.3, which kept my feet on the ground and helped my eyepiece performance. I used Mel Bartels diagonal calculator and I am pretty sure it led me to a 2.6" diagonal. ( https://www.bbastrod...om/diagonal.htm ). The secondary holder and spider came from AstroSystems. 

 

Here is a link to a few more details, and some other scopes. I think Webs.com is going to yank their free websites soon, kind of like Yahoo did several years ago. I'll add a link to my old Yahoo page that's still in the Wayback Machine.

 

https://donscopes.webs.com/

 

http://web.archive.o...es.com/astro1z/




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