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Merope - engineering a low compromise compact 16" dob

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#176 Millyon

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Posted 22 October 2016 - 02:13 PM

Just now seeing your build. Awesome Dob.



#177 clusterbuster

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Posted 13 November 2016 - 06:07 AM

Truly MAGNIFICENT. The whole Telescope is amazing, beautiful job on the varnish, and those are the sharpest setting circles that I have ever seen. Easily the BEST designed and made Telescope that I have ever seen. You are a WIZARD among craftsmen Sir.

What a treat !

Mark


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#178 WSMIKE

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Posted 21 November 2016 - 04:36 PM

First time I've seen this thread, AMAZING.  What are you doing for a step ladder?

 

Cheers

 

Mike



#179 Bob4BVM

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Posted 21 November 2016 - 05:13 PM

First time I've seen this thread, AMAZING.  What are you doing for a step ladder?

 

Cheers

 

Mike

After seeing Merope, It would not surprise me if Jonathon built some anti-gravity shoes !


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#180 Greg S

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Posted 27 November 2016 - 11:34 PM

I was just looking through some big dob builder sites and came across Wait Research's new line of Renegade scopes.  They are remarkably similar to Merope in engineering and construction.

 

 

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#181 brucesdad13

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Posted 28 November 2016 - 02:14 AM

It doesn't hold a candle to Merope ;) 

 

I was just looking through some big dob builder sites and came across Wait Research's new line of Renegade scopes.  They are remarkably similar to Merope in engineering and construction.


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#182 Oberon

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Posted 28 November 2016 - 07:14 AM

Gordon Waite makes beautiful beautifully finished telescopes and Merope's stylistic similarity to the Renegade has been noted before, I think by Pierre Lemay who brought it to my attention. I like the way Gordon laminates ply to built up his structure in layers, similar to Merope, only taken to the next level utilising CNC. And of course the altitude bearings wrap under a circular looking mirror cell. Look closer however and the likenesses are fairly superficial, and particularly so from a structural engineering point of view. I can't help but notice a range of key design decisions that appear to be made without appreciation of the impact on structural stiffness, and the consequences are inevitable. The Renegade pictured above must surely be floppy and bouncy, and would seriously benefit from some re-engineering as outlined by Albert Highe. Which is a great shame because it looks so neat.


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#183 Pinbout

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Posted 28 November 2016 - 08:57 AM

I don't like the look of the renegade.

 

I think the mirror sits too high in the "mirror box." That raises CoG making the alt bearings too large. Front collimation would also help lower the mirror that also  lowers CoG. 

 

With smaller alt bearings the cross brace "D" thingy would not be impracticle. 


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#184 Greg S

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Posted 28 November 2016 - 11:52 PM

I noted the similarity as I'm starting to like the built up plywood idea as it's so easy to get things laser cut or cnc routed these days.

 

Merope is stunning and in a different league than the Renegade.   I like the Renegade too as it's beautiful as well in a more simplistic style.


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#185 Fly2High

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Posted 12 December 2016 - 02:33 PM

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I am reading through Highe's book and he shows the 6 strut arrangement where there are 2 psudo trusses(made of 2 struts each) at the bottom and a single Truss on the top.  Basically, take yours and rotate it 60 degrees.

 

Why did you decide on this orientation instead of Highe's?  Was it to be able to use the mirror cell for cross bracing of the trusses since  this would make them actual trusses?

 

Does the mirror cell always put the 'triangle' in this orientation or could you have rotated it having the horizontal piece at the top?  I know that just sounds crazy to me and I would assume be needed if the trusses were Highe's way instead of yours?

 

Do you find it stiff enough or do you get some bounce and vibration?

 

Frank


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#186 m. allan noah

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Posted 12 December 2016 - 08:49 PM

 


 

I am reading through Highe's book and he shows the 6 strut arrangement where there are 2 psudo trusses(made of 2 struts each) at the bottom and a single Truss on the top.  Basically, take yours and rotate it 60 degrees.

 

Why did you decide on this orientation instead of Highe's?  Was it to be able to use the mirror cell for cross bracing of the trusses since  this would make them actual trusses?

 

Does the mirror cell always put the 'triangle' in this orientation or could you have rotated it having the horizontal piece at the top?  I know that just sounds crazy to me and I would assume be needed if the trusses were Highe's way instead of yours?

 

 

I prefer Jonathan's design, because it puts the focuser at the apex  of a triangle, and uses the mirror cell to connect the lower ends of the trusses. That is proper space frame design, and should allow some weight reduction. That said, if your upper and lower assemblies are stiff enough, you won't notice the difference, and could rotate the focuser or mirror cell to any position.

 

allan


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#187 Oberon

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Posted 12 December 2016 - 09:36 PM

 

 


 

I am reading through Highe's book and he shows the 6 strut arrangement where there are 2 psudo trusses(made of 2 struts each) at the bottom and a single Truss on the top.  Basically, take yours and rotate it 60 degrees.

 

Why did you decide on this orientation instead of Highe's?  Was it to be able to use the mirror cell for cross bracing of the trusses since  this would make them actual trusses?

 

Does the mirror cell always put the 'triangle' in this orientation or could you have rotated it having the horizontal piece at the top?  I know that just sounds crazy to me and I would assume be needed if the trusses were Highe's way instead of yours?

 

 

I prefer Jonathan's design, because it puts the focuser at the apex  of a triangle, and uses the mirror cell to connect the lower ends of the trusses. That is proper space frame design, and should allow some weight reduction. That said, if your upper and lower assemblies are stiff enough, you won't notice the difference, and could rotate the focuser or mirror cell to any position.

 

allan

 

 

Albert Highe's design can be seen here.

Alan is right.
 

I don't know why Albert didn't connect his truss points the way I did, because he is certainly aware of the engineering benefits, but looking at the picture linked above it may have been because of his special user requirements, i.e. he may have found the truss tubes interfered with his seating position. In any case he knows perfectly well what he is doing, so I'm sure he compensated by making his pseudo-truss LTA stiff enough.

Incidently, the reason for putting the cell triangle with the horizontal section at the bottom is to improve clearance when tilting in altitude.

 



#188 Fly2High

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Posted 13 December 2016 - 08:40 AM

Jonathan,

I look at your work as a lesson of great engineering and love the design.  I hope to use as much as I can from it for my own. 

 

By the way, as I read further and reread the truss section I noticed I overlooked the fact that Highe indicated that the orientation does not matter as to the location the 3 trusses are made.  this validates your design, not that it needed it.  It is only for my feeble knowledge that it was needed and my own education.  I hope I did not come off poorly.  I was only trying to learn from what little knowledge I have been gaining and to better understand the choice you have made and why.

 

I truly find  your scope, as I learn more engineering, is not only very well design but is truly a work of art.  You are a scholar and a craftsman.

 

Thanks
Frank


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#189 Fly2High

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Posted 13 December 2016 - 09:03 AM

Looking at the two designs, maybe Highe built his with the trusses offset from the mirror cell because he still does collimation at the cell instead of with the truss tube lengths.  I am not sure if the trusses attachment through to the mirror cell would make collimation easy.   Instead of using the mirror cell for the lower short strut of the truss I think he is using the mirror box to act as that support in a psuedo truss arrangement.

 

Does this sound reasonable?


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#190 Oberon

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Posted 13 December 2016 - 09:09 AM

Well spotted. That makes sense. He must be adjusting his cell from the front of the mirror box presumably on the lower pair only. That would prevent him from aligning his triangles. 

 

Another reason to use adjustable trusses!


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#191 Fly2High

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Posted 13 December 2016 - 09:57 AM

Once I saw your adjustable trusses, THAT was the first design element I wanted to incorporate into my own scope. 

 

But it is only one of many from your scope that I will use :)



#192 Kiwi72

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Posted 25 February 2017 - 01:40 AM

Just stumbled on this thread, what an awesome scope. Your workmanship is absolutely incredible.

I have always wanted to convert one of my box dobs into a truss style and Merope has given me the push to get it done! 

Can I ask what wall thickness aluminium tube you used on your design?

Is there a formula to work out diameter and wall thickness required for a certain length of truss tube? I guess its a balance between weight and stiffness?



#193 Oberon

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Posted 25 February 2017 - 02:00 AM

Thank you!

The wall thickness is 1mm, pretty standard for thin walled tube, and suits the tube plugs sold by Moonlite.

CF would be better though...if you can afford it. I wouldn't want the trusses any heavier, just stiffer.



#194 Jeff Morgan

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Posted 25 February 2017 - 02:59 AM

I was corresponding with one of the tech support people at DragonPlate (Kris Park IIRC) and most CF tubing has a modulus very similar to aluminum, 8-10 msi. Lighter yes, stiffer maybe not. They do offer a high modulus tube that is 30 msi. About $285 per tube, last time I checked.

 

Another argument for the hexapod over the classic design  :grin:



#195 Oberon

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Posted 25 February 2017 - 03:49 AM

Sheeeesh! 

 

I've seen a very thin walled stainless steel tube in my hardware store and wondered whether or not it would be lighter and/or stiffer than aluminium. Steel is stiffer than aluminium by weight, so maybe...


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#196 jtsenghas

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Posted 25 February 2017 - 06:36 AM

Sheeeesh! 

 

I've seen a very thin walled stainless steel tube in my hardware store and wondered whether or not it would be lighter and/or stiffer than aluminium. Steel is stiffer than aluminium by weight, so maybe...

Maybe. I replaced an aluminum robot end of arm tool with a steel one of of half the wall thickness to make it stiffer and lighter. It would be more stable dimensionally, too. 

 

I'm just wary that with such a thin walled aluminum tube to begin with, a lighter stainless steel one may be entering the realm where the first failure mode may be buckling.


Edited by jtsenghas, 25 February 2017 - 06:43 PM.


#197 polaraligned

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Posted 25 February 2017 - 01:03 PM

 

. Steel is stiffer than aluminium by weight, so maybe...

No, it is not.  The specific modulus of aluminum is higher than that of steel for all common alloys.  If you compare commonly available thin walled tubes, 6061 aluminum vs 304/316 stainless, you will find the aluminum has a 6% higher specific modulus.  So for the same outside diameter tubes with the same weight, the aluminum will be stiffer.  The aluminum will perform far better in buckling also, due to a thicker wall.  This is why you don't see too many aircraft skinned in steel.  The steel only has the advantage of lower cost and a lower CTE.  



#198 Oberon

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Posted 25 February 2017 - 06:39 PM

Interesting. I'll look further into that as I've been assurred by engineers quite the opposite (in context of professional telescope construction when I asked why steel not aluminium). Obviously some subtlety here I'm missing, aside from the risk of local buckling.



#199 racunniff

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Posted 25 February 2017 - 07:05 PM

Steel has a lower coefficient of thermal expansion depending on alloy. And it is not as vulnerable to work-hardening (although I don't think that should be a major issue with telescopes that don't suffer significant bending cycles). And it holds paint better :-)



#200 TopQ

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Posted 20 April 2017 - 08:23 PM

Very Nice

All I can say




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