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Alternative Dome Geometries

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#1 ssmith

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Posted 29 September 2018 - 10:47 AM

I have been doing a lot of research into home observatories and I am drawing up plans for my own backyard creation.  I am leaning toward a R-O-R design but in my searches I came across several alternative dome designs which would appear to be much more easily construct-able than the traditional spherical dome - the barrel dome and a cylindrical type dome.  These basic geometries could be further simplified to polygonal shapes eliminating the curved surfaces entirely making them more easily constructed by the average DIY'er.

 

CN barrel dome 1.jpg

 

CN cylinder dome 1.jpg


Edited by ssmith, 29 September 2018 - 02:46 PM.

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#2 brave_ulysses

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Posted 29 September 2018 - 11:23 AM

something different, probably an order of magnitude more difficult to construct

 

https://www.cloudyni...1-calotte-dome/



#3 astrodog73

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Posted 29 September 2018 - 07:04 PM

something different, probably an order of magnitude more difficult to construct

 

https://www.cloudyni...1-calotte-dome/

Just one order? lol.gif

Looks like a beast to waterproof.....



#4 gregj888

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Posted 29 September 2018 - 08:41 PM

I was pretty take with the Calotte dome for quite a while.  Sealing the opening is a bit of an issue, but a cap on a hinged arm would do it.  If willing to give up a piece of low sky you could part the opening under a cover. 

 

My current favorite is this one.  A little like ESO and about as easy to make as I've seen.  You could skip the metal frame and reinforce with wood.  

 

 https://www.youtube....h?v=FR5WsE8gGSk



#5 jp071848

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Posted 30 September 2018 - 09:12 AM

Thanks for starting this thread Steve. I'll be following it with great interest, as I'm of a like mind with you, interested in a dome but hesitant about constructing curves. What are the thoughts onone of these more "modern" concepts, such as a very much smaller, squared off version of one of these beasts, with few or no polygons. I believe that, historically, domes are preferred because they minimize total weight/volume, but as you note, are much harder for some of us to build. For me anyway. A simple rotating box on a ring based on one of these maybe?

 

153637-004-9DD30674.jpgGiant-Magellan-Telescope.jpg

thelargesyno.jpg



#6 gregj888

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Posted 30 September 2018 - 10:15 AM

JP, check the youtube link above, as close as is reasonable to what you are thinking.    I  started playing with it in cad... can be made with very little waste at 120" and 90.75"  probably others.

 

Attached File  dome.PDF   19.34KB   57 downloads



#7 brave_ulysses

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Posted 30 September 2018 - 11:24 AM

cn member jblockyou had an excellent dome build thread

https://www.cloudyni...ervatory-build/

 

sadly the server hosting the images is no longer active. i contacted him a year or so ago and he said he still had them. you might try to see if he will repost them

 

good luck



#8 Rusted

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Posted 02 October 2018 - 02:38 AM

It seems that amateur sized domes have been sidelined by much cheaper RORs.

Yet RORs offer little real shelter for the visual observer but do enjoy a bigger window on the sky.

 

At its most basic: An observatory is a building which opens to give a [fairly] unobstructed view of the sky.

It can be thought of as an adaptable weather shield. This is usually achieved by rotation and opening observing slits.

Most of the internal structure is to support the weatherproof shell and the resulting wind/snow loads.

 

If the shell can be arranged as a self-supporting skin then no other structure is required.

This is the basic GRP or moulded plastic dome usually found in smaller "amateur" sizes.

It is usually provided only a few construction panels sealed and bolted together.

 

Using GRP [fiberglass] as a weatherproof cladding over a heavier structure seems strangely illogical IMO.

The GRP would be far better arranged as a series of connected mouldings to achieve a suitable shell.

The supporting structure becomes merely superfluous weight and considerable extra expense and time consuming.

The downside is the thickness of the GRP shell required as dome size increases.

 

The Calf Igloo is a dome in every sense and remarkably affordable at 4.2m Ø or 14' in real money.

It comes in three curved and cleverly designed panels but has the disadvantage of considerable weight.

This hardly matters if the dome is to be power driven and properly supported on suitably low friction rollers.

No doubt the CaIf Igloo could be got moving by human force alone.

Just a good shove to overcome its inertia and considerable moment. 

 

The one advantage of a gored dome is the ability to have complete drops from zenith to base ring.

However, the very [hemi]sphericity of a dome is a grave disadvantage using thin, stiff and flat materials and amateur tools and skills.

Custom car builders use forming "wheels" to literally curve flat panels but there can be few dome builders willing to go that route.

 

Which brings us to trapezium and triangular cladding panels of traditional dome shapes.

These provide the desirable, basic curvature for the [supposed] minimum material consumption of the basic hemisphere.

 

Trapezoid panels are far more user-friendly to the amateur builder who requires a sheltered window on the sky. Most do!

The tetrahedral type enjoys theoretical geometrical strength in comparison but requires a far more competent and creative builder.

The tetrahedral structure can be built lighter and in considerable sizes with the minimum of tools.

The downside of both is sealing the myriad panels. Many of which will be lying almost horizontal.

 

Any building with an asphalt roof covering requires structure to support the considerable weight and low strength.

As does roofing rubber. Think heavy! Think about the weight and thermal issues before embarking on a black 'felt' roofed dome!

 

From my own experience of building a trapezoid dome from wood and birch ply I'd seriously suggest an alternative.

Build one out of standard aluminium profiles and clad it with full drops of aluminium sheeting for weatherproofing.

 

The design readily lends itself to mass production and requires no special tools NOR ANY PANEL CURVATURE.

Even the base ring can be made multi-sided out of straight strips of metal with overlapping joints.

Laser or water jet cutting would provide a smooth ring if desired but the straight sided "gores" won't fit it without adaptation.

 

All that is required is a narrow, unobstructed, circular track for the support rollers. They won't know it's a smooth ring.

So have the base ring cut multi-sided too if you can afford it.

 

I used 16 sides and four tiers of panels @ 3m or10' diameter. This size is easily managed by a one man band with basic skills.

The ease and speed with which an all aluminium, trapezium paneled dome can be built should be obvious.

The trick is to ensure geometric accuracy to avoid cutting any individual component to size.

 

The barrel dome is also an obvious contender for the amateur's observatory.[See images above ;^]

Though it requires the edge profiles be rolled to a curve the covering panels are easily and gently bent without spherical distortion.

This observatory shape has the huge advantage of lots of headroom at "the shoulders" for observers on stepladders.

The flat ends could provide a normal doorway if erected directly on its base ring on the ground.

Surely an obvious candidate for the Newtonian/Dobsonian owner compared with any "normal" dome on a wall?


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#9 gregj888

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Posted 02 October 2018 - 10:29 AM

One thing the Geodesic dome folks found is that you don't want your roof/dome too light weight.  Wind going over the curved rood causes lift.  If the roof is too light and not constrained it will lift.  

 

 https://users.eoni.c...tzj/observ.htm 


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#10 PETER DREW

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Posted 02 October 2018 - 03:05 PM

One thing the Geodesic dome folks found is that you don't want your roof/dome too light weight.  Wind going over the curved rood causes lift.  If the roof is too light and not constrained it will lift.  
 
 https://users.eoni.c...tzj/observ.htm 


Much the same thing goes for most telescope shelters. Our large dome has 19 hold down points, and it needs them on occasion!
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#11 appicloudy

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Posted 02 October 2018 - 03:17 PM

As the recent builder of a 3m spherical dome, aluminium ring, ribs sheeting I would agree with Rusted on what he says. I found building my dome not that difficult  but I have the skills to build that shape.

 

What I do wish I knew 8 months ago was Rusted's trapezoid build, I would have built it from aluminium, outsourced laser cutting of the base ring and shutter ribs but then if you can cut wood you can cut aluminium - with the same tools. Yes, specialised metalworking tools would be better ( think a combination guillotine / folder ) and I believe much quicker than a wooden dome.

 

If the minister for war and peace would agree, I would design and build one now.


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#12 Rusted

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Posted 03 October 2018 - 01:44 AM

Minister of War and Peace? Any relation to The Head Gardener? wink.gif

 

The trapezium dome provides some challenges in choice of aluminium rib profile.

Peter's T-form provides weather proofing overlaps on curves but requires a rethink for bends between straight, vertical sections.

I favoured V-notching and bending of square tubing to close the notches for simplicity and low cost.

No welding required when reinforced with pop riveted plates across the joints.

The T-form cannot be cut and bent without leaving open notches on the outer surface.

 

 

notched ribs trapezium 2.jpg



#13 PETER DREW

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Posted 03 October 2018 - 03:22 AM

???. Never had need to cut T section for curving, a small set of driven rollers easily curved them during a couple of passes to achieve the correct profile.
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#14 appicloudy

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Posted 03 October 2018 - 03:23 AM

Hello Rusted

 

I think they are sisters!bow.gif bow.gif 

 

Invert the T - the skin would be riveted across the top of the Tee, the " leg " would point down. I wouldn't cut all the way through the Tee, just a notch for the bend, could be tig'd or a splint placed on both sides and riveted.

 

I would put a bend on the top of the skin - to do away with sections between the ribs, if the covering sheets could be cut close to size, I think you could do away with the ribs as well, a monocoque shell, given all the angles I think it would be strong enough. then you would only need a base and shutter ribs. 


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#15 Rusted

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Posted 03 October 2018 - 03:38 AM

???. Never had need to cut T section for curving, a small set of driven rollers easily curved them during a couple of passes to achieve the correct profile.

Sorry to confuse, Peter.

I thought my drawing explained what I meant.

I was talking about stepped trapezium dome ribs.

Not the usual curved ribs.



#16 PETER DREW

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Posted 03 October 2018 - 03:54 AM

Sorry to confuse, Peter.
I thought my drawing explained what I meant.
I was talking about stepped trapezium dome ribs.
Not the usual curved ribs.

Thanks for the clarification Chris. Curved profile with full sheet drops are so much easier to form, as you no doubt now realise. T section with "leg out" is relatively self sealing.
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#17 Rusted

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Posted 03 October 2018 - 04:15 AM

Hello Rusted

 

I think they are sisters!bow.gif bow.gif

 

Invert the T - the skin would be riveted across the top of the Tee, the " leg " would point down. I wouldn't cut all the way through the Tee, just a notch for the bend, could be tig'd or a splint placed on both sides and riveted.

 

I would put a bend on the top of the skin - to do away with sections between the ribs, if the covering sheets could be cut close to size, I think you could do away with the ribs as well, a monocoque shell, given all the angles I think it would be strong enough. then you would only need a base and shutter ribs. 

Hi Ed,

 

Thanks goodness they are separated by thousands of miles of rock.  smirk.gif

 

Regarding the monocoque trapezoid dome:

I was still thinking in terms of vertical and horizontal struts.

Simply overlapping joints between gores won't add much strength.

Though the gores could be brought together in a standing seam with adhesive [?] and pop riveting.

 

There being no curves involved they only need a notch between straight tabs along the edges.

Just like any paper/card model. No puckering or trying to force 3D on 2D materials.

These tabs could be bent with a B&D workbench and a couple of lengths of angle iron, or even boards in the jaws. 

 

If laser cutting is available the seams could be reinforced with slightly deeper, sheet metal ribs.

This could be trapped between the seam tabs for extra strength.

 

Most architectural metal covered domes have a skeleton even when standing seams are involved.

Perhaps the shutter ribs could provide extra stiffness without getting into silly metal sections? 

I wanted tiered shutters, like the panels on my dome, but decided to avoid the complication for now.

 

Just found this:

https://www.systembu...oofing/lokfold/


Edited by Rusted, 03 October 2018 - 04:23 AM.

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#18 roscoe

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Posted 03 October 2018 - 06:00 AM

For barrel-type construction - any sort that uses panels that curve in one direction, but remain straight in the other, an easily-accessible material is corrugated or formed metal roofing panels, used in many places in snow country as home and outbuilding roofs, and available through any lumber supplier - even the big-boxes have sources -

 

These panels, normally steel, have baked-on weatherproof finishes in many colors, including several whites and off-whites, and also fully developed seam-and-corner sealing systems, and while the big-boxes normally only stock them in a couple of lengths, the real suppliers can get panels pre-cut to the nearest inch up to about 36' long.... often delivered to your site by truck. 

 

Most real suppliers also have available 36" or 48" flat panels in the same colors, that are sometimes 8' long, and sometimes by-the-foot - depending on supplier.

 

Onsite, they are normally cut to final length or shape with an angle-grinder or bigger jigsaw with a metal blade.  The flat panels, which seem to be a slightly softer steel core, can be cut with ordinary tin-snips.

 

Fabrall is one US supplier I have gotten roofing from.....


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#19 appicloudy

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Posted 03 October 2018 - 07:04 AM

Hello Rusted

 

I thought with your background you would have heard of the Minister for War & Peace before.

 

I think ribs would be stronger but if the sheet had a fold along the top horizontal edge it would provide strength - the down side would be the sheet would have to be supported in the correct location while it was riveted, the aircraft industry used to use a  clecoe fixing device to hold aircraft parts in place while they were riveted. More complexity ( cost ) so vertical ribs would be better. I didn't use any horizontal ribs - so far so good.


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#20 roscoe

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Posted 03 October 2018 - 01:14 PM

For aluminum-to aluminum fastening, sometimes pairs of strong magnets can make a nice clamping system.


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#21 PETER DREW

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Posted 03 October 2018 - 02:33 PM

I held my panels in place against the T section by clamping mole grips on the leg of the T.
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