Dome Ring question

[el_draco (Rom)]

After much frustration, I'm finally starting to build an observatory and I am working through some of the finer design considerations as site preparation comes to conclusion.

One issue is the section to use for the dome ring. I initially considered 75mm heavy wall steel tubing rolled into a ring but I'm now wondering whether a "C" or "I" section would be a better option in terms of connecting dome ribs, rotation systems, rollers etc. Ring will be 3.2 m diameter. Advice on the type of rollers to use would also be valuable

Input most gratefully accepted.

Rom

[ChrisM]

Hi Rom,

I considered a few different turntable designs, but finally settled on that shown in this thread http://www.iceinspace.com.au/forum/s...ad.php?t=36222 where I've just added some more info on the turntable. Also refer to post#1 for a pic of the obs frame with top ring and wheels fitted.

The choice of wheels is important to help reduce rolling resistance, since you don't want the wheels to skid. Therefore, they either need to be of conical section or at least have a curved (rather than square) profile. I found hard plastic wheels, approx. 75 mm dia. with a curved profile, for $6 each at a local bearing / industrial hardware place in the local regional town. You need to consider the load capacity of the wheels, and allow for some of them not carrying load (ie. over-design the arrangement). If I recall correctly, the $6 wheels have a rating of at least 80 kg each. I used eight, and the combined static load is around 240 kg.

Hope this helps,
Chris

[Merlin66 (Ken)]

When I built the 3.2mtr "Octo-dome" many years ago I had the eight wheels (75mm diameter, as per Chris) on the top of the wall(s) and a 2" heavy angle ring screwed onto the bottom of the "dome" base ring.
I also needed to add radial wheels to guide the dome on the inside of the ring.
Sealing between the rotating dome and the fixed wall structure needs to be carefully thought out.

(EDIT: just visited Chris's thread - that's almost the same as I had! Works very well)

[el_draco (Rom)]

Thanks for the input(s). The thread referred to confirmed a couple of other aspects of my design. I'm designing at a 5/8 dome to improve water shedding and am keen to have a wide slit. I'm currently inclined to use a horizontally opening set-up as in the Hale observatory design so that I can vary slit width but want a full 1m opening if I can swing it. I reckon it will be essential with the set-up I have (C14 FLT 132 S.B.S.)

The insect problem is one I have yet to master. I have "jack-jumper" ants about and it's like a lightning strike when they bite. If that wasn't bad enough, I had an "inch and half man" ant face me down the other day when I was using the jackhammer. It was a close contest, but the little Bas?ard won.

The dome will be on a concrete base but the riser wall is about 65cm high made of 200mm X 45mm aged oak, (60 years), I bagged from a demolition site a few years back. I've built the octagonal frame walls and, even though they are only currently glued, can jump up and down on a single segment with impunity. Believe me, I ain't no tinker-bell either

Dome design is ongoing. Gut instinct says I should go for an "I" beam ring because of the symmetry; I have a quote for a 75mm x 150mm ring 3000 mm diameter and I am about settled on that. I am still open to suggestions though. I had also already decided on the 8 wheel concept, (thanks for the confidence boost), which is good.

Dome itself? Dunno yet. I like geodesic from way back but think trapezoid would be less complex and now find I can get big rings rolled pretty cheaply I may go "traditional".... Decisions, decisions!

the journey continues..

[Merlin66 (Ken)]

Rom,
What about an "Octo-dome"??
The eight sides will be about 4' wide each . A couple of upside down sliding door tracks across the top and a slide back top section ( I had mine sliding with a small pulley arrangement)
One "face" of the octagonal with double outward opening doors - for low altitude observing I just opened the doors and left the roof) plenty of opening for the 14" (I used a fork mounted 12" f5)
My walls were about 1.5mtr high and the dome 1.2mtr......

[el_draco (Rom)]

Originally Posted by Merlin66

What about an "Octo-dome"??

Hmmm, thought about it. A bit like flagstaff. The idea of not being so "obvious" has some appeal but the obs will be largely hidden and have 24/7 online video and motion detection surveillance so its not such an issue. I have also done a few experiments with "Top Hat" and can make angles easily enough for a more complex design. Still, not out of the question.