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New dome...picture heavy.


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On the 15th of November 2017, i took the first few wheelbarrows of soil out of the ground that I'd earmarked a few years back as the site for a small 2x2 meter domed observatory.
Work commitments and other priorities meant that i knew i wouldn't be able to complete it before end of 2017 season. So as there was no hurry, i set about at a leisurely pace, digging out a few wheelbarrows of soil whenever i had the time and the weather was pleasant.




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By the middle of December i was down deep enough to where i needed to knock a hole through a stone wall I'd put up some years back. The hole would eventually be repaired and used as the entrance to the observatory.
Once i was able to gain access straight into the hole, digging out became much easier. I just needed to dig down 25-30 cm's under the intended finished floor height, so that there was room to lift the floor up from hard pack ground. Once done i could dig out the hole for the pier foundation.




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I didn't really know how large this foundation should be so i settled for a hole a meter in diameter and approx 45 cm's deep. In retrospect i could have gone deeper but the truth is, it was tough digging in the hard pack ground. The pier would also be in reinforced concrete and i estimated the finished weight of foundation and pier to be well over 200 kilo's. The hard pack would also add to stability. - The pier now carries a C8+avx and is rock solid, so my foundations were adequate.
Once this was done, i covered the hole and waited for a break in the weather so that i could get some cement in. In the meantime i started on the walls of the observatory.
The initial design allowed me to to align the dome slit with the door so that i could open both and walk straight in without having to duck down.
This design was changed along the way as i saw that it would be overly complex plus bring weakness and instability into the finished observatory.







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My well over dimensioned mount bracket was made with a couple of brake discs and four lengths of 16mm threaded galvanised rods and painted down with black hammerite when finished. Its not the prettiest but it does exactly what it's supposed to do.
I used a section of 25cm ribbed hard plastic piping that i had kicking around to use as the pier. This was levelled into place with a large plastic cut down pot around the bottom that would also be filled with cement to add a buttress to the pier.
Because the ribbed pipe had a slight bend in it and was made of very stiff plastic, i needed some kind of bracing around the outside to force it straight.
This would all end up being cemented into the foundations along with the pier.
The long threaded rods which poked out of the bottom of the mounting bracket had thick wire wrapped around them to give the cement something to cling to. I had originally thought I'd just bend the bottoms but this was to hard to do with the tools i had.
There was also more metal in the pier than i've shown. I'm afraid there's many other details i also haven't recorded on camera. When you're in the flow of things, the weather is good and moving from task to task without really thinking about it...stopping and taking pictures kinda gets left behind now and again.




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The floor beams are now in place and it's time to make a start on the dome construction.
I received a set of plans for an 8 foot dome from a member here called Huw .. Thanks again Huw .. although the measurements were US and the dome was slightly larger than the one i intended.
I puzzled over the drawing for a while before i realised that all the measurements could be derived from the base and the 2 main ribs regardless of base diameter.
By this time the weather had turned foul so i brought the operation indoors.
The base was cut out of 12mm plywood sheets. I planned on using 4 layers for the static lower base and 3 for rotating upper base. Each layer was glued and screwed together.
For the upper layer i planned on a groove around the base which the rolling wheels that where to be fixed on the lower base, would roll in. This groove was finished off with 4 layers of epoxy fibreglass.







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I was now waiting for a few good days so that i could get the dome out of the house and up on it's perch but weather and timing were often off. I ended up erecting the dome, getting a tarpaulin over it, insulating the finishing the walls and finishing the floor...all without getting any decent documentation.






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It was now time to sort out the hole in the stone wall. I'm not a bricklayer and looking at the pictures, it's obvious. I should have knocked the hole larger to start with so that i had more room to find and fit stones so that i could have made the edges more contoured. Instead i used stiff boards and cemented the stones up to them. This gave me that straight edge to each side of the hole. I've since scraped away at it a bit but there's still to much straightness there. I'll probably revisit that eventually and change it.




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Once the hole had been finished i turned my attention to cladding the dome skeleton. I used 5mm plywood for this and it worked a treat.
Then i sealed all the joining edges with a bitumen sealer and then painted the whole dome with another layer of the same sealant.



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A worry i had had right from the start now proved to be valid. The tiny plastic wheels i had originally fitted on the dome base where woefully inadequate. I hadn't even started to fix the 2 layers of heavy bitumen sheeting i planned as weather proofing and already the dome where hardly able to cope with the strain. I don't know what i was thinking.
The running groove that the wheels ran in couldn't really be altered without stripping it all off and doing something different. I didn't fancy that so i shopped around and luckily enough found some much more suitable and robust wheels, that fitted perfectly into the existing groove. This operation also gave me the added benefit of increasing the height of the overlapping side skirts, giving me a much better rain protection. Once done, dome back on and bitumen primed. The dome rolls around very smoothly now.






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I've only got a couple of pictures from when i cladded the whole dome with an over and under bitumen sheet. This was the type that didn't need to need to be heated to be fixed, having it's own bitumen glue on the backside. It would have been good enough to fix this straight down on the bitumen primed plywood but instead, i coated the whole dome with another thick bitumen product which was allowed to dry. The bitumen sheeting was then stuck down on this. Once the whole dome had it's first layer on i then coated it all again with another layer of the same thick bitumen paste. This was also allowed to dry over the course of a few days before the top coat of bitumen sheeting was applied.



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I'm afraid i didn't really have any laid out plans for the rolling hatch and front plate. I kinda made it up as i went along. Nor did i get much documentation of the progress until it was basically done.
This is where I'm up to at the moment. I'll probably end up darkening the interior as well as a few other minor bits and bobs although it's all fully useable now. I've had first light and had a great time with Mars and Saturn being prominent albeit low.
It's definitely a game changer being able to just switch on and go without having to setup, align and eventually break down again.
So yeh, that's it....the next project is automating the dome and digging an even deeper hole by the side of my observatory to build an underground warm room although......i think the wife probably needs a couple of years rest to get over this build before i start on the next..













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This is a really good project and I bet you have a lot of fun using it. I almost feel that I have cheated by using a commercial glass fibre dome having seen the work that you have put into this! ? You cleanly dodged a bullet when you upgraded the support wheels and I like you concept of using a groove to centre the dome.

Have you considered over-painting the dome white or silver to help with heat soak? Might be worth considering this as it may help with cool-down.

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Thanks guys. Yes it has been fun to do although there's still a few things I've yet to finish..I've made a simple locking down system that I can use if it looks like strong winds might cause havoc. I plan on making this better although I've actually already been under the dome during strong blustering winds and lashing rain without any problems from wind or rain. I also turned the dome trying to find an angle where rain might make its way in but still nothing. I'm quite happy with it.

Still, there are occasion's when it really blows up. I'd rather both dome and hatch be over secure than under.

Yes you're correct with regards to heat build up and cool down. The black top really accentuates the heat gradient. The scorching weather we've been having has really highlighted the problem.

I've temporarily been using a large white sheet that drapes over the dome. It works surprisingly well but doesn't look especially attractive. A suitable white paint that'll hold on bitumen sheeting is already on the next shopping list....along with something I can use as cuttering, a glass dome to house a meteor cam, interior paint and some kind of locking mechanism for the entry door.

Chuckles....yes, I really don't know how I imagined those small wheels would be adequate for the dome. Facepalm!


Edited by skute
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10 hours ago, ShrewView said:

That looks amazing!

Really cool too that its part underground. I guess you have a lovely clear horizon so why build higher? Brilliant stuff. Thanks for sharing.

My thoughts too! 

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8 hours ago, Alan White said:

What a great record of your build and a nice explanation too.

One thing that concerns me is the soil against the cladding, will this not leak or rot in time?

I spent quite a while working out how to build the square base in such a way that it would last at least as long as I do. 

The cladding is steel sheeting so it'll be OK as long as the framework behind it remains solid. I brought those sheets at the correct length so I didn't have to cut them and break the sealant they are coated with. I've used 3x3 inch pressure impregnated wood for the framework which I know by experience, is very long lasting. 

The bottom supporting beams don't actually touch the ground and the 4 corner posts sit on bricks. Likewise the floor is also raised of the ground, with a damp membrane in both walls and floor.

I don't think it'll last forever but I'm sure it'll last me out as long as I stay on top of any repairs. 


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