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Hilltop Observatory


Jokehoba
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This week I started to put the dome frame together. I'd not appreciated how long it would take to calculate all the different curved sections required - making use of circle theorem and trigonometry - and then mark out and cut all the sections. So I've not got on as well as I had hoped.

I'd glued and screwed the dome ring, having mounted it on blocks between the main observatory walls and my patio. However, I soon realised that I would need all-round access in order to fit the plywood skirt, and later the GRP. So I had to move it all - half on the patio and the other half supported on a tower of blocks on the garden!

Anyway, the dome it taking shape. I have a sixteen more secondary ribs to cut. This is a departure from the original Sky at Night design, which only has three either side of the shutter opening. I wanted a smoother dome, so will have nine on each side.

More pics...

John

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  • 2 weeks later...

The wonderful British weather has hampered progress of late - dry weather not coinciding with weekends! And despite trying to protect the frame from rain (I now have a large tarpaulin that should be better) the thin plywood skirt has suffered. I may need to remove it and refit or replace it.

Still, there have been enough hours to plan, cut and fit the additional ribs. So the dome is looking more like a dome now. I've also constructed the two shutter sections. Now planning how to cut the hardboard panels without wasting too much. I'm going to make a couple of cardboard templates and see how they fit onto the dome before I commit to cutting the hardboard.

My kids ask "Maths... what is it good for?" It's come in handy for calculating all the ribs, ring sections and angles to cut!

John

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  • 2 weeks later...

Very nice work - well done and nearly there :-) How will you 'weather' (protect) the dome exterior?

Just back from a weeks holiday, hence the lack of response to the question. Had great weather but judging by the forecast for the next week or so, I should have spent the time at home on the obsy project! Oh well.

Probably not the answer to your question, but below is how the dome has had to be protected while I was away. Once the hardboard panels are in place, the whole dome will be covered in fibreglass matting and coated with resin, and then top-coated. 

I am hoping to be semi-operational by the end of August but I need dry weekends!

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I have now completed the hardboard panelling. Quite labour intensive... At first I attempted to make a cardboard template but I just kept cutting too much off and having to start again. Wasted a lot of time there! Then I tried attaching the boards temporarily so as to mark out the shape from inside the dome. This was better but man-handling 8' x 4' panels on my own was difficult.

In the end I opted for a combination of the two methods - cutting a slightly over-sized  dome panel and using this as a template for the other similar segments (see pic). Each then just needed a little cutting and trimming to make a reasonably good fit. Even so, you can see there are a lot of gaps to fill.

All I need now is a clear day to cover everything in GRP.

Getting there though.  :smiley:

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An update on progress... 

Last week a couple of friends helped place the dome on the main structure because I was sure, with ladders and a platform, I'd be able to reach all areas in order to cover the dome in glass-fibre and resin. However, I was overconfident and it quickly became clear that it would not be safe to attempt this. So I decided to move the dome back onto the patio so I could do a proper job of covering it. Shame, because with the dome in place, it looked like I was on target and I even started laying the flooring.

Unfortunately, I've had to take up the floor again to protect it from the rain, now that the dome has been removed.

Still, I'm glad I made the decision to remove it because it would have been very difficult to apply the resin and fibre-glass while up a ladder! Even with full access, it's been quite an experience! Sky at Night mag state this process is 'fun but messy'. Messy yes, fun no. Once the catalyst was added, it was a race against (curing) time to get it all coated and well worked into the fibre-glass. I didn't always get it right and it was only for the last third of the surface area that I got my technique right. I hope that the final top coat seals it all properly.

Note to self: Next time, go for a geodesic dome because measuring, cutting, trimming and fitting all those curved ribs and hardboard panels was hard work! In contrast, cutting out hexagons, pentagons and triangles would have been so much easier, and when fixed together they form a rigid, covered structure, ready for the fibre-glass and resin stage. Next time?

Photos show the dome's temporary position and ladders, and after fibre-glass and resin application. Still got a little to do on the main shutter. Hopefully at the weekend I will put the dome back on top of the main frame.

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The good weather at the start of the Bank Holiday gave me an opportunity to get the dome and shutters top-coated. Then I was able to get the dome back onto the main structure - with the aid of a small pulley and a ramp! Once in place again, I propped the dome up on blocks so I could refit the wheels. Then it was a relatively simple job of reattaching the thin plywood skirt and covering it in resin, glass-fibre, and a top-coat. 

The Bank Holiday rain was a good test of how well I had sealed the dome. There were a few leaks, but I hope I have patched them now. More rain needed...

I have hit a problem with the shutters though. Since the Sky at Night magazine plans omit to mention the shutter design and measurements, I had to estimate everything from the rather small photos. I fear I have made the shutter too wide, and perhaps not long enough. And finding the best place for the small wheels is a challenge. The shutter opens OK but on trying to close it, it snags on the top of the shutter opening. I suspect I need to get cutting more plywood to help guide it and move the wheels.

So, I thought I had the finish line in my sights but it looks like there are some design issues to fix! Oh, and a proper door.

John

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:icon_salut:

you've done a great job there, looking forward to seeing the finished item!

Thanks. I'm very pleased with how it's turned out and just want to get it operational. It looks clear tonight so I'm carting all the kit outside - in front of the observatory! - to see if I can get some good snaps of M31. Maybe this is the last time I have to set up and then pack away? Hmmm.

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Nice.  Might have to leave the computer open on this thread for the Mrs to see.  Tis a thing of beauty.  You must be very proud.

Cheers. I'm very proud of it. Though someone said to me the other day that they wouldn't like to be the neighbour who has to look out at it. Can't please everybody...

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Actually, I'm quite frustrated at the moment. I adjusted the wheel spacing on the shutter, which made everything more mobile. I was able to get it open, though it was not as smooth and as effortless as I had hoped. Then I managed to get it half closed but it snagged on the lip again. As I tried to realign it, it slid back open and fell to the ground! Fortunately no obvious damage.

Once it's fully open, the shutter is really just dangling - it completely comes away from the top of the shutter opening. It needs a track or some means to keep it attached and lined up square with the opening.

So I've removed all the wheels and I am back to the drawing board to try and figure out the simplest workaround or solution. I wonder if Mark Parrish had this problem? I don't think I will be up and running by the end of August now.  :sad:  (I'll settle for M31 for tonight. :smiley: )

I would welcome your thoughts everyone. I've Googled dome shutters and looked at a few pictures to try and get some ideas.

John

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Frustration over. I've decided to make use of wire rope to both secure the main shutter to the dome and to act as a kind of 'track' to ensure it doesn't snag anywhere. I'm also going to fix some skateboard bearings to the outside edges of the main opening as low-profile wheels. And I'm retro-fitting some plywood to the inside of the shutter - a smooth surface for the bearings to roll against. Plus a few other things to make it all work.

I've made a start but the skateboard bearings haven't arrived yet. But it's coming to together at last.

The pictures show some of these ideas - the inner surface to clear the bracing; one of two wheels at the rear of the main shutter, together with an end-stop (so the wheel isn't the end-stop!); and the cleats through which the steel rope will be threaded and attached to the dome. 

Next setting up the mount...

John

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Yesterday I also added some 15mm plastic tubing to the inside edges of the shutter to reduce the inner width and aid keeping the shutter centralised. The SaN design utilised a simple single pulley, but I decided to use a dual pulley system, which makes opening and closing the shutter less effort. I fixed the pulleys in place and threaded some polyester rope through them. Shutter opens and closes easily - and that's without the bearings!


I cracked on with assembling the pier plates. As can be seen from the pictures, I've probably overdone it on height with the threaded rod. It might introduce some flexure. I'll probably cut them shorter but I wanted to try things out ASAP.


Thought I'd take advantage of the clear skies and have a go at the Pacman Nebula...


Getting exciting now!  :grin:

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John

I would cut the steel rod as low as possible as the will be the weakest link in the pier structure. Would be a shame to have used all that concrete and still have movement on the mount. Great build to watch gives me hope with my own at the moment. Still suffering back ache from pouring the pier this weekend.

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John

I would cut the steel rod as low as possible as the will be the weakest link in the pier structure. Would be a shame to have used all that concrete and still have movement on the mount. Great build to watch gives me hope with my own at the moment. Still suffering back ache from pouring the pier this weekend.

Yep. You are right. I see the distance between your top and bottom plate will be quite small in comparison.

As I was building the base I kept thinking that I hadn't made the pillar tall enough, especially as I put in the floor joists.

I bought my NEQ6 secondhand (on a steel pillar) and used this as my working height (about 90cm to the base of the pier plate). Once the dome was on, I wanted to be sure I could get as close to the horizon as possible - hence the long rods. It's currently 75cm from floor to base plate, and another 20cm to the top plate. Is it enough?

As it is, I am mostly surrounded by hedges, houses and trees and can't see much below 30 degrees. So I can probably reduce the top plate height without losing anything.

Another thought was to add more height in concrete. I'm quite tempted by the idea.

Thanks for the advice.

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Thought I'd share first fruits from the last night. First photo from under the dome, NGC281, the Pacman Nebula. This is 41 subexposures of 300 seconds each at ISO 800 - 205 minutes in total - using a William Optics ZenithStar 71mm and x0.8 focal reducer/field flattener. Processed in Nebulosity 3 and then in PhotoShop. I've cropped the image. I took flats but I've not yet got them right - hence the gradient in the image.

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