The OzDave "Dome" Observatory - Planning

[OzDave]

For some time I've been thinking about building an observatory on a block of land we have out in the country. We have relatively low light pollution but ridiculous planning regulations, which I won't go into. The available space is relatively large (20mx10m), but we have other plans for the space too and so the maximum size of structure would be 2.5m square. I am thinking to go for a dome-like structure because I don't really have space for a roll-off and the site can be pretty windy at times. Therefore, the more closed structure of a dome seems to make a lot more sense to me at least. The problem with small domes though, is the head room available. So instead of a typical spherical design, I have gone for a rotating octagonal turret-like top half.

The whole design would be made from timber and plywood and would sit on concrete blocks and be secured to the ground with 4 sunken corner posts. The floor is designed with 6x2" joists and 12mm ply. The lower wall framing is 2x4" and the turret frame is 2x2 with 12mm ply for the roofing panels.

In terms of weather, we never get snow but do get heavy rains at times, and it is quite often very windy. So the design needs to cope with getting wet and not leak.

The image below illustrates the concept. Obviously the pier is missing from this, as is cladding and so forth, but for the purposes of discussion it is better to leave that off for now. I've not fully worked out the dome rotation mechanism, although I want to use rollerblade wheels mounted in fixed brackets. For the shutters, I was thinking of having them hinged so that they open as shown. That seems to be the simplest option compared to sliding or folding mechanisms, but I'd be interested in hearing any ideas people may have.

I need to complete the schematics so that I can submit them for council planning approval, but construction isn't really my thing and I know there are a lot of folks on here that follow the obsy threads and have a lot of useful advice to give. I would welcome any suggestions or comments about potential issues to help me finalise the design.

Regards

David

[teleskopjo]

Have a look at the Kielder Observatory website - similar concept.

[NickK]

Automate.. automate..

The main thing is to ensure the scope has enough space to move and you can fit around it with counterweights. Even if it's computer controlled, you'll find yourself sharing the space at some point.

[OzDave]

Have a look at the Kielder Observatory website - similar concept.

Oh interesting! They don't seem to have *that* many images of the actual structure though, but you can see some things from the different pictures if you look carefully.

[JamesF]

I would be tempted to make the roof pieces slope slightly to allow them to shed water rather than being horizontal I think. Any slight inward bowing of a horizontal roof would allow water to sit on top with potentially unpleasant consequences.

It might make the design a little more complex, but not incredibly so.

James

[OzDave]

Automate.. automate..

The main thing is to ensure the scope has enough space to move and you can fit around it with counterweights. Even if it's computer controlled, you'll find yourself sharing the space at some point.

Yeah, I fully intend to automate at least the dome rotation. I don't think I'll bother with the shutters though initially. I expect I'll want to reverse that decision later once I've been using it for a while and have gotten bored getting up in the middle of the night to close things up.

The external width of the building is 2.5m and the internal diameter will be about 2.3m. Having been in some other small observatories, I think this amount of space will be ok around my scope, especially if I make sure I can properly stand up without hitting my head.

I made the bottom bit square so that the corners can be used to stash equipment or fit a desk for computers etc.

The lower wall is 1m high in the current design. I'm considering making it say 20cm higher and the top turret 20cm less high so that desk space could be properly arranged with a normal sitting height and enough space on top of the desk below the dome ring.

[OzDave]

I would be tempted to make the roof pieces slope slightly to allow them to shed water rather than being horizontal I think. Any slight inward bowing of a horizontal roof would allow water to sit on top with potentially unpleasant consequences.

Yes, I was thinking of exactly that earlier on today. But I couldn't decide whether to tilt up the long edges of the roof parallel to the slit so water would run off perpendicular to the slit, or whether I should run a beam perpendicular to the slit on either side and then make a little apex on each side of the slit (so the water would run off front and back). Now that I've written that down, I'm thinking the first way seems easier and probably less prone to having water "fall" into the slit.

Would it be worth fitting some type of gutter?

[JamesF]

Guttering might be quite tricky to fit. Not sure how that would work.

James

[OzDave]

What do people think about the shutter hinging? Not shown in the picture, but I would make an overlap with some kind of rubber strip to seal it when closed. However, does it need to be anything fancier than simple hinges along the join?

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[JamesF]

What do people think about the shutter hinging? Not shown in the picture, but I would make an overlap with some kind of rubber strip to seal it when closed. However, does it need to be anything fancier than simple hinges along the join?

In many dome observatories the "front" shutter hinges down and is supported by stays rather than folding sideways. I think that probably makes more sense because it's not going to get flapped around by the wind or by the movement of the dome, and if you add catches to the sideways-opening ones to keep them still then you probably won't be able to close it up from the inside.

James

[k2k]

I guess ozDave's will be a little smaller than the VLT observatories but a similar shape on the roof.

This deals with domed observatories, but still interesing. There is a bit on sliding doors which may be relevant.

http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/PageMill_Resources/PUBLICATIONS/Designing%20%26%20Bldg.%20Domed%20Obs.pdf

My pier foundation gets put in next week

[Davey-T]

Have a look at the Kielder Observatory website - similar concept.

I think the scopes at Kielder are fork mounted (as are all, most ? of the big professional scopes) which makes them more suited to this shape of obs'y, EQ mounts tend to throw the scope around more at odd angles.

Dave

[OzDave]

Been doing a bit more design work, this time around the dome support ring and rollers. I've based the design on using 66(-ish)mm inline skate wheels for the support rollers and 72mm inline skate wheels for the guide rollers. Here are a couple of ring cross section images I knocked up in Sketchup, one a solid view, the other with X-Ray turned on to see a bit differently.

The dome rings (upper and lower sections) will be made from two layers of plywood cut in circular arcs, joined, and overlaid to produce rings approximately 4cm thick. The top ring will be slightly wider than the bottom so I can attach a guide rail on the inside and a facing rail on the outside to stop the wheels being exposed etc. Not shown in the diagrams, but I'll probably try to fit some kind of brush strip between the outer facing rail and the lower ring in an attempt to stop things getting in through the gap. I suspect that won't really work though.

One aspect I'm not really sure about is whether the cutouts from the lower ring that house the rollers will weaken the structure too much. I'm not really sure if that ring will be load bearing as such, or whether the walls that hold up the ring are actually the important thing for load bearing.

Another aspect I'm still puzzling over is how to actually mount the roller axles. The main rollers I thought would just have a groove cut in the ring surface to hold them. But maybe they need to be held down with a metal over-bracket or something. The guide rollers have a vertical axle and I'm not quite sure how to secure that in the ring. Maybe it is as simple as putting an 8mm bolt through the ring and countersinking the head underneath and having a nut on the top surface and then the roller on top of that.

Finally, I'm not sure how I would mechanise the rotation of the dome with this setup. Normally you would want to pick one of the rollers and drive that with a motor. Perhaps that can be arranged by using a slightly larger roller for the drive wheel and extending the axle out into the inside of the ring. Although the bearing setup would need to be totally different with the wheel being fixed to the axle and the axle riding in bearings so the motor can rotate it.

These might be some of those things you just can't really plan until you have the actual hardware to play with and try some things out.

I would appreciate any comments / ideas / feedback people may have.

Regards,

David

[ollypenrice]

Hi Dave,

I think this looks like a good idea. Dome shaped domes are hard to make and don't give much headroom round the edges in smaller sizes. Personally I can never see the point in them. The professionals have gone off them as well because of the phenomenon of 'dome seeing.'

I would set your flat roof on a single pitch tilted downwards towards the slit end. If you have a prevailing wind I'd always park it low end to the wind to get downthrust rather than lift. Waterproofing the slit flap would be easy if you had a raised lip around three sides and a flap which overlapped and had a baton round it the same size as the raised lip. The tricky bit would be calculating the slope on the sides of the polygon but you guys will have software packages for that!

Do the guide rings have to rotate with the roof? Couldn't they be fixed to the lower structure? Might this be easier?

How important is it to have a fixed lower building? Making a total rotator would be easier. We have nearly finished a very large roll off here and with big nylon wheels it rolls with one finger. The need to have a smaller lower roof than upper in your design adds a slight complication in terms of sealing because any leak at the rotation joint would be high and risky for the contents.

Olly

[ollypenrice]

David, sorry, I meant 'smaller upper roof than lower' in my last paragraph.

Olly

[Davey-T]

Can't spend too much time on designing David, however I find that no matter how many drawings I do once I start making something it becomes blindingly obvious what I should have done and the plan gets binned.

Don't get too hooked on one idea, be flexible and prepared to scrap ideas and start afresh.

Talking of not wanting a ROR roof I had one plan where the roof hinged up and back ending up standing vertically behind the shed to reduce the footprint, never got to build it though

Dave

[OzDave]

I had a word to a civil engineer here in Greece that will help me to get the planning application submitted. She reckons that "odd-shaped" buildings will attract suspicion, especially in a small rural village where the number of houses is < 25. There are gypsies that will drive by looking for things to steal. So if I really want an observatory, it would be best to disguise it as a garden shed, perhaps reinforce the corners with angle iron to make it a little bit harder for someone to just take a saw to. But if someone really wants to steal the wood...well...I'm not sure that can really be prevented without a major change in construction (i.e. bricks and mortar, which I'm not wanting to do).

So this is my latest design. The octagonal turret part is replaced with a pitched square rotating roof. This is not ideal because obviously as it turns away from square, the corners will jut out and it means that there are only 4 positions I can park it in. However, at night, when I'm using it, hopefully no one will notice. And during the day it will be parked squarely and will look normal.

The other major change is the shutter mechanism. I think I'll want to automate this, so the previous design with fold out flaps wasn't really going to work. Not to mention leak proofing and various other things. So the new design has a two-piece sliding shutter where one piece slides up inside the other and both pieces slide back over the rear third of the roof. This will provide a clear view of the zenith and make automating the shutter fairly easy with a cable/pulley system.

At the front of the shutter, where it meets the wall, I thought what I would do is round off the corners of the boards that hold the shutter runners and then fit a flap with a hinge. When the shutter is closed, the flap will fall down vertical due to gravity. When the shutter opens, it will hinge up and go parallel to the roof, essentially becoming a part of the shutter. The rounded corners will facilitate the movement of the flap. I've not drawn this yet in the picture. When securing things, I will latch the flap somehow internally so that it does not open.

The other thing to note is that I've designed things so that there is no need for an opening on the side wall. Since I really only do imaging, and there is not much point in imaging anything below about 25 degrees altitude due to haze and incredibly bad seeing, plus the fact that most horizons will be blocked until about that altitude anyway by surrounding buildings/mountains, I designed things so that the pier will be at a height so that it can get a clear line of sight to anything above about 25 degrees altitude. From basic math, I was able to work out that the scope can sit up to 50cm below the lip of the slit (measured at the wall). At the centre of the building, that translates to about 70cm of clearance, which I think will be enough. I will need to check this more carefully though and jiggle the design about if necessary until it works. In the "worst" case, I may need to add a small section of wall in the rotating roof part and extend my flap concept to cover the part that will be in the slit.

Let me know what you think. I'm fairly pleased with the shutter mechanism now, but not quite sure about the scope height vs roof clearance yet.

[OzDave]

Hmmm, it has just occurred to me upon reading my post that I'll probably want to add a third section to the shutter at the back for the two other pieces to slide up into. Otherwise water will run down the smallest gap at the back of the middle piece. If I make 3 sections fully nested, it should work ok.

[OzDave]

I think Olly was suggesting maybe doing a total-rotator instead. I could do that, except I want to have the building reasonably near a wall. If the whole building were to rotate, I'd have to move it further than I'd like from the wall in order to give clearance for the square to rotate. Since the wall is not very high, the roof piece can rotate as I've designed it and the building can still be near the wall.

The other thing I was thinking about is that this is a fairly complicated design. Why not just do a roll-off and be done with it? I may yet decide to do that, but for the time being I like the idea of the extra wind cover that this design should provide.

Why not make the building octagonal so it has a better rotational profile? Well, I want the extra space in the corners that a square will provide. Also, an octagonal building may look odd since most designs are rectangular. I don't want the building to attract undue attention.

[Gina]

I designed my obsy to look like a garden shed so as not to attract attention too.

[Davey-T]

I designed my obsy to look like a garden shed so as not to attract attention too.

My obs'y is a garden shed

Dave