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Latest News: The telescopes are in! Just need a break in the weather to get them set up now.

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Wednesday, 15 March 2017

The Dome (part 2)

In the last post we built the skeleton of a dome now we need to make it turn and make it weather tight.

It will be much easier to do the rest of the work if we can turn the dome to suit our stable work position (remember my observatory is on a slope, there aren't many places I can safely put a ladder!) so the first job is make it turn.   There are a number ways to make that happen: 
1. You can use non-swivel castor wheels, but you must make sure that they are very accurately lined up at a tangents to the ring otherwise you will find it difficult to turn or worse the dome will derail.
2. You can use swivel castor wheels but reversing direction comes with a swing of the wheel which could be difficult with the weight of the dome and if they don't all reverse simultaneously the dome could again derail.
3. You can use transfer bearings.  These have the advantage of universal movement with no backlash but they do need a strong running surface (as I discovered during the build).

I went for the transfer bearings for that feature of universal movement. You can get them very cheaply from Amazon.
Transfer bearings
I put one of these under each joint in the base ring of the dome, initially I only placed them under the joints in the lower of the two layers of plywood but the ring gradually began to sag at the joints in the top layer so I later added a second set to support all the joints in the ring. Also initially I ran the bearings directly on the plywood of the running ring but then every engineer I know on Facebook told me I was an idiot so I cut up a load of sheet aluminium and screwed that onto the running ring to make a better running surface:

Bearings on the aluminium running strip.
Great! So now we can push the dome around in any direction we want... Including right of the ring...  We need some way of keeping the dome on track and since the plan is reuse and upcycle where we can:
Three of these should do the trick!
Skateboard trucks ahoy! They work pretty well too:
video
Round and round we go!

In that video clip you can see that I have stapled a membrane skirt to the base of the dome, this was to try and stop wind/rain coming under the dome, unfortunately I made it too tight so it tends to ride up and slip between the dome and the running ring, at some point I will redo this with some slack in it but it is currently relegated to a "fettling" job for when I can be bothered.

Moving on swiftly we have arrived at another decision point we can no longer ignore: What is the outer skin going to be made of?

There is no doubt that fibreglass is the material of first choice, its relatively cheap, easy to shape, very strong, very durable, weather tight... However, for me it has one major downside:  it is very unpleasant to work with.  The glass fibres are extremely irritating requiring the use of extensive protective clothing; the resin is also quite nasty, very sticky and difficult to clean up spills.

I have had enough experience of working with small amount of fibre glass in the past to know that I wanted to avoid it if at all possible, this wasn't easy, I spoke to every architect, builder, engineer and artist that I know desperately trying to source an alternative product eventually, inspired by a visit to Castle Drogo in Devon I started to investigate scaffold shrink wrap.  I was initially somewhat dismayed to find that it is normally only sold on huge roles covering hundreds of square metres, I only needed about fourteen square metres so this seemed pretty wasteful... But I figured nothing ventured nothing gained so I called Tufcoat and very kindly they agreed to sell me a "small" offcut.  This product may not have the strength and multi decade durability of fibreglass but it is undoubtably a lot nicer to work with and is very lightweight, it is designed to last at least five years (by which time I will likely have moved house!) and I figure that it can be pretty easily patched if needs be!

We had a fun time fitting the skin which took pretty much a whole afternoon:
video
You can see that I have put a layer of breathable membrane over the dome first, this is very important: without insulation or the membrane you are going to get a lot of condensation forming on the inside of the skin which could be bad...

After fitting the shrink wrap we fixed the running rails for the shutter, these were made from three metre lengths of "L" section aluminium (left over from the mk1 observatory).  To get them to curve nicely we cut away most of the short side of the "L" leaving four tabs equally spaced along the length so we could fix them to the dome with screws.

Then the next morning was spent blowtorching it just enough to shrink it, not enough to set fire to it!  I'm not going to lie to you, it was a really nerve wracking task to start with but it turned out that you could get pretty gung-ho with the torching, so we did! 
My dad, just about managing to avoid setting the observatory on fire!

An interesting demonstration of Boyle's gas laws,
guess where the phase transition from liquid to gas was happening?

We now have a waterproof skin on a dome that spins!  This thing is really starting to come together!   Now we just have to do something about that 1.5M wide hole in the middle of it... We need a retractable shutter.

Sunday, 5 March 2017

The Dome (part 1)

Up until now nothing we have built as been particularly complicated but we are about to get into moving parts and multi-plane curved shapes, this is going to get complicated and we need to make a lot of decisions about how to build this thing.

We have already established in The Floor that I have built a structure too big to easily get a prebuilt dome (although a later suggestion that came to me after I finished my dome that it would be worth asking aircraft manufacturers for QA failed radomes... Too late for me but if you know someone that works at an aircraft factory ask away!) 

We will need to decide how to skin this dome in at some point but right now we can start building the framework.  The frame can be thought of in two parts:
1. The base ring (this matches the running ring we built in the the ring)
2. The upper works

We will build the ring while we think about the upper works.   

I still had the router jig made up so cutting out the ring sections was pretty straight forward (and much quicker now that the process is so well practiced!).  I built the ring out of two layers of 9mm plywood * with the joints overlapping.  As the ring is exactly the same size as the running ring I built the base ring using the running ring as a template which dramatically increased the speed of manufacture and reduced the anxiety about whether the shape was accurate or not.   As with the running ring I built the base ring in two parts and then joined them together using nuts and bolts so that it could potentially be split in half and transported if needs be in the future.

* As a foot note I will say that I have subsequently found the base ring sagging substantially between the castors, two layers of 9mm keeps the weight low but will need lots of support to keep it's shape, it may well be better to use thicker plywood or three offset layers to do this job, or more castors to support all the joints which is where the sagging occurs.

Now the upper works.  You need to decide based on your planned usage and space constraints what the upper works are going to look like in terms of both its shape and the materials you are going use.  There are a lot of considerations and options all of which make different compromises.  I will go through a few with you now:

The Shape

1. The aperture shape.  

Are you planning to automate the movement of the dome? Do you think you ever might want to?  If the answer to either of these questions is yes then you have to make the aperture a hemisphere, all the automation systems you are likely to use assume that the opening is a hemisphere, if it isn't then your scope is going to end up looking at the inside of your dome.  If you are not planning to automate the rotation then the upper works do not have to be any particular shape, build whatever shape you like / find easiest to make!

2. The sides. 

The aperture may have to be hemispheric but that doesn't mean the sides have to be!  If you have sufficient head room you might want to consider panelled sides rather than curved as they may be easier to make.   If head room is an issue near to the sides then you will need curved walls.  There are several ways to build curves, some stronger than others: You could build more plywood arcs, strong but fairly heavy; geodesic structures can be easily put together out of a variety of materials but require a lot of component parts to be built to pretty tight tolerances; tent poles can be bent and wedged in place.

I went for a hemispheric aperture and curved sides.


Building the upperworks

I am going to assume for the purposes of this section that you, like me, are building a hemispheric dome, if you are building any other shape, good luck to you, let us know how you get on!

Framing the aperture

One more outing for the router Jig!  This time using two layers of 6mm plywood instead of 9mm just to keep the weight down.   As we are building a hemispheric dome we can again use the running ring / dome base as a template so assembly is pretty quick, just remember that this time we don't need a full circle.  How much of a circle you do need depends on a couple of factors which interlink to some extent:
1. How wide an aperture do you want?  If you are motorising the dome you might want a relatively narrow aperture to keep the wind out when you are observing, if you are not motorising then you may want a wider opening so you don't have to push it round so often.
2. How tall do you want the final dome to be?  The wider the aperture the lower the final dome will be, this might be a concern if like me you have built quite a large building, at 1.5 metres tall sat on top of a 1.5 metre wall my observatory would be three metres from floor to ceiling which means I would need a step ladder to do anything to the roof.

I decided to make my aperture 1.5 metres wide to keep the overall height manageable.  Accurately marking out the aperture on the base is critical to the future functioning of the dome, fortunately its also really easy!  First up, find (or cut!) a bit of timber as long as your aperture is wide and mark the halfway point across the full width on it. Then run a bit of string from the outside edge of the base ring, across the centre point of the pier at the centre of the observatory and on to the outside edge on the opposite side.  Take your bit of timber, line up the halfway mark with your string and mark off the ends of the timber all the way across the plywood from inner edge to outer edge of the ring.  Now all you have to do is layout your jig cut arcs between the marks you have made and fix the pieces together.  Note that you will need to trim the ends of the arcs to make them sit flush on the surface of the ring, you can use the position lines you drew on the ring to mark out the angle required as it is precisely the shape you need to cut! Once the glue has dried you need to raise the arcs upright and fix them in place.  You could use hinges or metal brackets to do this, I went a bit more agricultural and used eight big blocks of wood (two each end of each arc) fixed in place with big screws driven up through the base ring and even bigger screws driven all the way through from one side to the other.  Its not particularly pretty but it is effective!
Here we see Rybes doing his best "Kilroy" impression, to his right you
can see one of the fixing blocks used to hold the upright arcs in place.
Next up we need to build the sides.

The Sides

To build the sides you need something to support and shape the outer skin (more on which later!).  If you are enjoying using your router jig and like doing geometry you could calculate some new arc radii and build a series of plywood ribs, there are some advantages to doing this: the arcs are self supporting so they won't put any stress on your structure other than their own weight.  However I didn't do this. By this point in the build I was running low on plywood and what was left would be needed for later jobs, also I really didn't want to buy any more and besides I was pretty fed up of the router and wanted to do something a bit different.  I used tent poles.

Tent poles have quite a lot to commend them to this task, they can be had cheaply (£30 on Ebay got me a box of 100 + odds and sods poles), they are easy to cut to length (although you should definitely wear gloves when doing this, fibre glass is really irritating to the skin!), very flexible and very easy to assemble.   The one major down side is that your structure has to hold the poles in compression indefinitely so you are going to need to take action to prevent significant warping of your structure.

I built a grid of upright poles running from the aperture down to the base ring and braced these with an arc running from the base of the uprights at each end and inclined at about 45 degrees.  I then attached the arcs to each other using some string, a Square Lash and some PVA glue to make sure the string never moves again.

To anchor the ends of the poles I simply drove screws through the plywood structure where I wanted the ends of the poles then slotted the ends of the tent poles over the screws. If you are going to emulate my methods please be sure to use the metal ferules to make the junction. Although tent poles are hollow you should resist the temptation to drive a screw into an unsupported pole, the pole will almost certainly split along its length unleashing the glass fibres within causing immense irritation in both senses of the word, trust me on this, I learned the hard way so you don't have to!

So now we have the skeleton of a dome that looks like this:

Notice the bracing timbers linking the uprights to the base ring and the timber bracing across to the other upright?  That's the sort of thing you need to stop the poles warping your dome. The large panel you can see at the left side of the dome is also helping to brace the structure but is mainly there because that is the bit of the aperture that will not be sliding anywhere and so can be fixed in place to improve the weather tightness and overall all rigidity of the dome.

That's it!  We have built a dome!  Well done, now we have to make it weather tight, spin and open and close to order... that's quite a job list and we are going to tackle two of the three in the next post.  See you over there.