The Ring

There are not many parts of an observatory that are truly critical to its operation but the running ring is one of them:  It must be strong; it must be round; it must be level and it must be centred.

This then is the most important thing we have yet worked on, we need to get it right.

I spent a lot of time fretting over this part of the build.  

The only feasible material to build the ring from is plywood.  There is no way you can get the entire ring out of a single piece of plywood, plywood comes in sheets eight feet by four so the ring needs to be cut in sections then joined together to form the whole circle.   If your budget allows there are many companies that will laser cut sheet plywood in whatever forms you need, this is significantly quicker and vastly more accurate (and therefore easier to assemble) than cutting it yourself, however my budget did not allow so I had to cut them myself.

As I would have no choice but to buy the plywood I did actually try to plan out how much to buy,  I broke out Autocad and sketched out the arcs I needed on a template the size of a sheet of plywood, I figured I would get nine arcs from each sheet, and that I would need 10 arcs to make a complete circle.  I wanted 3 layers of plywood so I would be needing four sheets.

Now we meet another of my friends who can do Things.  I was talking over the problem of accurate building the ring with my friend Rob and described how many arcs I thought I would get out of a sheet and he said "I don't think so... get me some paper, I have a pencil" What followed was a lot of complicated maths ending with the declaration of "you'll get thirteen."   Well now we had to settle it didn't we?

See that big grin on his face?  Yeah, that was as he started drawing the thirteenth arc. Never bet against a maths professor (he really is)!

As you can see in the above photo we had a bit of a production line going that day and after a few hours we were able to layout the first layer of "Rob's Ring":

Not bad, its the most circular thing we have built yet!

So that was the first layer done.  Having lost my helpers I modified the drawing jig we had set up to take a router instead of a pen this made the process a lot quicker but I did have a couple of minor accidents.  The jig works by having the main plank slide up and down against a peg.  Depending on which way you are swinging the arm the torque of the motor will push the arm against the peg or pull it away, if you are swinging from the wrong side the router will dive away from your intended cut on the torque of the motor.  I found this out the hard way.  The way I set mine up I needed to start the inner arc cut from the right hand side and the outer arc cut from the left that way the torque was pushing against the stop on each cut which meant I didn't have worry about following a line anymore.

So after a few hours of routing we have the the following piles of wood:

It took a couple of weeks to glue this lot together I built it up into two semi-circles which (with an eye to any future house moves) I then joined together with nuts and bolts so that they could potentially be separated later on.

Looking through my photos archives it seems that I never took a picture of the completed ring.  That's probably because I came to feel that it was something of a millstone round my neck.   Having built a strong, round circle I now spent months worrying about how to get it level and centred.  If I can't achieve those two things the observatory simply will not function.  The whole summer went by and I hadn't committed to fixing it down on top of the walls.

Towards the end of the summer I was describing my worries about this task to another friend, Richard, (Richard is a builder but he has so far studiously avoided getting involved with this project!) he asked me one simple but oh so important question:

"Do you still have the routing jig?"

Of Course!  how could I not have seen this gloriously simple solution?   All I needed to do was wedge a bit of timber into the pipe in the centre of the observatory and fix the jig arm to this at the right height, then as I turn the jig arm round I can immediately tell when it is off centre and adjust!  Even better, if it turns out that the final ring isn't properly round I can just turn the router on and make it round in situ!  How had I not thought of this before?!   

Richard also pointed me towards frame packers as a way of levelling the ring.   So I set up a post in the pier, spun the router jig round a few times, nudged the ring this way and that and within half an hour had a centred ring!  Amazing, several months had gone by while I worried about this task and here it was, done between getting home from work and having dinner.   

The next job was to level it up, I used a laser spirit level for this, the technique went something like this:
Step 1: place a piece of wood on the top side of the ring
Step 2: shine the laser level at the piece of wood
Step 3: mark the level of the laser and the position on the ring on the piece of wood
Step 4: move the piece of wood to a different location on the ring
Step 5: repeat Step 2, 3 & 4 until you return to your original position
Step 6: identify the difference in height between the lowest and the highest point
Step 7: pack the low point up until it is the same height as the high point
Step 8: go round the ring packing any gaps which are wide enough to cause sagging
Step 9: go round with the laser level again make any adjustments as needed

With the levelling done (which took a couple of hours) I went back around with the jig to make sure everything was still centred, then I screwed the ring down on to the top of the walls.  

What a moment that was!  The sense of relief!  No only did this moment stiffen my resolve to get on and finish the project it also significantly stiffened the nature of the walls, now there was no movement in the walls or the floors, now we have a load bearing structure upon which we can place a rotating dome!

The Walls Go Up

Up until now we have essentially built a deck, now it's time to go up!  But how far?

There are competing design criteria here, higher walls means a taller door which makes for more comfortable ingress and egress but walls that are too tall either impinge on the field of view of the telescope or require a taller pier which then means you need steps to get to the eyepiece which can be problematic in its own right. I figured the most important things is to be able to use the telescope with the least difficulty possible since I will spend a lot more time using the scope than the door!  For me this meant the scope being about 1.6m above the floor in the horizontal position so I set the wall height at 1.5m to allow for the running ring and the rolling structure of the dome.   

Here we find yet another reason not to build a decagon... You have to build more walls!

There are a huge number of ways to build walls depending on your skill level and appetite for doing carpentry.  To keep things as simple as possible I opted to build simple butt jointed frames, one for each section of the decagon plus six more for the storage pods giving a total of sixteen panels, four joints on each panel: sixty four joints to be made... Best get on with it then.

As I have mentioned before my carpentry skills are rudimentary, this and the fact that I needed all the walls in place quickly so they could be fixed together led me to use nail plates to join the timbers rather than trying to make nice joinery.   A word of caution though, whatever method you chose, make yourself a jig to work to, it is all too easy to make your frames skew when you are bashing away with a lump hammer!   Conscious that some of the spans were bigger than others I also added supporting timbers in the middle of some of the frames.

Each of the wall frames was in turn screwed down onto the floor then to its neighbour in the picture you can also the temporary bracing timbers used to steady the frames while I worked on them.  Because these frames will not be visible from the outside I wasn't overly worried about the occasional gap between frames, these will be covered by the exterior cladding later on, what is important is that they be plumb, that is they must be straight up and down otherwise you will set yourself up for big problems when it comes to supporting the dome.

I would also like to take a moment to talk about the storage pods here, I made these the same height as the rest of the walls, that was a mistake, a much better idea would be to make them a little lower so that you can add a sloping roof to them I have had to put flat roofs on mine which may be a problem in heavy rain, we shall see!

All in all the walls went up pretty quickly, they may not be the prettiest but they are up and now we can move onto the most challenging part of the construction: the running ring.