The Design

Now, I would love to show you a whole bunch of beautifully detailed 3D cad drawings showing every dimension and jointing detail for the whole project and a rendered fly through video... I do own a copy of Autocad so I really aught to be able to do that.  In fact I spent several hours trying to produce those drawings but in the end I realised that learning how to do the drawings was probably going to take the same amount of time as the building of the thing and so I gave up on CAD and essentially started building from a picture in my imagination.

As yet no one has devised a way of printing out the human imagination so this post isn't going to feature many pictures!

Throughout this blog there is going to be a recurring theme of me essentially making stuff up and thereby making life somewhat more difficult for myself!  When I am telling you about these decisions in this blog I am now doing so with the benefit of hindsight and so may well wander off for a while exploring why this decision made life more difficult so that should you be planning to emulate my work you won't also emulate my mistakes!  I would strongly encourage any readers planning any project to research carefully what other people have done when building similar things and then try to work out the unspoken reasons for why things have been done in that way,  as a starter for 10 lets consider the basic shape of the Observatory mk 2:

It's going to be round! 

But now lets just think about that statement.   I am using recycled dimensional timer: 2"x4", these are famously not curved materials.  So although the final shape will need to have a circle for the dome to turn on clearly the supporting walls are not themselves going to be a circle.   What shape are they going to be?

Well, a square, while easy to build is going to look a bit odd, will lead to lots of wasted space and wide overhangs which will need to be supported somehow.   At this stage I was doing a lot of Googling and found that most self build observatories use an octagon as the base shape, if you don't know what an octagon is - or even if you do - here is a picture for you:

Octagon

That shape - I'm sure you will agree - is significantly closer to a circle than a square is and so I applied a slightly deranged sort of logic and came up with the idea that adding more sides would make a shape that even more closely approximated a circle and this would be "A Good Thing".   Now I am sure that some of you have realised that I have taken a slightly shaky step towards the concept of infinitesimals here and are wondering just how far I descended into that rabbit hole... Well not that far really.  I quickly realised that adding more sides involved doing a lot more carpentry for rapidly diminishing returns in that each increase in the number of sides only brings us a little closer to a circle so I settled on a decagon.  Phew! We have avoided complete madness (sort of).

Lets have a look at a decagon:

Decagon

There is no denying that shape is significantly more circle like than the octagon, what a good idea this is.

Here is where I am going give you - my dear reader - a really important piece of advice:

If you are going to build an observatory, build an octagon.

But why should you build an octagon?  This is a teachable moment in finding the unspoken reasons for doing a thing in a certain way.   Lets look again at those two shapes:

Octagon

Decagon

Now consider that you are going to have a pole in the centre of this shape where you are going to mount your telescopes, through which you cannot pass a supporting beam and that you need a floor to walk on which needs to be suspend above the ground so that you don't make the telescope vibrate when you are walking around.

Have you seen the problem yet?

Let me draw in some lines for you, first the octagon:

Four beams, four simple right angle joints and a nice big space in the middle for the pier to go in.

Now for the decagon:

Oh dear, it all started so well with the two red beams spanning all the way through missing the mid point, just like the octagon... things don't go so well when we start looking at the rest of the floor though do they?   We can't use full spans because they all cross in the middle where the telescope pier will be!  We could of course stop where the yellow lines intersect the red but now we have 6 joints to make with three different angles to cut  and no mutual load bearing, those two red lines are taking all the load with nothing to prop them up in the middle.   There are of course other arrangements we could use but they are all much more complicated to build than the octagon and all have less than ideal load bearing configurations.   Trust me on this, build an octagon.

Of course I didn't realise any of this before I laid out the foundations because, well because no other description of a self built observatory I found explained why an octagon was a better shape to use and I am not a professional builder so had no prior experience to call on...  

So a decagon it is 

Now how big should it be?

My biggest telescope is one metre long, so the maximum radius that the telescope can possibly sweep is one metre, but in reality we don't mount telescopes like that so really a two metre diameter observatory should be suffcient.

With those fundamental design decisions made I set off gleefully to mark out the foundations.

At this point I hadn't given much thought to the design or construction of the dome and I think I will cover that in a separate post because that is a significant piece of work all by itself!