The Shutter

I'm not sure I have got the shutter right yet, of everything I have built this is the part most likely to get taken apart and rebuilt.  That said, lets take a look at what the shutter is there to do and some options you could consider.

What is the shutter for?  Well its simply there to stop the rain getting in!  It doesn't need to be airtight but it needs to provide run off for the rain and snow.  The catch is you need to be able to get it out of the way so you can see out when you want to!  There are as always some compromises to be made which require you to think about how you are going to use the observatory, the main issue is that on a hemispherical dome the largest single piece shutter you make is half the dome (ie from the base to the zenith), but if you do that you will not be able to observe the zenith of the sky which is a problem, it leaves you with a blind spot.  Now you may not think this is a serious issue, after all nothing stays at the zenith for long, the earth's rotation will bring things into view soon enough, however the zenith is the point where you are looking through the least of Earth's atmosphere so this region of the sky is where you get the clearest and most stable view of the stars so it is quite important especially if you are interested in astro-photography.

So assuming you, like me, want to be able to observe the zenith you need a shutter that is in at least two pieces.  There are many ways to achieve this of varying complexity. Because I have a hedge and a couple of trees in close proximity to the observatory I didn't want anything protruding beyond the building itself so that restricted my choice down to a nesting shutter in two pieces such that one rolls up under the other, then they both roll down the back side of the dome together.

Critical things to keep in mind:
1. The shutters failing of the rails would be bad for the dome and your nerves at eek o'clock in the morning.
2. The shutter jamming on the rails would be equally frustrating at eek o'clock in the morning.
3. The shutters blowing off the dome in the wind would be bad for the dome and everything inside it regardless of the time of day.

Considering the above you will need a mechanism that allows the shutter to roll on the rails, self corrects any off centre movement and doesn't jam and because I was still following The Plan I built several of these:

Using some sheet aluminium, some small nylon wheels and some nuts and bolts I built a roller / rail guide / safety hook in one, the wheel runs on top of the rail, the hook goes under the rail to stop the shutter lifting and the side is a primitive guide stopping any sideways movement from derailing the shutter.  The lower section of the shutter has one of these on each corner, the upper section has two at the rear only with the front pair replace by the same hook / rail guide arrangement but no wheel so that the lower section could move without hitting a wheel.  Then I built a test jig using these wheels and some more aluminium to check whether this would work.

Did it work?  Well, sort of.   The wheel hooks looked effective enough but the test rig revealed that the uprights that form the aperture of the dome were not perfectly parallel, at the apex of the arc they were about 15mm closer together than at the base.  This was annoying because it meant that with the available adjustment in the length of the wheel axle and the width of the rail: either the shutter would derail at the apex or jam at the base.   Annoying but easily fixed, I unscrewed the rails from the top of the dome, added 7mm of packing on each side and screwed the rails back on.

Now did it work?  Oh boy did it!  Standing on the ground I gave the test rig a good push and watched in astonishment as it shot up the rails, over the apex, down the other side and launched itself off the other end of the rails and into the hedge!  With that I was confident I had got the geometry right so set about cutting a plywood skin to cover the test rig.   

I initially thought that 6mm plywood would be bendy enough for this but as it turned out the radius of the dome was significantly smaller than the natural curve of the 6mm plywood so I had to kerf the board.  Kerfing is a technique whereby you use a circular saw to make a series of parallel cuts half way through the board to increase the available flexure, its tedious but cheaper than buying more ply! 

Kerfed plywood attached to the test jig 

Then it was rinse and repeat to make the second shutter (but using 3mm ply for the skin which was bendy enough).   It was about this point that I discovered its not easy to juggle an angle grinder when its turned on and it is inadvisable to try!   A quick trip to A&E to put my thumb back together 

Remember kids: a falling [insert dangerous thing here] has no handle, don't try to catch it!

and some help to get the shutters in place and we have a weather tight dome!

And with that the building is complete!  We have done it!  Now all we have to do is fit the mount and the telescopes and frankly after all we have achieved to get here that is not going to cause you any problems!  Join me in the next post to see some sparks flying.