Does Saturn have ears?

This year Jupiter and Saturn are fairly nearby in the sky so just a few weeks after Jupiter went past opposition so did his father.  As this would be the closest approach to Saturn this year (and also its highest position in the sky) I decided to take a picture... and just for a bit of fun I thought I would show you all why Galileo famously described Saturn as "As a planet with ears".

In the top image I used a high frame rate camera recording at 23 frames per second for 3000 frames, I then used software to select the best 300 frames and combine them to allow me to produce a relatively sharp image that clearly shows the rings as distinct from the planet.  You can also see the Cassini division which separates the darker 'A' ring on the outside of the disk from the brighter and wider 'B' ring.  Above the disk of the planet you can make out Saturn's shadow being cast over the rings and conversely where the rings cross the planet you can make out hints of the shadow of the rings cast on to the cloud tops of Saturns atmosphere.

But of course Galileo didn't have a high frame rate camera, or a computer capable of sorting and stacking the frames, he also had no prior knowledge of what he was looking at!  

The lower image is the same as the upper one but blurred to simulate the relatively poor optics and the effect of the atmospheric "seeing" that Galileo might have experienced at the eyepiece.  

The video clip below is an unprocessed export of part of the video I recorded through my telescope, keep in mind that this video clip is much sharper than the view through Galileo's telescope would have been due to the size of my telescope compared to his.  Its not hard to see why he was a bit confused!

Jupiter Posing

Neither of my telescopes are particularly suited to planetary observation, the focal lengths are a bit too short really but sometimes the planets are just to tempting to ignore, especially when they are at "Opposition" (the time in the year when a given planet is opposite the sun in the sky and therefore at its closest approach).  Such was the situation for Jupiter last month so I took this image of the King of the Planets:

I am very pleased with the outcome!

I should point out that this is not a single frame snapshot.   This picture was produced by combining several hundred frames extracted from a video and combined to produce a much sharper image than could ever be achieved with a single frame. 

First and most obvious thing to notice is the colour banding: the equatorial, tropical and temperate bands are all clearly visible along with the "Festoons" which are the the crenelations visible along the contacts between the light and dark bands.  These Festoons are essentially storm systems and occasionally they develop into full blown cyclone storms, the most famous of which is the Great Red Spot which I think is the dark spot in the upper left quadrant of the planet.

Some of you may be thinking that the GRS is in the southern equatorial belt, why is it in the upper section of the planet?  That is because my telescopes invert up and down, so Jupiter's South pole is at the top of the image! It would of course be very easy to flip the image over but I like to preserve the view through the eyepiece :) 

Taking Pictures of the Moon

Now that the observatory is working reliably I decided it was time to try out the cameras.   I decided to keep it simple for the first few images in order to avoid too many frustrations with remembering how to get all the equipment working so I took some greyscale images of the moon:

Here the prominent crater with the large central peak is Theophilus. I love the "pearls in space" effect of the sun on the mountain peaks below the main crater along the terminator (the line separating night and day) but I am most intrigued by the subtle relief features in the upper right.   The dark patches on the moon are known as Mare (latin for sea) as they were thought to resemble or even possibly be seas of water.  These days we understand them to be vast planes of volcanic lava, those relief features only become visible to us when the terminator is very close by and we can see shadows being cast across the surface.   What we are looking at here are volcanic domes and wrinkle ridges left behind as the lava cooled. I will be looking to find out more about those features and perhaps make more detailed studies at a later date.

I also took this image further along the terminator:

This is the western part of the sea of serenity with crater Posidonius prominent at the top of the frame (some of its rilles are visible too which I am pleased about), the striking snake like feature is "the great sinuous ridge" or Dorsa Smirnov.  This is an example of a wrinkle ridge which form when the lava that formed the Mare cool and contract, some of these features can extended for many hundreds of kilometres across the moons surface.

It is very easy when studying the moon to get distracted by the impact craters, they tend to be large, bright and dramatic but they don't tell you much about the moon itself.  The craters are the result of foreign objects hitting the moon but the volcanic features: the domes, the wrinkle ridges, the cinder cones and rilles tell the story of the dynamic moon that was, how she formed and cooled. So to me the more subtle features are the more fascinating to examine. 

Solar Safety

One of the things I can observe with my 80mm refracting telescope is the Sun.  I use a white light filter for this (the shiny metallic film in the picture below) which rejects something in the region of 99.99% of the incoming light.  The problem in my observatory is that the refractor isn't alone, it is mounted on a dual mount bar with a much larger (200mm) reflecting telescope. Unfortunately I lost the dust cover for the reflector some years ago so pointing the small scope at the sun whilst using the dual mount meant also pointing the large, unfiltered telescope at the sun, this is potentially extremely dangerous not only for anyone who might accidentally look into the wrong eyepiece but it could also easily set fire to the observatory!

So I made a dust cover out of left over scraps of plywood, which made me happy:

Now I can safely observe our nearest star :)

Installation

Last steps! All this time since we laid the foundations the pier has patiently been waiting for its scope and in this post it is going to finally arrive.

To get to the telescope we are going to need to start in the middle, work down then return to the top, all this will make sense shortly so stick with me.

In order to easily operate a telescope mount (and therefore the telescope on it) the mount needs to be fixed to a level surface.  How neatly did you set that pillar? how square is the cut end? Do you think the mount is going to be level fixed straight on there?  Probably not, so we need a levelling plate (two metal plates mounted one on top of the other such that the bottom one is fixed to the pier and the top one can be adjusted on three screw threads to get level.) Then we need to attach the levelling plate to the pier which - because welding things above you is a little fraught - will require you to cut a small section of pipe to weld to the bottom of the plate so you can do this upside down and outside the very flammable building you have just spent 18 months building! 

Cutting and shaping the pipe extension (levelling plate base visible to the right)

Welding the pipe to the levelling plate

Once you have joined the levelling plate to the pipe you can weld the pipe to the pier and you are ready to mount your telescope!

And there it is! My 8" Newtonian atop the HEQ5 pro mount (white bit) bolted to the levelling plates, welded to the pier, set in a cubic metre of concrete sunk into the ground beneath our feet, we are ready for first light.

Dome open and scope pointing, we have first light!

And with first light comes first capture:

Waxing gibbous moon on the night of first light

The plan is complete! We have a working observatory built from recycled materials and a fair dose of ingenuity!  If you decide to undertake your own build please make sure you get guidance on how to use any tools you are unfamiliar with and try not to shed as much blood as I did!  Above all don't be a slave to my design, experiment, adjust and build the observatory that suits your needs and your location.  Good luck and Dark Skies.

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.

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:

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:

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.

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.

Cladding The Walls

I almost forgot!  I know I ended the last post saying it was time to build the dome... but patience, we are not quite finished with the walls yet!

With the ring finally fixed in place the walls were now strong enough to clad with their outer skin.  Up until now I had worried that adding the outer skin would put the walls at considerable risk of getting blown flat by the strong winds that frequently whip up the valley here so I had left the structure open.   Now it is time to start making the observatory weather tight.

An often overlooked feature of observatories is that the atmosphere inside is going to pretty closely match that outside, this means that we need to mange humidity as the temperature rises and falls through the day, we don't want rivers of condensation running down the walls or dripping off the roof!  So the first step in cladding out is to fit a breathable membrane around the outside of the framework, rolls of membrane can be found inexpensively at any builders merchant and is happily very easy to fix in place with staples.  One word of caution here: the membrane isn't very strong but it does make a very good sail... only fix as much membrane as you can clad over in the time available otherwise you will end up decorating your surroundings with shredded membrane and having to fit it all over again!

The next step is to cover the membrane with the outer skin of the observatory, for this job I am going to use more pallet planks, fixed vertically (one screw at the top, one in the middle and one at the bottom) and overlapping to provide a water tight and wind tight outer skin.  I built this up in two layers, the first layer had alternating planks then a gap, then another plank:

On the left both layers of cladding are in place, on the right only the first layer has been fixed.  The breathable membrane is the grey fabric that can be seen through the gaps

The second layer of planks were fixed to overlap the gaps in the first layer.  You could do some joinery at the angles of the wall but I found that a perfectly satisfactory weather seal can be achieved by simply butting the planks in the first layer then overlapping the joint with a plank on the second layer.  The cladding does not have to be 100% water proof, the breathable membrane will stop any water that manages to work its way through the joints, we are just aiming to prevent the wind driving water inside.  As long as you leave an air gap between the cladding and the membrane the walls should be weather tight.

Working in this way I was able to completely clad the outer walls with just a couple of days work using about 320 planks of wood recovered from pallets which I got for free from the local builders merchants and various building sites nearby *. If you want to be really hardcore about your upcycling you could straighten out all the nails you pull out of the pallets and use those to fix the planks... I drew the line at this and fired in about 1100 screws to hold the planks in place.  It is advisable to drill pilot holes in the pallet planks before putting the screws in, the planks are often quite brittle and easily split if you don't.

Now the walls are finished we really can move onto building the dome :)

* Always ask before taking anything, even if it looks like scrap.   If for no other reason than you will often find that you get offered other materials to take away too!

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.

The Floor

With the foundations laid it was time to get on with the floor!    I stood inside the decagon and tried to imagine what this whole structure would be like and I found myself thinking "its an awfully small space..." So I decided to make it bigger,  I laid out the floor joists with a half metre overhang giving a new overall diameter of three metres.   Structurally this isn't a particularly great idea, it creates the opportunity for a lot of movement in the floor but it did result in the foundations becoming almost entirely invisible so now the observatory appears to float above the ground with no visible support which is a pretty cool effect.  It later turned out that - again - this was a decision that would have serious consequences later down the line:  There are quite a few commercially made products that have two metre diameter domes: oil stores, sceptic tanks, geodesic green houses to name a few.  Small composite manufacturers will happily lay up two metre diameter hemispheres for you in fibreglass... but not so with three metres.  There are very few commercially made products with three metre diameters and the few that are available are hugely expensive to the point of out costing the rest of the build by an order of magnitude.  This decision basically committed me to building my own dome from scratch and that (as we will see) was not easy.

I had a friend over to help and together we set about laying in the floor joists and cutting all those awkward joints I illustrated for you in "The Design"  and after a few hours work this is where we were:

The "Pole star" stage

Then we had a tea break.  

I had planned to use the 18mm plywood that had formed the sides of the Observatory mk 1 to make the floor. The plywood itself would have added significant strength to the structure however it turned out that most of this plywood was not in good condition and (because it had been nailed to the timbers rather than screwed) most of it was too badly damaged to reuse.  So the tea break discussion focussed on what to do next.  Again the easy way out of the problem would be to just buy new plywood but a quick call to the local timber merchant revealed that sufficient plywood would cost a couple of hundred pounds and I wasn't keen on that!  Then Ryan's eyes alighted on the pile of pallets that I had recently collected for firewood.   "Why don't we use those?" he said.  Well why not indeed, wasn't that "The Plan" after all?

The first of many such piles to be converted to building materials

Well, I don't know if you have ever tried to dismantle a pallet without breaking the wood but it is not easy!

It is very annoying when this happens!

The rest of the afternoon (and most of the next week, ah who am I kidding, most of the next year!)  was spent trying work out an efficient way to separate the deck boards from the runners without breaking them.   I'm not sure we ever found a really good method but a Gorilla bar turned out to be good enough and if you have enough pallets then you can get away with breaking a few planks along the way, it is all useful. If I can't build something from it it can always go in the wood burner to keep me warm!

After I broke up a lot of pallets Ryan came over again the following weekend and we set to laying up the floor:

Looking good!

While this is going to give it an undeniably great look it was clearly going to take a long time to do, which isn't a huge problem. What was a big problem was that the spacing between the joists was too wide to support the planks sufficiently at the outside edges and they bowed alarmingly under loading.

This is the problem with repurposing materials for jobs they were not designed for and modifying your design criteria on the fly.  The joists would have been fine with the planned-for structural plywood floor but it was clear we needed some serious reenforcing if we were going to get away with using pallet wood instead.

Fortunately some of the runners from the pallets were a good size to make the extra joists and more carpentry ensued:

Lots more wood added to the pole star

Here you can see the extent of the reenforcing needed, each original radial joist has had an extra piece of timber on both sides to allow sufficient timber to attach the boards to and each "wedge" has had an extra radial joist added.  We also added plywood cross ties underneath the joists to provide some measure of load sharing to reduce the flexing at the outside edge.

Tests showed that the new arrangement provided adequate support for the pallet deck boards and we really started to motor along by the end of that day we got to here:

Notice the extension at the far side of the central ladder, this is to become a storage pod later on.

This was pretty slow going because every plank had to be laid in place, marked up, taken up, cut to shape, replaced and fixed in position.  There are ten wedges and two ladder section, each section contains sixteen planks so that is around one hundred and ninety two planks, each end had to be marked and cut at both ends, so that's three hundred and eighty four cuts, and that's just the first layer, we did a second layer to add strength... seven hundred and sixty eight individually measured and cut piece of wood. 

about two weeks after we first started laying the floor the first layer was complete and looking good

Layer one done!

It was now that I had a floor that I could walk around on that I recognised how annoying it would be to have things stored on that floor so I added further extensions to the joists to allow the construction of two storage pods to keep all the ancillary equipment in.  I am very please I did this and I heartily recommend that if you build your own observatory you give some serious thought to where you will store things when they are not in use,  it very quickly gets tedious tripping over boxes in the dark!

Having by now gotten quite good at the process of laying the floor the second layer went down fairly quickly, here we see my son helping me glue the boards of the second layer in place:

And with that done we were ready to start building the walls!

The Foundations

Having gathered the materials it was time to start.  First I built the two metre diameter decagon I decided on in the design phase, then I called on my neighbour.

My neighbour is a landscape gardener (I find it very useful to have friends who can do Things, as we shall see later in the project!) and coincidentally he had rented a mini digger and an auger for his own project and agreed to help out and dig the foundations for me.  Here he is scrapping the vegetation cover off the building plot:

Once the ground was cleared we marked out the position of the supporting piles and the centre point for the pier and then dug them out with a hydraulic auger (in the background you can see the decagon propped against the hedge).

Interestingly we found that the auger easily lifted large rocks out of the holes but would jam when it encountered a pocket of smaller fist sized rocks.

Having dug the holes we then filled them in again!  This time with concrete and some fence posts which were too warped for Martin to use in his work.  And here it is:

Here we can see another consequence of choosing the decagon over the octagon.   Due to the un-compacted nature of the soil and the inherent inaccuracy of the machinery we couldn't get a post under each joint in the ring, instead we had to go with every other joint which is less than ideal as it allows a lot more flexure in the bearer ring and a lot more stress on the unsupported joints, hence the reenforcing plates you can see sandwiching each joint top and bottom.  Had I built an octagon we could have had a post under each corner and a lot less faffing about with the joints (but even now I still hadn't realised the real problems that lay ahead in using this shape).  But nevertheless we have reached a milestone, we were out of the ground!

The central pier is worth talking about, in total it was two and a half metres long, of which a metre is buried in the ground.  We cut slots in the side and placed reenforcing bars through these slots and then poured about two thirds of a cubic metre of concrete into the pit making sure it went into the tube through the slots, once up to the level of the ground we poured more concrete into the tube to fill the inside to the same level as the outside.  Safe to say this pier is not going anywhere anytime soon.   Once the concrete hardened we had some fun playing tubular bells on the pipe, but since vibration is a very bad thing in a telescope mount I then filled the pipe with sand and grit to damp down the ringing to a dull thud when tapped.   The final touch was to skim some fine cement over the top of the concrete block and summon the family.

All hail the mighty pier!

The foundations are done! Onto the next phase.