Dome build: Dome base ring trials.

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Monday pm:  I pottered on with the dome base ring this afternoon. Which I laid on top of the 2x8s to check overhang etc. The problem was trying to ensure the ring is perfectly round when the large mounting is in the middle of the dome. I can't run a straight edge across and the tape needs to bend around the asymmetric mounting.

So I progressed by adjusting the position of the ring relative to the 2x8s. There is about an inch protruding at the post miters beyond the ring which can be trimmed with the jigsaw. Though I'll wait for the wheels to arrive before making any final decisions. The wheels would seem to fit the center of the track without falling off the 2x8s.

Note that the ring will  be raised 195mm or 8" by the inverted wheels. The dome base ring will not be lying on the 2x8s but on the wheels. The 2x8 octagonal 'ring' is only there to sturdily support the inverted wheels at the correct radius. The radius of the octagon posts is too small for mounting the wheels directly.

I'll need to add timber brackets to the octagon posts on the undersides of the 2x8 miters for wheel support. Just as I did to support the main beams/joists for the observatory floor and veranda.

Perhaps I should add some suitable decoration to the bottoms of the brackets on my band saw this time? I simply mitered the lower brackets last time but they didn't look very exciting. Better, though, than a straight 90° cut-off. 2"x4" is undersized while 4"x4" is too heavy looking. Most of my sawn 4"x4" has split badly since purchase. I may look for some planed 3"x4" at the timber yard. Even 0.5m/ 18" adds up to 4 whole meters or 12' with eight lengths.

I wonder whether I should clad the observatory walls before fitting the brackets? If I fit them first I shall have to miter cut the plywood sheathing carefully around all eight brackets. This would demand considerable neatness and accuracy in cutting and might easily lead to leaks.

If I fit the timber brackets afterwards they will need solid packing to fill the empty void where the plywood meets the 4x4 octagon posts t an angle.

I will have to clad the outsides of the posts with shallow isosceles triangles anyway to give the cladding something solid to be fixed against. The back side of the brackets may need to be sawn into a shallow V to match the cladding at the posts. I notched and bolted the posts for the beam brackets but the loads are much lighter for the dome wheels. So bolting the wheel brackets may be enough.

Tuesday: Bought a length of 3x4" planed timber for the brackets. I may need to countersink the coach bolt heads or even for the nuts. I bought a load of 7" climate coach bolts and nuts and still have lots left. My lifetime collection of coach bolts may still come to the rescue if I can find enough of the right length.

Wednesday: Drizzle. Waiting for the wheel courier to arive.

Click on any image for an enlargement.

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Dome build: New clamps and wheels.

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Monday am. I had ordered 18 new Rawlink 15cm/ 6" spring/glue clamps despite having badmouthed my original Rawlink clamps. Now I am delighted to report that the new Rawlink clamps are a vast improvement! They have the same shaped handles but now have nicely grippy, rubbery pads. Making them very easy to use. They also seem just as powerful but with slightly shorter jaws for better leverage when opening them.

I think the spring wire is slightly smaller diameter so it may be improved steel for greater lightness. The previously loose jaws are now properly pinned so they can't fall out like the originals. Making them excellent value compared to the pricey and much weaker Bessey clamps.

Having opened my parcel of clamps I painted the new, 2x8, wheel support ring with the Safeway, mineral/water wood treatment. I have been delighted with the very even, silvery-grey appearance of the treated timber. The bare larch floor boarding has weathered to a very similar appearance. It may not be to everybody's taste but I like the "abandoned barn" finish.

I finally ordered 8 off 160x50mm nylon wheels today in pressed steel forks to support and rotate the dome. Each, individual wheel has a capacity of 350kg.  That's 770lbs! Which would easily carry the full weight of the dome all by itself. Each wheel has ball needle roller bearings for easy movement. The hard wheels should avoid flats forming if the dome isn't turned regularly.

I liked the slightly increased height of the fork to provide a marginally taller dome. At nearly 6.5" diameter I imagined these wheels would roll more easily over inconsistencies in the track than my earlier choice of 5". My eye level, when standing on the observatory floor, would then be just above the intended base ring height.  So a horizontal view of the distant woods is still possible without needing a ladder to reach the eyepiece. 

Shopping around online provided considerable savings. The most heavily advertised Danish dealers on Google are wholesale only. However, they do not make it remotely OBVIOUS that they will not deal with private customers! I wasted considerable time on their websites picking and choosing! As it was, I found better wheels with higher load capacity for about the same money from a much more friendly dealer. They should be with me on Wednesday.

Click on any image for an enlargement.

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Dome build: Dome weight and wheel load.

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It has been sunny but very windy all day after the storm. So I have been getting on with capping the octagon walls with mitered 2x8s. Only one post was out of level. My stock of 2x8s is of two different sizes! 

A bare segment/gore weighs just over 6 kilos or 13lbs.  So, 16 gores = 13 x16 = 208lbs.

45x45mm horizontal struts weigh 1.1kg per meter [0.7lbs per foot]

Vertical struts: Pi x 1.6 x 8 = 25 meters for the 8 vertical runs [half circumferences] over the dome. If the vertical struts weigh about the same per meter as the horizontal struts then [say] 27.5 kg = 60lbs total.

4mm birch ply weighs about 15lbs per 5'x5' sheet. [2.25m²]
Area of a hemisphere = 2.Pi.r² = 6.284x1.6² = 16m² / 2.25 x 15 = 106lbs for the ply cladding.

Plus any reinforcement around the obs. slit and shutters + hardware + paint. Say 500lbs total for the entire dome?

Support and rotation wheel loading:  500/8 = 62.5 lbs static load per wheel.

Hard, white, 5" Polypropylene industrial wheels with ball or roller bearings and fixed forks have a load capacity of 150kg = 330 lbs.

6" PP wheels capacity is  250kg = 550lbs. The price difference between 6" & 5" wheels is 2:1.

I had ordered 18 new Rawlink 15cm/ 6" spring/glue clamps despite having badmouthed my original Rawlink clamps. Now I am delighted to report that the new Rawlink clamps are a vast improvement! They have the same shaped handles but now have nicely grippy, rubbery pads. Making them very easy to use. They also seem just as powerful but with slightly shorter jaws for better leverage when opening them.

I think the spring wire is slightly smaller diameter so it may be improved steel for greater lightness. The previously loose jaws are now properly pinned so they can't fall out like the originals. Making them excellent value compared to the pricey and much weaker Bessey clamps.

Having opened my parcel of clamps I painted the new, 2x8, wheel support ring with the Safeway, mineral/water wood treatment. I have been delighted with the very even, silvery-grey appearance of the treated timber. The bare larch floor boarding has weathered to a very similar appearance. It may not be to everybody's taste but I like the "abandoned barn" finish.

I also ordered 8 off 160x50mm nylon wheels today in pressed steel forks to support and rotate the dome. Each, individual wheel has a capacity of 350kg.  That's 770lbs! Which would easily carry the full weight of the dome all by itself. Each wheel has ball bearings for easy movement and the hard wheels should avoid flats forming if the dome isn't turned regularly.

I liked the slightly increased height of the fork to provide a marginally taller dome. At nearly 6.5" diameter I imagined these wheels would roll more easily over inconsistencies in the track than my earlier choice of 5". My eye level, when standing on the observatory floor, is now just above the intended base ring height.  So a horizontal view of the distant woods is still possible.

Shopping around online provided considerable savings. The most heavily advertised Danish dealers on Google are wholesale only. However, they do not make it remotely OBVIOUS that they will not deal with private customers! I wasted considerable time on their websites!

Click on any image for an enlargement.

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Dome build: Observation slit mock-up.

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The observation slit provides the view of the sky from within the enclosed confines of the dome. Its minimum width is important for a number of reasons. Not least the difficulty of keeping an object in view as the sky passes across the slit over time. Though in reality it is the rotation of the Earth which gives this illusion of sky movement from East to West.

If the slit is too narrow then the dome must be moved so often it becomes a bore. Many amateurs prefer to drive the dome to avoid having to monitor and manually rotate the dome during observations. Those who image from a remote location will demand a dome rotator which closely follows the telescope's field of view. This will change depending on the altitude of the object being imaged or studied.

For simplicity I have chosen a slit width of 60cm or 2' which is the same as the normal gores. The mock-up was to see how such a slit, or rather its framework, would affect the rest of the ribs.

The gores on either side of the slit have no need of an adjoining rib so these nearest slit can simply be left out. Though the opposite side of the adjoining gores will still have a normal rib which stops at the slit framework. The two gore's horizontal struts can be carried right over the intervening inch to stop at the slit framework. To be fixed there as if they were normal ribs. Though the miters on the slit end will be at different angles to normal ribs. 

As mentioned previously, the slit framework must be strong enough to compensate for the missing struts which would otherwise cross the slit. More than that, the framework carries the tops of all the ribs where they are cut away to allow viewing at the zenith. [i.e. The slit must be open overhead.]  So none of the ribs actually reach the dome's pole. In the image above I have pulled the base ring forwards by a 30cm [one foot] to simulate the open zenith. The arched sides of the slit's framework have been extended to compensate. Perhaps a foot [30cm] extra clearance, for zenith viewing,  is too much?

It would be safer to match the slit framework to the complete dome skeleton to ensure geometric accuracy. The top of the slit can be left square. Or a rounded collar could be formed from laminated plywood glued in enough layers for adequate strength. Weatherproofing at the zenith is obviously important as any leaks will fall directly onto the mounting or telescope, or both.

Click on any image for an enlargement.

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Dome build: Progress at last?

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A morning of light showers didn't put me off working on the dome. I had to replace all the 57cm C2s before I could add all the short gores to the three, full length ones. There was a minor panic when I realised I couldn't pull the resulting skeleton into a circle of the correct diameter. 

It seems the architect had decided to make only 15 gores to save on materials but forgot to tell the foreman or the labourer. Problem solved by adding a couple of extra struts for decoration where the observation slit will go.

By the time I had finished lunch the dome was enjoying heavy drizzle. I rapidly unclamped it all and put it in the octagon with a tarpaulin over the top. I have ordered more Rawlink clamps online. I know I keep saying it but they offer remarkable clamping pressure for small change. Yes, the jaws fall out all the time and the widely splayed, slippery plastic handles are very hard to grip.

Nevertheless, for my purposes in holding gores together, they are ideal. The Bessey clamps, for comparison in the same size, are very weak and need at least two or three, to hold as well as a single Rawlink. I never had any of these spring clamps before starting the observatory. They made dome construction so quick and easy I should call it Insta-dome. Or perhaps not. The entire dome can be erected and dismantled again in under five minutes.

What's next? I need to mark the observation slit on the gores. Then add side reinforcing ribs to replace the missing strength from, quite literally, cutting the dome wide open on one side. I was advised to stiffen the dome with cladding panels before attacking the ribs.

The opening must be left without any obstructions even up over the zenith. That means a considerable loss of strength without really stiff, additional materials built right around the opening. The dome is very likely to sag around the slit unless it is well support with vertical edge ribs. Conversely, I can't add these parallel reinforcements without the dome being supported evenly.

The gore on the left shows the latest dimensions and angles.

It also means cutting 4mm birch plywood panels on the table saw. Which means working outside to have room to swing a 1.5x1.5m [5'x5'] sheet freely. The largest [lowest] panels are 60x50cm but they'd have to be cut from 50cm wide strips. Three strips from each sheet. I'll lose a little bit of that width to the two saw kerfs as I divide up the sheet into three. Working outside needs dry weather. Or a temporary roof to work under.

Perhaps I'd better get some white, lightweight tarpaulins to hang a sloping canopy off the octagon and shed. Otherwise I shan't get much done in steadily deteriorating weather. It would be easy to run a 2x4 or 2x6 off the veranda to support the side furthest from the shed. A sturdy post will hold up the corner using truss plates. The shed will be providing support on the other side.

It needs to be windproof as well as waterproof and absolutely must shed rainwater easily. Rather than collecting it in heavy puddles ready to destroy the entire structure. One of my earliest memories was of a neighbour trying to push an enormous puddle off the roof of his tarpaulin or canvas "garage." Which was hung between two buildings. Unfortunately the length of timber he was using to prod the puddle pierced the canvas and he and his motorcycle were drenched! I remember being fascinated by the bright, lens-like puddle in the roof. History does not record whether I was also soaked.

The latest iteration of the dome has the upper C4 [35cm] struts fitted near the tops of the [still] shorter gores. Some to the ribs have taken on a slight set from being damp and twisted by earlier struts of the wrong length. They will soon respond to being straight again. Next step is to extend more of the ribs towards the pole.

By the strangest coincidence I saw a telescopic, big bale loader on my morning walk today. I was just discussing these machines as a means to lifting my completed dome into place. Though I still think I will lift each gore/segment effortlessly up the ladder using my hand-cranked, boat winch. Perhaps pairs of gores will be quicker to build with and more stable on the ladder. The important thing is to have rehearsed the entire build down on the ground before lifting anything. 

I can build a derrick off the pier with a pulley on top. Then the gores will remain upright throughout the lift and just slot neatly into place on the base ring while still safely restrained. Their forward balance point will ensure they don't tip backwards. Access, via a stepladder, through the open observation slit should be fine for working on the top of the dome.

I have received the white, 4x6m lightweight tarpaulin I ordered to form a work shelter. The 18 extra Rawlink spring clamps should be here tomorrow. I am using storage tubs for carrying the clamps about now. Bags were hopelessly awkward and bringing them in an out by hand meant lots of trips, due to the bulk. I can't believe how many clamps it takes to hold the dome together when I had none at all before starting. I need lots more to be sure the dome is safe as I build it up on top of the building. The gores should be mostly clad by then so I don't want 'bits' blowing away across the fields!

Click on any image for an enlargement.

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Dome build: Construction access?

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I have already discussed the difficulty of getting any sort of crane in the back yard/garden. Our narrow drive runs very close to the house and is bounded by trees embedded in a tall, shared hedge on the opposite side. The rear view mirrors on my modestly sized car brush the hedges going both ways.

Then I remembered my 5m, 15' roof ladder I had made up after the great storm of '99. When I desperately needed safe access to repair the damaged roof myself, working alone. 

I had bought half of a wind-damaged, two stretch ladder and had converted the good half into a roof ladder with an add-on, ridge hook and wheels kit. Aluminium crossbars bolted on the underside allowed it to lie safely flat on our heavily ridged roof.

I just need to fix a rigid stand-off to the octagon walls for holding the top of the ladder. This will provide safe access to a perfect working height with good ladder width for sliding complete gores aloft. The dome will still rotate inside the top of the ladder. Allowing single gores to be safely added at each station as it rotates. Scaffolding would not have helped much because I'd still need to get the gores up there somehow.

Needles to say my new strut lengths were totally wrong! A recalculation showed that C3 should have been 36 not the 51.5cm of which I had just cut another 14 examples! Despite three gores fitting nicely, side by side, the new dome went together in a huge spiral! Eek! 😨 Wrong again!

Finally, I decided to trust the trigonometry which required a re-measure of a full gore to be safe. This confirmed my original measurements for Radii and the altitude angles of the struts were correct. I even used several different methods to check and all the numbers seemed to confirm each other.

Running through my "sums" again showed I had made some silly errors of entry on my calculator. Or made 'typos' onto the computer drawings. So the longer C3 struts had to be shortened again. To 48cm not 36! Though the 40cm C4 top struts still seemed not to fit well. Regardless of all this, I still managed a half dome which fitted together and had the correct diameter and height. All the ribs now lay flat against each other without obvious strain and needed far fewer clamps. So, progress at last? We shall see when I get a full dome, with full length ribs, clamped together. Until then, your guess is as good as mine. It's all been taken apart again and gone under the tarpaulins overnight. Two days of rain or showers are forecast.

Click on any image for an enlargement. 

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Dome build: Horizontal strut trig calculations.

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A member of the Cloudy Nights Observatory discussion forum has kindly provided guidance to calculating horizontal strut lengths using simple trigonometry. 

My own memory of trigonometry was so old and rusty I had no idea where to start. Once provided with the simple formulae and helpful drawings it was a matter of entering a few figures in my dusty, scientific calculator and pressing a few buttons. If only I could be excused typos! I made a mess of calculating R3 and C3 which threw everything off.

I have underlined the corrected figures and they make a dramatic difference!

Original "made to measure" strut lengths are on the left under Orig. Calculated lengths on the right after subtracting the 1.8cm thickness of the ribs. The differences are not exactly trivial! I shall now cut a couple of new struts for the complete, full height gore to see how it compares.

Having cut new struts to the calculated lengths they made no sense. The ribs were curved sideways and the spacing at the tops of the ribs was about a foot!

I have now returned to my more generous "made to measure" strut lengths. The glue on the plywood patches for a second, full length, gore/segment is drying over lunch. A third gore will be made this afternoon to the same generous strut lengths and the rib extensions glued.

In the image above the base ring has been placed underneath to ensure the gores were following the correct radius. A plank on the wall fixed the center height of the ribs at 1.5m. [5'] The ribs lie nicely flat together without gaps between them. The base has been carefully checked for level.

At this point I have run out of clamps. The timber yard had not restocked with the Rawlink clamps as promised. Despite their numerous drawbacks the Rawlink clamps offer plenty of clamping pressure for the modest investment.

A couple of contacts have kindly suggested alternative means of building my dome accurately. For which information I am very grateful. I think I have been too afraid to continue after a couple of minor failures. Though I have certainly not lost the will to succeed. I had rather lost my nerve without any real insight as to how to improve the strut dimensions. I seemed to be shooting in the dark. My trepidation was largely due to  a fear that I'd have to cut each covering panels individually to match each facet on the trapezium dome's skeleton.

I have now realised that with a complete gore completed I can check the radius directly at each strut out from the shed wall. This will be a good check for the calculated radii and perhaps explain where I went wrong with the calculated upper strut lengths. The bottom and second struts match both the physical and the calculated lengths. Things go awry on the third and fourth strut level.

The triangle represents the latest gore dimensions and angles. In reality the gore is curved away from the viewer. Later, panels will be laid over the gores to achieve an angular profile. Note: I have underlined changed dimensions and angles compared with earlier versions. R is the Radius, measured from the shed wall to the outside of that particular strut. I also lifted the base to achieve a true 160cm radius at the center of the strut and show the difference with hyphens. The larger angles on the left are direct altitudes measured from the wall base.

I can also set up the three gores at 120° to each other to see how that looks. Once I have confirmed I am on the right track I shall mass produce more struts to the new lengths. It is now certain that I shall need to buy more 9mm [3/8"] birch plywood to complete the upper ribs. Which is a bit of a shame because most of them will be lopped off halfway once the observation slit demands it. I shall just have to use the "waste" as local reinforcing material.

Click on any image for an enlargement.

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Dome build: More dome musings.

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In theory I ought to be able to measure the circumference at different heights on the dome and divide by 16 to find the strut lengths. If only it were that simple! The polygons involved are not true circles and the shape of the dome is not perfect until it really is perfect.

I could ignore the polygons and run my long cloth tape measure around the dome resting on panel pins driven into the ribs at the correct 'latitude' for the struts. [Bottom of the intended strut where it is longest] The panel pins would stop the tape from slipping downwards. That might be worth a try with bare ribs with only base struts. Variations in rib spacing would not affect the results. I'd just need to remove 16 ribs from the hip, waist and chest measurements.

The observation slit is treated as the last gore going in either direction. As that demands the dome is cut away this would nullify any variations. I just need to make the dome as even as possible before attacking it with the tape measure. This allows me to cut struts of equal length for each ring. Based on real world figures for the circumference at each 50cm higher level. Better than guessing by measuring each gore and averaging the difference as I have done so far? Not really sure.

I am aware that I could glue the ribs together in pairs for greater stiffness. The horizontal struts could be held in place by skewed screws passing through two ribs from above and below the adjacent struts. They don't have to have screws straight into their end grain through just one thickness of rib.

I have to constantly keep in mind the difficulty of getting useful structure up onto the top of the octagon walls. Not to mention the added height of the supporting wheels and both base rings. [Walls and dome] Anything too heavy or unwieldy will make it simply too dangerous or unnecessarily difficult. Particularly in windy weather working well above the ground.

A quick mock-up of the octagon using lightweight tarpaulins and a garden net. It was wet and windy so I had to time my shot carefully. I haven't clad the building with plywood yet because I thought it might interfere with building the dome up on top.  

One injury from heavy lifting could put an end to the entire project. I still tend to try and lift heavy things as if I were a teenager. In middle age I would lift ridiculous weights and did all day, most days when I was landscaping a very rocky moraine, by hand, right next to the house. There was no room for a digger even if I had wanted one. I wheel-barrowed away the results of my exhausting digging with a navvy's pick and shovel. Of which I wore out several of each.

Rocks up to 3' across were rolled away some thirty yards to provide a level lawn or terrace. Larger rocks were moved constantly backwards against the rising and receding bank of highly compacted gravel yet to be attacked. I removed probably 5x15x3m of rock hard gravel that way over a couple of years. Every, single teaspoonful was hard won. It took years for the damage to my shoulders to recover.

Repeated lifting, bending and moving about is still keeping me fit. As is the rough, country walking and reduced cycle rides. But there are obviously limits. I have no plans to become a weight lifter in a gym just to become strong enough to lift the complete dome with one hand! I'll settle for carrying one complete gore at a time up a ladder.

Wednesday: A grey, dull day with the threat of rain in the afternoon.

The octagon mock-up with a rather bright green dome painted on top, roughly to scale. The planned colour is 'dirty' pale sage. The observatory walls will be dull, grey-brown plywood not green as shown here.

A rest day from the dome for consolidation of theory and practice in my mind. I need more strut material which means taking the car out tomorrow morning in the rain. I keep cutting new struts from previous iterations/lengths but inevitably this uses up the longer lengths. Which then have to be replaced.

Without measuring the actual dome I am fairly certain the previous struts were not long enough. By pulling in the gores too tightly, lower down, this 'tipped' them upwards and outwards at their tops. So the supposed hemisphere became more pointed towards the pole by at least 15cm, or 6". Not quite your Kremlin dome but similar.

Click on any image for an enlargement.

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Dome build: Mistakes galore!

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A photo of the redesigned dome at dusk. Twelve gores out of sixteen and it is hardly more than a half circle. The diameter is about eight inches more than expected! Eek!

The ratchet strap was my attempt to pull the sides in. It worked but opened up gaps between the ribs at mid height. The full length ribs have also risen by about six inches at the dome's pole/zenith.

I have covered the dome over for the night to have a rethink about what I am doing.

Perhaps the new strut lengths still aren't remotely optimum? I'll clamp more upper rib extensions in place tomorrow to see what happens.

Tuesday: Spent the entire morning removing struts and adjusting what was left.  Back to squeezing the base into a 10m polygon. I made a simple height gauge with an alloy pole and disk of plywood. Both the ladder and the plank were too bulky where the ribs wanted to rest. Added more rib extensions sandwiched between the gores. This adds about 6" to the circumference!

Later I made more gores from the spare rib arcs. I don't think the remaining struts are the correct length or have the right angle cut on them. The 2nd and 4th ring of struts certainly weren't correct because they forced gaps between gores and made the full ribs too high at the center. Even now the ribs are all over the place on center height.

Measuring and averaging the widths between the ribs, at the standard strut heights, seems sensible. though it assumes the existing struts are the correct length! Too many unknowns!

The latest image shows the result of averaging the spacing for relaxed, gores without lower and upper struts. One complete gore has four, new horizontal struts. It meant cutting yet more, but longer struts with careful attention to the compound miter angles. Fortunately I can sneak up on the required length from below thanks to the tapering of the gores. I kept cutting and trying until the strut would slide into place without distorting the ribs outwards. Then a final check for rib flatness against the shed wall.

Tomorrow, if it is dry, I shall glue the top extensions to more ribs and try them with the new strut lengths. Unless I build at least two gores each time and clamp them together, it is difficult to judge the results. Even then it takes a few more gores to discover the finished dome diameter and geometry. Visualizing the spherical geometry involved from any changes does not come naturally to me. No doubt it gets easier by one's third or fourth dome. Unfortunately I am not about to make a career of it at my age.

Click on any image for an enlargement.

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Dome build: Sample gore.

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Before I launched into mass production of the new horizontal struts I thought I'd better build a complete, sample gore.

I cut off the extension ribs at the angle at which they joined the lower ribs. Then added a 4"x8" 9mm plywood, reinforcing patch across the inside of the slanting joint.

You can never have enough gluing clamps!

The outsides of the ribs are joined side by side so the patch could only go on the inside. When the gores/segments are joined and glued to complete the dome this will further reinforce these joints. It should be emphasized that these 9mm [3/8"] ribs are merely guides for the horizontal and vertical struts. It is these battens which will provide the real strength in the dome's skeleton.

I couldn't see any other way I could assemble a lot of loose struts into an accurate dome without some guidance. It was a fortunate decision because, even with the help of the ribs, it has proved a difficult geometric task to achieve a uniform dome.

Black polythene was sandwiched between the spare ribs clamped to the outside of the gore to avoid them becoming stuck fast by the white 'Outdoor' wood glue. These temporary outer ribs acted as guides to ensure the arcs were correctly followed by both upper and lower ribs. The actual joints were hidden inside the plywood sandwich while the glue dried.

  In the image on the left I have ringed the patch-reinforced joints just above the 4th horizontal strut.

Having carefully measured the dimensions and angles of the new struts I spent a couple of hours cutting new, shorter lengths from the originals. Only the 57cm second struts had to be cut from fresh stock of 45x45cm. Chopping them off straight at 58cm was easier than trying to cut compound miters on each length. The shorter lengths were much easier to handle using the end stops on the DeWalt, miter saw stand.

As these stops are so abysmally inadequate in design and manufacture I have packed the stop heads with softwood batten to give G-cramps something to grip. I could then add various lengths of 2x6 as the true end stops on top of the metalwork. Otherwise the DeWalt end stops only allow very long lengths to be measured off and are far too floppy to be accurate within half an inch. Marking the outside faces of the new struts with pencil was vital. Since none of the compound miters was equal in angle. Rotating the struts between cuts would have resulted in completely wrong angles on each end.

Then next task is to drill and countersink the ribs at 50cm centers for the new struts.

Click on any image for an enlargement.

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Dome build: New struts, please!

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As Friday remained dry, I built a single gore leaning against the shed. A batten was screwed to the shed wall at 1.5m [5'] to provide the dome's inside height measurement. The outside of the bottom strut was placed at 1.6m from the shed wall. I clamped two spare arcs to meet at the batten on the shed wall. Then re-fixed the original, horizontal struts at higher positions to bring the 60cm level to 55cm in width. This was the average of measuring all 16 gores on the previously competed, strut-free dome.

Trial gore/segment with struts at 50cm [20"] centers: The aluminium speed square allowed me to measure the angles and new strut length to be placed at 50cm center height from the ground.

I then trimmed a new, "middle" strut until it fitted snugly without distorting the ribs. I shall now make a longer, upper strut to fit above the middle one. Followed by a new gore using these new strut lengths to ensure they fit together side by side. 

Saturday: Constant fine drizzle but I decided to continue with building a standard [pattern] gore. The horizontal struts were reset at 50cm from the ground and then at 50cm centers above that.  Note that this spacing is measured in straight lines rather than circumferential because the plywood cladding panels will be flat. The image text gives the general idea.

Both ribs of the gore/segment rest nicely flat against the shed side as a rough check for lateral curvature. The loose arcs were clamped as usual to extend the gore to the batten ledge on the shed at 1.5m [5'] high. Because of the large observation slit cutout, [60cm or 2' wide] the top rib extensions will be much shorter in practice.

Although the rib extensions look correct to the eye there is considerable leeway before a full and true arc of a circle is achieved.

As the dome's circumference across the flats is 10m then the full rib arc should be C/4. Or 2.5m measured across the flats from the ground up to the zenith. The rib's top section beyond the top horizontal strut must also be another 50cm to reach the pole. This is vitally important to avoid the gores drooping when fixed together side by side. So the lower ribs must be tilted back until the final 50cm dimension is reached. This is achieved by sliding the upper rib extensions outwards [telescopically] while still resting on the 1.5m high support ledge.

The total lengths of the ribs must be correct. Otherwise the ribs won't come together to meet at a single point exactly at the pole. Then all the struts will be of the wrong length. This is easier to visualize if the gores are imagined to be rolled out flat on the ground as a long thin, isosceles triangle drawn on paper.

The total rib length [or gore height] must match the dome's polygonal circumference/4. While the struts must fit snugly between the two straight sides drawn from the base to the apex at the correct distance apart. [50cm or about 20"]

Having typed this I panicked and had to go out and check the new gore up against the shed again. The "top end" looked much too short on the image above. Fortunately the real thing measured only an inch too long. This was measured along the ribs themselves. When, in fact, it should be measured on a perpendicular from the horizontal base strut. Which in this case makes the gore/segment the correct length if I had measured it properly. Phew!

Better the gores are too long, rather than too short. As this ensures the upper arc of the dome is not slightly "pointed" at the top. [Lanceolate.] With the danger of the dewshield clearing nicely at the zenith but striking the dome ribs at lower pointing altitudes. Clearance is further complicated by the large lateral offset of a German equatorial mounting on either side, east and west of the pier. I'm hoping it all comes right in the end. Or I shall have a lot of firewood to get us through the winter!

Click on any image for an enlargement.

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Dome build. Veranda access and octagon top rail.

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While 'playing' with plywood arcs up on the platform I made a couple of discoveries. I could make a sizeable hatch, or rather, double doors, in the octagon wall at the top of the interior stairs. This would give me easy access to the veranda. The height to the top of the octagon posts is not too tortuous for a fairly fit [elderly] person to negotiate as needed.

The stiffness of one's back is the ultimate arbiter of ability to pass under such a hurdle. The width of a low door is also vital if the the person navigating it can turn sideways to speed access and egress. Simply by managing a short sideways bow with the support of the stair handrails. Practice may help! A narrow door would force a longer, stooping, forwards shuffle. With the far greater risk of standing up while still under the door lintel.

There is really no need for the top of the hatch to be hindered by the present cross brace below the heavy top ring of timber reinforcement. [see below] A horizontal strut could just as easily reinforce the gap below the lintel.

I would not wish to enter the dome routinely by these doors but it will not be necessary. A normal sized, exterior door will be provided in the octagon wall down on the ground floor. Leading to safe, aluminium steps, with handrails, to reach the observatory, on the 1st floor, via a hinged, drop-down hatch. Only by closing the hatch can one access the veranda door. This is deliberate to avoid nose dives [no doubt with tuck, two half somersaults and twist] on returning to the observatory from the veranda. Nobody can hear your last scream in darkest, rural Denmark. Particularly in the middle of the night!

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I tried laying 2x4, 2x6 and 2x8 timbers across the tops of the octagon's upright posts. 2x4" is too narrow to allow full miters to sit neatly on top of the 4x4 posts. Unfortunately I have little in the way of 2x6 left of any suitable length. Though I do have plenty of 2x8 looking for a useful purpose.

I could cut arcs on the outer edges of 2x8s. Which would be lag screwed to the tops of the octagon posts. Then an upstand of thin plywood, attached to these outer curves could help to seal the lower edges of the dome. Closing off the midriff gap where the support and dome rotation wheels will sit, otherwise fully exposed.

This stack of rib arcs has six missing as they are being used on the trial gore outside. Do not underestimate the sheer amount of work and quantities of materials in a dome.

A skirt on the lower dome 'hem' is traditional to throw rain, dew and melt-water clear of the supporting walls. The upstand, rising from below, will aid this weather-proofing while still providing airflow ventilation to help avoid interior condensation.

Timber brackets, attached the the upright posts, would provide greater stability for the top ring of mitered 2x8 timbers. These same timbers would help to support the dome rotation wheels. As well as tying the tops of the octagon posts even more firmly together. All I need now is a stretch of dry weather to be able to work outside.

A useful suggestion on the CN forums was to clad the gores with plywood prior to cutting other ribs and gores away to form the observation slit. The plywood will stiffen the dome and avoid it sagging when structural material is removed.

Click on any image for an enlargement.

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Dome build: Monsoon season!

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Tuesday is another wet day. In between showers I removed the 180mm telescope [7" f/12 refractor] and saddle from the mounting. Then improved the mounting's weather protection after the counterweights had swung down to their lowest point. One of the Skywatcher style tube rings had a sheared hinge screw. Suggesting I need to take greater care in risking my telescopes on these rings. The spindly castings at the hinge do not inspire much confidence either. Drilling these out to fit larger hinge screws [or stainless steel, hex socket head bolts] might easily weaken them!

I have now checked the level of the pier relative to the octagon. The present 2x4 "handrail" [bracing] is 4cm [1.5"] above the pier's plywood top plate. With the tops of the 4"x4" octagon posts another 10cm [4"] above that level.

I still need to check the octagon post heights with a water level before adding a new, 2"x8" octagon top rail, laid flat. Eight dome rotation wheels will be inverted on top of these over the posts for support. All of which helps to increase the height of the dome to provide improved, internal, telescope clearance at the zenith. Though this will not alter the diameter elsewhere, of course.

Wednesday was another wet day. I brought back more 45x45mm timber for making longer struts but it was far too wet to uncover the dome for taking any measurements.

Thursday: I just looked out at another cloudburst at breakfast time. To see the dome skeleton uncovered by the wind. A dome is not the easiest item to cover with undersized, lightweight tarpaulins. I don't have room for a tarpaulin tent-garage in any of the sizes on offer. Since they are sold as vehicle garages they are always much longer than I need or have space for. A 4m x 4m square one might just have worked. I'd need room to move around the dome or it would have been completely pointless. My old car needs room to reverse before heading down our long drive.

Hanging a loose tarpaulin from the octagon and shed, for shelter, would not have helped much. Even if the wind had not torn it down it would not have been big enough to offer much protection. A tarpaulin over the dome itself does not provide anything useful for working on the dome exterior in changeable [wet] weather. The struts and fixing screws have to be fitted from the outside.

Today, I decided to dismantle the complete dome and store the arcs and struts to dry out in the shed. It took only quarter of an hour from start to finish thanks to Torx screws and my rechargeable driver/drill. It just wasn't worth persevering without a single tarpaulin to quickly cover the entire dome in one go.

I discovered that toe-nailing adjacent struts with screws is best achieved with round head wood screws. Countersunk [Csk] heads love to dig in and jam on the previous strut when being removed. Round head screws cannot dig in whatever the angle. Worth knowing before I toe-nail screw all the struts in adjoining gores for greater strength. I had used a mixture of screws to fix the gores together and wasted time, in pouring rain, struggling with the flat heads of the countersunk screws.

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Dome build: Look, Ma! No struts!

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The image shows the dome skeleton with both sets of upper [horizontal] struts removed. The extended ribs are all resting loosely on blocks on the post at 1.5m, 5' in height.

As can be seen from the orange ratchet strops, I am trying to pull the dome into a smaller circle. Hopefully to close all the initial gaps between the gores from complete re-assembly without struts.

The "circumference" of exactly 10m includes the flats across the ribs. So is not a measure of a true circle, but is, of course, a 16 sided figure or polygon.

10/16 = 0.625m per side [or gore width.] My lower struts are 60cm in maximum length. So the ribs are adding 2.5cm or 1" each. Since they are only 9mm thick and doubled it ought to be 18mm or 3/4". 16 x 6mm [difference] = 96mm or 4". There is obviously some extra spacing spread around the bottom ring of ribs and struts. My long, alloy measuring pole shows an excess on the diameter too.

The upper dome is slightly distorted because I have trapped the rib extensions between the lower pairs of ribs. The upper and lower ribs ought to be continuous but it is far more convenient [and stronger] to clamp the upper ribs in a rib sandwich. Though this does not affect the width at the base of each gore where it is all pulled tightly together.

It turned out the dome was oval but I was able to pull it  back to round with another ratchet strop across the widest measured diameter.

Faced with a bunch of wobbly ribs I needed a datum point at which to measure for new horizontal struts. So I just measured up 60cm from the base struts and marked the ribs. Then measured horizontally between the marks and wrote down each measurement. Dividing the total by 16 gave me my average distance. This turned out to be 55cm. Rather longer, by 2cm, than my original lower struts at 53cm.

The only way I can reuse them is to raise them considerably. Which I really don't want to do. For both reasons of economy in cladding plywood and appearance. I shall just have to buy more 45x45mm batten and make a lot of new, much longer, lower struts ti be fitted at the 50cm height. The 53cm struts can be used in the next ring upwards after trimming to avoid waste.

Click on any image for an enlargement.

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DeWalt's crappy, DW2735P Flip drill and driver kit.

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Thanks to DeWalt's substandard crap I wasted an hour and a half of precious sunny weather. What passes for a pilot drill, countersink and reversible driver swallowed the driver bit. The tiny super-magnet had obviously come loose in the body. Causing it to shift enough to allow the tip of the driver bit to be a full 1/4" below the nose of the holder!

I struggled with every tool I had to try and remove the Torx bit without success. It was far too deep and there was nothing to grip in the tiny space available. Four pairs of tweezers were sacrificed in the battle.

In the end, I removed the drill holder and countersink and drove a cut off hex key down its throat. It took quarter of an hour of hammering with the body held tightly in the 3-jaw chuck of my lathe. Finally the driver bit came out of the other end with the magnet still clinging tenaciously. Then all I had to do was wrestle the hex bar free.

The image shows how far down the throat the driver bit went. The damage from being tightened in the 3-jaw lathe chuck had to be filed away to allow the body to re-enter the DeWalt chuck.

My added packing is slightly too long so the bit is protruding more than it should. I shall fix that for what it is worth. The DeWalt flip drill/driver kit costs only $22 on eBay in the US but cost me over £40 equivalent here in Europe including delivery! $53 equivalent! 

Without the magnet in place there is no bit retention so the driver falls out repeatedly. Sadly I have no idea if something vital fell out of the body. The bit never wanted to disappear inside previously. Not until it suddenly vanished from sight! I had used the tool for several days before it broke so badly. The driver bit had fallen out onto the ground several times during normal use. The pinch screws are incredibly short and are actually hollow rings. Hex inside and threaded outside. Lose one and you'll never find it again!

So, thank you, DeWalt, for designing and producing such a pathetic piece of expensive crap, as badly as only you know how!

Click on any image for an enlargement.

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Dome build: Segment mock-up for a slightly raised dome.

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Saturday, the 7th Oct. Rain all day again!

Meanwhile, my shed tidying has made just enough room for two segments to be arranged at opposite ends. With base ring arcs clamped between them to simulate a full dome with 1.5m center height. I checked the base ring diameter and dropped a plumbline from the center or north pole. This ensured the segment bases and tops were equally spaced along the floor.

The lower horizontal struts still seemed to fit fine in their original positions. While the upper struts needed only an inch or so upwards movement on the ribs. I used screws to fix the ribs quite loosely at the pole to a solid vertical batten. This was to avoid altering the relaxed state of the ribs if i had clamped them. Which would have changed their straightness from base strut to pole. Which, in turn, would have altered the required length of the horizontal struts needed to separate them.

The vertical batten provided firm support. With just enough spacing, at the pole, to match the added rib offset from using base ring arcs. Which were deliberately clamped to the the outside of the real ribs for checking whole rib alignment.

Naturally, I can't be sure that this new rib spacing would equate to a complete dome of the correct height. I shall have to change all the segments and then build a dome from the modified segments. Just to confirm  they will all fit together properly. This should be quite a quick job with a rechargeable screwdriver.

Sunday: The image shows the first dome assembly after moving all the top struts upwards. I had run out of clamps at this stage. So I toe-nailed screws on most of the horizontal struts to free up clamps for the extended ribs.

Many of the ribs look as if they are sagging. Now I have to try and lift all the ribs up to the top of the dome. I have already screwed blocks to the upright center post to provide support for the rib extensions.

The second image shows the dome after I have removed all the upper struts and applied all 16 rib extensions. I was all too obvious that the struts were badly distorting the gores. There are still wide gaps around the lower struts do they will have to go next. I hope the ribs remain stable! Or I shall have to clamp them in place between two disks.

The bottom braces set the diameter of the dome so they must stay. I should really have started with full length ribs and gone on from there. Fortunately, tomorrow looks fine again. So I can continue removing struts until the ribs are all perfectly straight. Then I can decide what to do about lightly trimming and refitting the struts further up the ribs.  

Click on any image for an enlargement.

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Dome build: I [finally] have a plan!

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Following discussion on the Cloudy Nights Observatory forum it seemed sensible to put together a plan of future operations. I have to admit that I have not been giving the project's serial progress enough thought. In fact I have been concentrating on getting the dome construction completed before any consideration of actually getting it up on the octagon's walls above the platform.  

The dome's base ring will be built on top of horizontal 2x6s fixed flat on the tops of the octagon posts with lag screws. The large surface area will provide flat and even support for the ring after checking all round with a water level. Shims below the 2x6s are more sensible than fine adjusting the heights of the posts with a saw. No dome rotation wheels will be fitted at this stage.

The telescope and mounting will be removed to safety. A flanged pipe will then be fixed on top of the bare pier to just reach the inside dome height and then carefully centered. This arrangement will act as a strong support and location for the later dome re-assembly in place on top of the octagon.

Completion of the dome's ribs, observing slit and struts must all take place down on the relative safety of the ground. Though I might build some simple supports to lift it to a more comfortable working height. Wooden blocks will have to be used anyway to level the base ring on the sloping and undulating lawn. So an improved means of support makes sense. The dome will need protection form the weather. I seriously considered buying a PVC tent-style garage and selling it on afterwards. The problem is a serious lack of ground space unless the car is parked elsewhere than its familiar bald spot.

The completed dome skeleton will be dismantled into single segments for ease of handling. Each segment will be carried up, singly, and placed on the completed base ring. Weight and physical manageability are vital factors in safe and easy assembly working well above the ground. I shall work as much as possible inside the dome from sturdy, 5' high builder's stepladders with crossbar stabilizers for feet. 

There is a slight problem with the gores/segments being quite narrow so I can't easily reach between them. Though this shouldn't be necessary as it mostly involves clamping and screwing the already assembled segments in place. Which is hardly taxing provided I can easily reach the top of the dome.

The 2' wide observation slit will be wide and parallel offering improved access.

Working down on the ground it is obviously much easier to work from the outside. The dome is much too low to work easily from below when down on the ground. I could work on final assembly from the veranda but the risks of a fall are greatly increased. Though I do own a proper safety harness if I am suitably tethered. 

Once the dome skeleton is completed I shall clad it in its pre-cut plywood panels. Now the dome is nicely stiffened it can be jacked up at intervals onto wooden blocks. Then the wheels inserted between the base ring and flat, 2x6 wall sill. Work on cladding the dome might be easier if it were able to rotate but the advantage is probably minimal compared with having a flat and stable base.

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Dome build: Working with a full set.

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A surprisingly better day than forecast allowed me time to finish the remaining 7 segments. It went quickly enough when I used the countersinking pilot drill. Then drove all the screws half way through before entering pre-drilled holes and fixing the struts. It's the sort of job you get better at just as you finish.

The image shows the result of having all 16 gores/segments to play with. I had yet to confirm the true height as lunch intervened. A short shower of less than two minutes teased me half way through.

The spare arc from the base ring will join another to test the curve's full height over the pole. With all sixteen gores connected by the horizontal struts there isn't much room for error. The dome becomes far too stiff in all planes to allow much flexure.

The stepladder shown here is 1.5m [5'] tall at the top rungs. Which is the correct height for the inside of the finished dome. I laid an alloy plate on the top rungs to give me more area on which to rest all the extended ribs.

I'm calling this one "Total chaos!"

Unfortunately it soon became a total mess. This was because the base ring radius and length of the spare arcs changed from one segment to another. Spring clamps were flying off everywhere as I tried to add new rib extensions and pull the segments outwards. Short, sharp showers did not help.

There is only one way I can think of,  to sort out the dome's height issue. I shall just have to cut some more 9mm [3/8"] rib arcs. Then make each rib its correct length to just reach the pole. Only then can I check if the dome's geometry is true. It seems a waste of expensive plywood because the arcs have no need to reach the pole due to the large [2' wide] cut-out of the observing slit. However, I can always use any extra arcs for reinforcing the rib lap joints later. The weakest part of the dome will be the upper sections during snow loads.

The forecast is even worse tomorrow with high winds as well as rain. It was horribly wet but not very windy.

A dome which fails to reach the correct height can be physically raised to give more headroom. Using taller walls or even a much thicker base ring. Which both amount to the same thing. Or, the gores can be slightly expanded as they rise to stop them meeting at a point [the intended pole] which is much lower than planned. The horizontal struts can be pushed upwards to achieve this at the cost of taller, upper panels. The alternative, if the panels must remain as designed, is to cut longer struts and keep those at the same height.

The dome shrinks when hastily clad in a lightweight tarpaulin for yet another, heavy shower.

There are slight advantages, for myself, regarding perspective with having slightly taller, upper panels. My low viewpoint inevitably shrinks the upper panels as seen from the garden below. This is because the dome is lifted, higher than usual, on a low 'tower.' Moving the struts upwards, towards the pole, will save me the cost of replacement materials and the time in cutting all new lengths. First I shall have to make the upper rib extensions to see exactly what is needed. 

I am referring only to trapezium paneled domes here. Conventional, gored, hemispherical domes are affected in similar ways. Though the horizontal struts do not dictate physical bends between dome panels. Normally the gores would be sheathed in continuously and smoothly curved sheet material as far as is possible to avoid horizontal joints.

Thursday 5th October: Heavy, continuous rain this morning so I tidied the shed/workshop. I need to be able to work outside on the dome to have enough space to move.

Click on any image for an enlargement.

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